National Physical Laboratory

I L. R s See note inside cover SHIP REP. .100

October 1967

NATIONAL PHYSICAL LABORATORY

SHIP DIVISION

HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

A, Silverleaf and J, Dawson

(Reprint from Trrn'sactions of Royal Institution of Naval Architects VoL 1091967)

A Station of the Ministry of Techno]ogy Crown Copyright Reserved

Extracts from this report may be reproduced provided the source is acknowledged.

Approved on behalf of Director, NPL by Mr. J. A. H. Paffett, Superintendent of Ship Division Reprinted from RINA TRANS., APRIL 1967, Vol. 109, No. 2, pp. 167-1 96

SUMMER MEETING IN GERMANY

l2m-16TH JuNr, 1966

THE SCHIFFBAUTECHNISCHE GESELLSCHAFT E.V. TiINSTITUTE OF MAItme ENGINEERS THE INSTITUTION OF ENGINEERS AND SHIPBUEDERS IN SCOTLAND ThE NORTH EAST COAST iNSTITUTION OF ENGINEERS AND SHIPBUILDERS THE ROYAL INSTITUTION OF NAVAL ARCHITECTS

HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

By A. SILVERLEAF, B.Sc. (Member of Council),* and J. DAWSON, B.Sc. (Member)t Read in Munich on June 14, 1966, The Right Hon Viscount Simon, C.M.G. (President R.J.N.A.), in the Chair

Summary This paper discusses some of the hydrodynamic features of medium size and large merchant ships intended to operate at speeds higher than those general today. The ships considered are bulk carriers, tankers, cargo liners and passenger vessels from about 400 ft. to 1,000 ft. in length with service speeds from just below 20 knots to above 30 knots and which may have propelling powers up to about 100,000 h.p. on one or two shafts. Power requirements in calm water are considered and criteria in the form of a boundary speed and hydrodynamic efficiency factors are introduced. These criteria are applied to fine form cargo liners and large, full form tankers and bulk carriers at relatively high speeds.Some effects on propulsive efficiency of varying propeller diameter and rate of rotation are examined, and the possible advantages of contra-rotating and ducted propellers are discussed. Some of the factors which affect the performance of high speed ships in waves and their manoeuvrability and steering qualities are then described.These include freeboard requirements and the influence of ship length and speed on pitch and heave motions in specified sea conditions. The effects of bulbous and ram bows on resistance in calm water and on seakeeping behaviour are also discussed. Finally, possible future developments of unorthodox high speed merchant ships are briefly considered. These include ships designed for super-critical operation, submarine tankers and cargo ships, and very high speed displacement ships.

Infroduction although not very much larger than her predecessor, is very During the past twenty years there have been many remarkablemuch faster.Indeed many modern cargo liners, with service changes in the size and composition of the merchant fleets ofspeeds above 20 knots, are among the fastest merchant ships the world, and in the sizes and service speeds of the ships which afloat on a basis of speed-length ratio, and have hull forms even form their largest groups. By about 1949 the total active worldfiner than those considered suitable for passenger liners and fleet, excluding the U.S. reserve merchant fleet, had replaced ferries. wartime losses and overtaken its total pre-war size of about Many of these changes in the size and speed of ships have 70 mithon gross register tons (or about 100 million deadweightbeen accompanied by changes in hull form proportions and shape tons).In the next fifteen years the active world fleet doubled inand by marked alterations in the characteristics of propellers. size, and at the end of 1965 totalled about 160 million g.r.t. (orAlmost all of these have been natural developments and exten- about 220 million tons dwt.). The composition of the fleet alsosions of earlier practice, and there have been few abrupt breaks changed considerably; the proportion of tankers and dry cargo in the steady evolution of ship forms and propulsion devices ships increased substantially, and the average size of ships in in the continuing attempt to maintain and improve the standards these dominant groups also increased markedly. of hydrodynamic efficiency.It is reasonable to suppose that The service speeds of ships have also changed during the past the size of many types of ship will continue to increase and that twenty years, probably more than in any previous similar period. service speeds will rise further; indeed, speeds appreciably higher As the size of the largest tankers has increased tenfold (fromthan those general today are likely for several important classes about 20,000 to 200,000 tons dwt.), so the service speeds of the of ship. Will it be possible to satisfy shipowners' future demands fastest tankers have tended to rise slowly from about 14 knotsfor higher speeds without introducing quite novel hull forms and to about 16 knots. On the other hand, the pre-war cargo tramppropulsion devices? To what extent can present design methods has been steadily superseded by the cargo liner of today, which,provide good hydrodynamic performance if much faster ships become essential to maintain economic competitiveness? The Superintendent, Ship Division, National Physical Laboratory.aim of this paper is to present some hydrodynamic data f Head, Ship Design Branch, Ship Division, National Physicalwhich will be of assistance in answering these and similar Laboratory. questions, and which will help shipowners decide in which 167 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION direction, and to what extent, progress is both possible andrelationTq 2-08 - 2 CB.An independentanalysisby desirable. Hughes(2) of N.P.L. model data showed that Dawson's relation The ships which dominate world merchant fleets are tankers,gave a reasonable indication of the limiting value of ® for a bulk carriers and dry cargo liners from about 400 ft. to moregiven hull form up to speeds for which the mean wave resistance than 1,000 ft. in length, and their present service speeds are, invaries as the sixth power of the speed and can be defined by the general, from about 15 knots to less than 25 knots.This first although Hughes pointed out that it overall assessment of future ships is concerned primarily withformula ©= y ships of these classes having somewhat higher service speedstended to underestimate the limiting speeds for finer forms. from just below 20 knots to 30 knots and above. A more recent analysis of N.P.L. data, including further results for forms both finer and fuller than those originally available, has yielded a relation which endorses Hughes' com- Power Requirements in Calm Water ment. The boundary speeds for about 100 representative single- Before considering in detail the hydrodynamic design featuresscrew forms (excluding tugs and trawlers) and about 50 twin- of high speed merchant ships, it is desirable to define what speedsscrew forms (excluding ferries), selected for good performance, are to be regarded as high, and to establish efficiency criteria bycovering a very wide range of values of block coefficient and which to judge the performance standards of present and futurelength-beam and length-draught ratios, and including values ships. for two fine single-screw forms (CB 050 and CB 0-525) specially designed to provide information for this purpose, were carefully Boundary Speed examined.Although other parameters are doubtless important in determining the boundary speed, this analysis suggested that The concept of "maximum economic speed from hydro- block coefficient may be taken as the dominant parameter for dynamic considerations" has often been used as a measure ofboth single-screw and twin-screw forms having block coefficients high speed for a hull form, though generally without any clearfrom 050 to 0-86, and that a common simple relation involving or precise definition.Formulae of the type Tq = a - b CB,speed, length, and fullness only can be useful for preliminary whee Tq is the Taylor quotient or speed-length ratio V/,/L, design purposes.Within the ranges of length-breadth and C is the block coefficient, and a and b are constants, have beenlength-draught ratios indicated on Fig. 2, an acceptable relation much used to calculate the highest speed at which it is "wise"for the boundary speed is to drive a hull of specified fullness; the original Alexander formu1a(' and its later variations are well-known examples of T = l-7 - l4CB . . . . (1) such formulae. During the past few years several attempts have been made at N.P.L. to analyse available model resistance dataand this will be taken to indicate the "normal upper speed" for to derive a more up to date relation of this kind, though nota form of specified fullness.Speeds above this boundary value necessarily of this simple form.In these analyses the vaguewill be regarded as "high speeds" for the purposes of this paper; concept of "maximum economic speed," which cannotbe ships which operate at speeds above their boundary speed will defined in terms of hydrodynamic factors alone, was replacedbe defined as "overdriven."Fig. 2 illustrates relation (I) for by a rather more precise definition of a "boundary" speed. Forthe boundary speed in dimensional terms; it facilitates deter- any given hull form, the Boundary Speed is defined as thatspeed minatic'n of either boundary (maximum) speed, boundary below which the resistance coefficient does not vary greatly and (maximum) fullness, or boundary (minimum) length separating above which it begins to increase rapidly.Although it has notthe "normal" and "overdriven" regions. yet been possible to express this definition in exact mathematical An alternative way of presenting relation (1) for the bourdary terms, it was found that, for most hull forms, thisboundary speed is shown in Fig. 3. This gives the value of the displacement- speed could be derived with reasonable precision from a curvelength ratio L/(0 -01 L)3 at any boundary speed Tq for different of resistance coefficient in terms of speed coefficient (such asvalues of the product of the length-breadth and length-draught or Tg), as indicated in Fig. I. ratios; greater values of displacement-length ratio correspond © or C, in terms of tooverdriven conditions. Typical values of the product An early analysis by Dawson, quoted by Hughes,2> of N.P.L.(L/B) (L/T) are below 80 for trawlers, around 120 for cargo model data for loaded single screw ocean-going vessels withliners and tankers, and froni 160 to 250 for passenger liners. normal bows, led to the relation Tq = 1-63 - 1-3 CB for the Other ways of defining the boundary between normal and boundary speed, similar in form to the original Alexander overdriven regions have also been examined; one of these which has been used by designers is a formula of the type proposed by Posdunine as quoted by Baker,t3giving the minimum "economic" length for given speed and displacement as

L=24[V/(V+2)]2i.)I3 . . . (2) This formula has been found to have very limited validity. OVt RON IVE N

REGION Hydrodynamic Efficiency Factor NORMAL RENSTA?&C! The simplest and clearest measure of hydrodynamic efficiency OCF,!CICNT S PE E N S is the power needed to propel a specified displacement at a set speed; the long established Admiralty Coefficient, and the Telfer merit factor as modified by Saunders,4 are overall criteria of this kind.However, the required machinery power, or propeller delivered horse power, isinfluenced by the type of machinery if this controls the propeller rate of rotation, since this affects the attainable propeller open water efficiency. Conse- quently, a better measure of the hydrodynamic efficiency of a SPE,O COCrrIC!tnr hull form and its appendages is obtained by eliminating propeller FIG. 1 DERIVATION OF BOUNDARY SPEED FROM TYPICALRESISTANCE CURVES open water efficiency from the criterion, but retaining factors 168 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

r 0-3 08 0- S V/it FI-L 06 0-2 .7- l-4 30 04 0-IS 0.4 0-6 0-6 C9 .0

V l(ÑOTS

AR0) MATERANGE I0 SINcLE SCREW TWf N SCREW L/B 64-7.7 65 8.5 23 - C8 0.54 0.80

FIG. 2.BOUNDARY SPEED FOR SINGLE AND TWIN SCREW ShIPS

such as or F for a very wide range of good quality hull 3O forms.This gives, as desired, a criterion involving only speed, displacement and power which is considered to provide a useful basis for assessing the quality of a hull form and for estimating power requirements. Although length is not explicitly involved in this relation, it is necessary for direct comparisons to refer the hull resistance coefficient © and appendage resistance factor (1 + b) to a standard reference hull length this has been taken as 400 ft. and the values of the factor H in Fig. 4 are for 400 ft. which can be expressed as

H400=26 0-5 (g) - Single screw

00 for cg) = 1-2 to 26 (4) H400 = 238 0-5 - Twin screw for (j') = l4 to 2-8 FIG. 3.MAXIMUM DISPLACEMENTLENGTH RATIO AT HYDRODYNAMIC BOUNDARY SPEED Values of H for other ship lengths can be derived from the correction factors also shown in Fig. 4, which are sufficiently which take account of the interaction effects between the hullaccurate for preliminary design and assessment purposes. Some (including any appendages) and the components of propulsivetypical values of speed-displacement constants are given in efficiency.An efficiency criterion of this kind which gives anTable I. overall assessment of the hyd.rodynamic design of the hull and For many hull forms for which propulsion experiments have pendages is a Hydrodynamic Efficiency Factor defined by been made at N.P.L. at speeds up to about 20 per cent beyond the boundary speed, values of H were calculated for '7D/'7o 71-t 7R speeds below and above the boundary speed.It was found H (3) (1 ±b) (1 + b) ( that the ratio H/HB, where H8 is the hydrodynarnic efficiency s factor at the boundary speed and H that at any other speed, decreases steadily as the speed increases, and that this ratio It was found that, for speeds at or close to the boundary speedH/HB is generally independent of hull form and of the absolute given by relation (1), the values of this hydrodynamic efficiencyvalues of H8.This variation with speed is shown in Fig. 5, factor H varied consistently with a speed-displacement ratioand provides a starting point for assessing the power require- 169 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

20 VALUES 0F '1460UT = 7- .4 C0

SINSLE SCREW / H.00.260E ®

/f TWIN SCREW '2 N405 2. 30 - OS

10 H -

l-4 IS IN 20 2.2 2.4 2-6

2-5 30 % 3.5 4-0 4-5

0-4 OS F, 0-6 0.7 0-0 10 VARIATION 0F H WITs SHIP LENCTH V/v

FIG. 5.EFFECT OF SPEED ON 1-IYDRODYNAMIC EFFICIENCY AND lOS POWER H, P and j0 are values at any speed V HB, PB and soB are values at boundary speed V 400 600 000 1000 200 special features of the hull form, the value of H0 may be t. estimated more accurately from Figs. 6 and 7. FIG. 4.HYDRODYNAMIC EFFICIENCY FACTOR AT BOUNDARY SPEED

2-T TABLE I 5.0 TYPICAL VALUES OF SPEED-DISPLACEMENT CONSTANT AT THE BOUNDARY SPEED I-B

H400 Ship Type i V/z F I'S

Largetankerorbulkcarrier .. 2-4 1-4 04 .4 Coaster ...... 2-6 l5 O-45 Dry cargo ...... 3-2 1-9 0-55

Refrigerated liner . . . . .- 33 19 0-55 Trawler ...... 3-8 22 065 .5 Cargo liner ...... 4-1 2-4 065 Vehicle ferry ...... 4-2 25 07 FIG. 6.EFFECT OF CHANGES IN RESISTANCE COEFFICIENT ON HYDRO- Passenger liner ...... 45 26 O'75 DYNAMIC EFFICIENCY FACTOR AT BOUNDARY SPEED Single screw ships ments for "overdriven" ships intended to operate at speeds appreciably above the boundary speed and higher than those general today. The marked drop in the hydrodynamic efficiency factor H at high speeds is a first indication of the penalties which such speeds impose; a further penalty is the inevitable drop in propeller open efficiency, discussed later, also shown in Fig. 5. u .4 Hull Resistance Coefficients 'Z The values of the resistance coefficient © at the boundary speed clearly vary with several form parameters, but it is of some 1.0 interest to note that a value of © 071 (Froude basis for length ° 400 ft. L) is a reasonable first approximation at all values of block coefficient.The values of H400 in Fig. 4 correspond FIG. 7.-EFFECT OF CHANGES IN RESISTANCE COEFFICIENT ON HYORO- broadly to © 0-71; if the resistance coefficient is known to have DYNAMIC EFFICIENCY FACTOR AT BOUNDARY SPEED a different value at the boundary speed, perhaps becauseof Twin screw ships 170 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

It is of interest and practical design importance to know whathas suggested that it is possible to provide a first estimate of proportion of the total resistance is wave resistance; estimates propeller open-water efficiency at the boundary speed in terms of of this depend on the methods used to separate the total block coefficient.Fig. 9 gives approximate values of these open- measured model resistance into viscous and wave components water efficiencies for propellers designed for N 120 rev.Jmin. at and to convert these into ship values.The recent analysis byL 400 ft. (or n,/ L = 40 for all values of n in rev./sec. and L in Hughes(2 is considered to represent the most satisfactory methodfeet); these values can be expressed as: of separating these components, and this has been used to derive m (120) = 098 - 0-55 CB - Single screw (5) estimates of "good" values of the wave resistance coefficient and (120) = o-90 - 0-33 CB - Twin screw at the boundary speed.Fig. 8 gives the relation between this wave resistance component and the total resistance; althoughAn indication of the drop in efficiency when the propeller rate this will vary slightly with shiplength, it shows that, at theof rotation is increased is given by the correction factor boundary speed, for full forms (CB 080) the wave resistanceso that: . (6) is about 20 per cent of the total, while for fine forms (CB 055) o(N) = o (120) + &o . . the wave resistance is generally at least 40 per cent of theThe effect on these efficiencies of changes in ship speed above total resistance.At overdriven speeds the proportion of theand below the boundary speed is shown in Fig. 5. total resistan due to wave-making is greater and increases rapidly with speed. Preliminary Power Estinlates The delivered horse power is readily given in terms of the hydrodynamic efficiency factor H and the propeller open-water efficiency m by the relation Propeller Open Water Efficiency 1 (1 + x)213 For most ships, unless the screw diameter is severely restricted, y3 . . (7) the open-water efficiency of the propeller is largely independent 427 0H of the overall hydrodynamic efficiency of the hull and appendagesin which the load factor (1 + x) is the performance prediction as defined by the factor H.Limitations on the draught of afactor linking model and ship powers (as defined in Ref. 5), and ship or, for very large vessels, those imposed by propellerz, V and H are the ship displacement, speed and hydrodynamic manufacture may restrict the propeller diameter to considerablyefficiency factor respectively.The possible significance and usefulness of the criterion H for power estimates are not affected by the method of extrapolation used to derive the resistance coefficient for the ship from that measured on the model. The 0.6 absolute values of H will, of course, depend on the method by which the ship resistance coefficientis obtained, but power C. $ estimates will not be affected because the performance prediction factor (1 + x) will also change correspondingly. 0.4 The effect on power requirements of increasing speed above the boundary speed can be readily estimated from the hydrodynamic efficiency ratio H/H2 and the propeller open water efficiency ratioO/OB in Fig. 5.These together give a power ratio P/P2, in whichB and P are respectively the shaft powers C.? at the boundary speedB and at any other speedV, in the form P o. t y'3 (HB\ 'VB) 'ii) ') (8 o- This power ratio for single and twin screw forms is also shown in Fig. 5 and typical values are given in Table II; these show FIG. 8.APPROXIMATE RELATION BETWEEN WAVE RESISTANCE AN]) that to increase speed 10 per cent above the boundary speed TOTAL RESISTANCE AT HYDRODYNAMIC BOUNDARY SPEED demands an increase in power of about 50 per cent, while to provide a 20 per cent increase in speed the power must be almost two and a half times that needed at the boundary speed. less than the optimum value; in other cases the propelleropen- water efficiency depends primarily on its rate of rotation and Higher Speeds for Cargo Liners and Tankers on the required thrust and speed of advance, which, at a known ship speed, can be broadly related to resistance and hull fullness. Fine Forni Cargo Liners Examination of data from propulsion experiments at N.PL. in The general criteria developed in the preceding sections can which the propeller diameter was close to the optimum valuebe used to examine some of the problems which may occur if TABLE II EFFECT ON CALM WATER POWER OF HIGH SPEED OPERATION

Speed ratio ...... V/V2 08 0-9 1-0 1l l-2 (V/V2)3 051 073 1-0 1-33 173 Hydrodynamic efficiency factor .. H/HB I 08 1 Ø5 I Ø 090 074 Propeller open-water efficiency ratio-. o/oB Single 101 i-oi io o-98 0-94 Twin 1-01 1-01 1-0 Ø.99 0.97 Power ratio ...... P/PB Single o-47 0-68 1 -0 1 -51 2-48 Twin 0-47 0-68 10 l-49 2-41

171 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

0.8 ''oATTc= 17-I4 C

TWIN SCREW 0.7 CB (l2o) - -.

0.6 SINGLE SCREW 170.98_0.55 CB 05 05 06 0.7 08 09 CB

0'l (w) = (12o)+

o .TS

-0.I SS

-02 100 ISO 0O 250 N (iev/mfri)

f I I I 20 40 60 80

FIG. 9.-PROPELLER OPF.N WATER EFFICIENCY AT HYDRODYNAMIC BOUNDARY SPEED the service speeds of certain classes of ship are raised abovedraught.For such a ship the maximum block coefficient to those current today. The modern cargo liner is an outstanding avoid overdriving is about 059 at 20 knots falling to 050 at example of this trend, and for some time there has been a growing about 23 knots, and to even lower nominal values at higher need for design data for hull forms suitable for this type of ship. speeds.Since a block coefficient less than 050 is unlikely to To avoid excessively "overdriven" conditions these forms haveprovide either sufficient cargo capacity or adequate stability, block coefficients less than 060, and for such fine shapes it isthis value has been taken as a practical lower limit, and thus at often difficult to reconcile the conflicting demands of low power all speeds above 23 knots it is not possible to avoid overdriven and adequate initial stability to provide a safe working margin. conditions.Preliminary power calculations have been made Available information has recently been surveyed and assessedfor both single and twin screw ships of these dimensions for by Moor(6); this includes N.P.L. data from designs to meetservice speeds up to 30 knots8 using results derived directly specific requirements of owners and builders and from others from N.P.L. experiments with a form of block coefficient 050. specially developed by N.P.L. as parent forms for two B.S.R.A. In these calculations, summarized in Fig. 10, the hull resistance methodical series (CB 060 and CB 055) to provide information coefficients for speeds above the boundary speed (VB 23 knots) on the effects of systematic changes in principal form para-are estimates for forms designed specifically for these higher meters.However, recent experience has shown that data arespeeds and are thus less than the values derived directly from needed for even finer forms, and two further parent formsthe N.P.L. parent form. Although there are several important (CB 0525 and CB 050) have been developed independently at differences between single screw and twin screw hull forms, including the effective hydrodynamic length and the possib.Llity Largely because of port limitations, many recent high speed of increasing the effective length of a twin scrcw form by having cargo liners have closely similar dimensions.It is thus possiblea transom stern, these have not been taken into account in these to suggest that a reasonable "basis" ship to represent anpreliminary estimates, thus giving the same hull naked resistance important group in this class for the next decade or so will havefor both single and twin screw forms.However, the total dimensions about 530 ft. 78 ft. breadth, and 30 ft. load resistance of the twin screw forms is 10 per cent greater than that 172 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION of the single screw hulls to allow for the drag of the shaft supports. particularly if the power output of a single engine is raised For both single and twin screw hulls it was considered pos-above present levels, there is clearly a field for other forms of sible to accommodate propellers of diameter 225 ft., and itpropelling machinery. was found that the propulsive coefficients are greater for twin Since the high powers needed to maintain speeds of 25 knots screws than for a single screw, while the optimum propeller rate ofand above may make it economically impractical to operate rotation is significantly lower for twin screws; these differencescargo liners of 530 ft. length at these speeds, some further are direct consequences of the lighter loading of each of the twinsimilar calculations have been made for larger ships having screws. The overall result, as shown in Fig. 10, is that the required dimensions 650 ft. 95 ft. breadth, and 38 ft. draught. For machinery powers are essentially the same for single or twinsuch ships the boundary speed is about 254 knots at block screw propulsion.It is of interest to note that power estimatescoefficient 050, and about 244 knots at CB 0525.Conse- made using the simple method embodied in relation (7) and inquently, these calculations were made for speeds from 24 to Figs. 4, 5, and 9 over the speed range for which they are valid30 knots for CB O5O and CB 0525. The results, summarized give results which agree closely with those in Fig. 10. in Fig.11, show some remarkable and perhaps unexpected It is possible to draw some tentative conclusions about futurefeatures.At 24 knots the powers required for the larger 650 ft. trends from Fig. 10. A cargo liner of the size and fullness con-ships, with displacements approximately double those of the sidered here could have a service speed of 25 knots with aboutsmaller ship, are about one-third greater than for the smaller 35,000 shp installed; although this is a high power, itis not 530 ft. ship. However, the difference narrows as speed increases, excessive even by present standards.However, service speedsuntil at 28 knots the powers are substantially the same for both above 26 knots appear unrealistic using machinery havingsizes of ship, and at 30 knots the larger ships actually require present power-weight characteristics, and even marine gasalmost 20 per cent less power than the smaller ship.These turbines would probably be impractical for the powers neededrather surprising comparisons emphasize the severe power to maintain 28 knots or more.The differences in optimumpenalty imposed by raising speeds signifiòantly above the propeller rates of rotation may well also influence the choiceboundary speed, and, of course, strongly suggest that, if cargo of machinery for higher speed cargo liners; while direct driveliners are to have service speeds above 25 knots, they should be diesel engines may be suitable for speeds up to 25 or 26 knotslarger than the present typical vessels of this class.

140000 200 SS TS 120000 + ESTIMATE FROM SO EQUATIOÑ (7)

(rev/ 111n)

100000 loo

60000 SO

HORSE POwER 60DOO

40000 15

2O00 ©SN

o OS 22 24 26 28 SERVICE SPEED (rics)

FIG. 10.-PoWER ESTIMATES FOR HIGH SPEED CARGO LINERS Ship 530 ft. X 78 ft. B mid. x 30 ft. T mid. x 050 CB Single screwTwin screw Appendage resistance coefficient(l+b) l0 1l Performance prediction factor (1 + x) 094 097 Service power allowance (l+y) 12S I25 Transmission efficiency (T) 098 098 Propeller diameter (ft.) 225 225 Prçpeller, No. of blades 4 4 173 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION Power estimates for the larger 650 ft. ships made by thefor CB 080 is higher than that for C 0825, the estimates have simple method previously described again give values whichbeen based on forms having CB 080, and the corresponding agree closely with those in Fig. 11, which were derived quitedisplacement and typical deadweight curves are also shown in independently using detail model resistance and propulsion data.Fig. 12, the deadweight-displacement ratio varying with size of ship according to information derived from shipbuilders' data. Full Form Tankers and Bulk Carriers The hull resistance coefficients used in the power estimates are Although for most tankers and bulk carriers under con-typical values forfull forms with good performance when struction today there is no definite relation between size andoverdriven. speed, it is reasonable to consider typical service speeds as It has been suggested that for very large tankers and bulk about 15 knots for ships 500 ft. long and 20,000 tons dwt.,carriers the propeller diameter and rate of rotation can have a increasing slowly to about 17 knots for mammoth vessels overstrong effect on power requirements, particularly if manufacturing 1,000 ft. in length and close to 200,000 tons dwt. As shown limitations result in the use of a screw well below the optimum in Fig. 12, these speeds are generally above the boundary speeddiameter, or the diameter is less than the maximum which could for full forms with block coefficient O80 for lengths up tobe accommodated because of the need to match diameter to high almost 800 ft., but below the boundary speed even for veryshaft revolutions. The maximum diameter of propeller from full forms with CB 0-825 at lengths above 900 ft.Althoughhydrodynamic considerations alone depends on draught aft hydrodynamic factors doubtless affect the service speeds adoptedwhen loaded and in ballast, on the need to have a cruiser stem by owners for tankers and bulk carriers, they do not appear toreasonably well immersed, and on the need to provide adequate be the decisive factor.It is therefore possible that service speedstip clearances above the keel line and below the immersed cruiser may be raised if other factors indicate that this is economicallystem.Detail examination of these factors suggests that, for justifiable, and some estimates have been made of the hydro-ships longer than about 500 ft., the maximum ratio of propeller dynamic consequences of raising the speeds of large tankers updiameter to mean load draught can generally be two-thirds. This to and beyond the boundary speed. gives a maximum permissible diameter from hydrodynamic con- Fig. 12 shows typical breadths and draughts of full formsiderations of about 30 ft. for ships of approximately 800 ft. in vessels built recently or at present under construction; althoughlength; since 30 ft. is presently regarded as the maximum dia- the proportions of some ships are appreciably different, it ismeter of propeller which can conveniently be manufactured, this considered that these values are sufficiently representative tocould mean that larger ships are at present penalized to some be used for general power estimates.Since the boundary speedextent by having to fit propellers of restricted diameter.

140000 SS

20000 / 7 N N

I- 100000 / / so CB 050 CB 0525 // + E5TIMA1E FROM / EQUATIOÑ (I) 80000 / + H05E POWER o00o / /, e- -r/ 40000

e- 20000 10 ©sN ©SN 0-5 24 26 28 30 24 26 28 30 SERVICE SPEED(Inots) Fia. 11.POWER ESTIMATES FOR HIGH SPEED CARGO LINERS Ship 650 ft. 95 ft. B mid. x 38 ft. T mld. Single screw Twin screw Appendage resistance coefficient(l+b) l0 l-1 Performance prediction factor (I +x) 0-89 094 Service power allowance (1 +y) 125 125 Transmission efficiency ('i-r) O-98 O98 Propeller diameter (ft.) 275 275 Propeller, No. of blades 4 4 174 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

o0000 power, transport efficiency increases steadily with ship size, even though the service speed naturally drops as ship size increases. The single curve of hydrodynamic transport efficiency in Fig. 14 can be taken as a starting point for initial estimates for a wide range of large, full form ships, and as a basis for comparing the qualities of different designs the variation in transport efficiency 200000- o with speed shown here agrees with the power ratio values in Fig. 5 and Table II. The curves of optimum propeller diameter for N I 10 rev/mm. Dw - ISO in Fig. 13 show that propellers of diameter up to 30 ft. are (tons) - adequate even for the largest tankers for service speeds up to the 1Go boundary speed; this is above 19 knots for ships 1,100 ft. in ,80000 - length, and is thus appreciably higher than present service speeds, and would demand powers probably greater than could be 40 transmitted by a single screw ship.However, the results of the B other power estimates for propellers free to run at optimum o rates of rotation, summarized in Fig. 15, show that some improve- (ft) monts in propulsive efficiency are possible using larger propellers than those indicated in Fig. 13.Some tentative general con- 100 clusions from these estimates are: T IO For propellers having diameters 30 ft. and above the ()40 maximum attainable open water efficiency may be higher ¶8 than the best possible when N is fixed at 110 rev./min., V 20 but these large propellers must run at less than 90 rev./min. if appreciable gains are to be achieved. 6 The advantage in open-water efficiency decreases sharply as ship speed increases; the ratio of open-water efficiencies 4 falls by about 10 per cent for 3 knots increase in speed in almost all cases. 12 The gain in propulsive efficiency with large, slow running 400 600 800 000 1200 propellers is less than the gain in open-water efficiency, L (;t) and in some cases very much less.This is due to the FIG. 12.TANsRs AND BULK CARRIERS, TYPICAL DIMENSIONS AND very strong influence of propeller-hull interaction effects SPEEDS which vary markedly with diameter-draughtratio. Detail knowledge of this effect is necessary before the To examine this point, estimates were therefore made for a full consequences of any change in propeller diameter series of full form ships up to 1,100 ft. in length for different and rate of rotation can be accurately assessed. combinations of propeller diameter and rate of rotation.Since Any gain in propulsive efficiency also decreases as ship most large tankers and bulk carriers have direct drive diesel speed increases. engines running at about 110 rev./min. an initial set of estimates (y) Gains of more than 10 per cent in propulsive efficiency was made for this propeller rate of rotation, ignoring any pos- appear likely only for the largest ships (L> 1,000 ft., sible limitation on propeller diameter due to manufacturing D > 30 ft.), and then only if the propeller rate of rotation capacity.Further estimates were then made assuming that the is reduced to about 70 rev./min. maximum diameter was limited to 25 ft., as was the case not long (vi) If the speeds of very large tankers and bulk carriers are ago, to 30 ft. as at present, to 35 ft. as may soon be possible, increased beyond their boundary speeds, there would and finally to 40 ft., accepting that such large propeller diameters appear to be less advantage in departing from propellers might lead to optimum rates of rotation considerably lower than suitable for present direct drive installations.Indeed, it those of present direct drive installations.In those cases where might be possible to obtain some of the gains now often the ratio of propeller to draught was less than two-thirds, considered dependent on increasing propeller diameter approximate values of wake and thrust deduction fraction were and reducing rate of rotation by shaping the afterbody estimated from the results of model propulsion experiments with to give favourable propeller-hull interaction effects at screws of varying diameter-draught ratio; these values gave hull smaller diameters and higher revolutions.Nevertheless, efficiencies which decreased significantly as propeller diameter although not significant in altering the basic power increased, principally because of the decrease in wake fraction requirements for higher speed tankers and bulk carriers, with increasing diameter-draught ratio. the differences due to present propeller restrictions are The results of these power estimates are summarized in Figs. 13, sufficiently important, even at current speeds, to justify 14, and 15 in forms considered useful for preliminary design special efforts to develop methods of manufacturing and purposes when assessing the hydrodynamic and other conse- handling larger propellers and of enabling them to run quences of increasing ship speeds.Fig. 13 gives the results of more slowly than is customary today. theinitialestimatesfor fixedpropellerrateof rotation N 110 rev/mm. The curves of constant speed and length give approximate values of the hydrodynamic transport efficiency Propulsion Devices for Cargo Liners and Tankers LW/dhp (or its reciprocal, the specific power dhpJL V) for speeds It has frequently been suggested that higher propulsive effi- from09 VBtoabove 11 Vforshipsoflength 800 ft. to 1,lø0ft.;ciencies can be obtained with a ducted propeller or with contra- these demonstrate again that increasing speed above the boundaryrotating propellers than with a single orthodox propeller. These speed involves not only sharp increases in power, but a markedsuggestions were examined recently at as part of a drop in transport efficiency.This point is also illustrated byfeasibility study into the use of geared medium-speed diesel the curves of constant power, which show that, for a given engineengines for cargo liners of the type considered in the previous 175 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

IZO 13 N CURVES OÊ CONSTANT N SPEED AND LENGTH N N 'N N too - NN\ S. N N 00 NS. N /dbp N

GO N N N -'__ N N? ...N 03 S.'I0 D ','- BOo 20 00 - N

80 l0

/dhp OPTIMUM PROPELLER

GO RATE OF ROTATION CURVES OF COÑSTANT loo POWER AÑO LEÑH oO N 28 --- 35 40 --- 80 I Ve V21 - _s - ___9_Q_ -- 30 O 9 Ve - 0 (ci) 25 OPTIMUM OPELLER II E IS 19 20 il DIAMETER DR N ITO V" v (kneEs)

20 FIG. 15.-EFFECT ON POWER OF C}IANGES IN PROPELLER DIAMETER AND RATE OF ROTATION FIG. 13.HoRoDyNAMIc EFFICIENCY OF LARGE TANKERS AND BtYLK CARRIERS Contra-Rotating Propellers L800-1,lOOft.: CB080 The gains in propulsive efficiency possible by replacing a V 15-21 knots: N 110 rev/mm. single propeller by a coaxial pair of contra-rotating screws have been demonstrated several times by the results of experiments and calculations published during the past fifty years. One of the clearest and earliest accounts of the principal effects is that by Luke,t10 while interest has recently been revived by work in the United States11 and elsewhere.The main conclusions of these investigations are: There is little difference between the "open" efficiencies of equivalent single and contra-rotating propellers designed to absorb the same power at the same ship speed and at about the same rate of rotation. How- ever, as the screw loading increases there is a growing advantage in favour of the contra-rotating pair, and this can be appreciable for the conditions in which higher speed cargo liners will operate. The interaction effects between hull and propeller are generally more favourable for contra-rotating screws than for a single propeller.These effects can be expressed by the ratio of overall hydrodynamic propulsive efficiency (iD)to propeller open efficiency (); improvements inD/iØ of more than 15 per cent have been V/v reported, particularly for full form ships, though the potential gain in propulsive efficiencyD is generally little more than 5 per cent. FIG. 14.-VARIATION OF HYDRODYNAMIC TRANSPORT EFFICIENCY WiTH There is some evidence that, at the same rates of rotation, SPEED the diameters of optimum contra-rotating propellers are slightly less than the equivalent single screw, and section, and for tankers and bulk carriers up to about 750 ft. this accounts for part of the improvement in interaction in length.The results of this examination, and of subsequent effects.There is no evidence that, for the same dia- work at N.P.L., are summarized here. meters, the optimum rate of rotation for a contra- 176 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

system is significantly different from that of the equivalent single screw; consequently, the gains possible LOAD FAOrOR with orthodox marine propellers by increasing diameter (i+ x) and reducing rate of rotation will also apply to contra-080 ..0 80 rotating screws. ' 85 1-00 Thus in many cases a contra-rotating propeller system on a I IO single shaft can have a higher overall propulsive efficiency than070 an orthodox single propeller absorbing the same power at the FROUDE AAL't5s same ship speed.This is principally due to more favourable LOADED CODlTION interaction effects between the propellers and the hull.These differences in efficiency have not been large enough to induce060 any general use of contra-propellers for merchant ships, pre- FORAVEP.AE TRIAL CONDIrIONS A sumably because of the mechanical problems and extra costs LOAD FACTOR O 085 SHOULD BE USED involved.If these objections are removed, a close examination of contra-rotating propellers may be justified. 4.s 150 155 160 185 170 175 V-knots Ducted Propellers For ship propellers operating at high loading coefficients, as Fia. I 6.PRopulsioNANALYSIS WITH DUCTED PROPELLER SYSTEM in tugs when towing or fishing vessels when trawling, the advantages of enclosing the propeller in a duct which accelerates the present the master ofa ship frequently finds it necessary to reduce inflow have been appreciated for many years.However, the speed in heavy weather because pitch and heave motions become operating conditions of some large tankers and bulk carriers excessive.Recent studies of ship motions at N.P.L.'3 have already appear to be in the range where ducted propellers mayindicated that the ship characteristic which most influences be useful and if service speeds are increased, this will be more pitching motion is ship length, and that the effects of variations likely.There are three main possible advantages; for a given in speed and block coefficient are small, though heaving motion thrust the rotor of the ducted system may be smaller than the is influenced by ship speed as well as length.The calculations conventional open propeller, although the overall diameter mayand measurements (both model and full scale) on which these be the same; a substantial proportion of the total thrust can beconclusions were based were generally for speeds below the transmitted by the duct, thus reducing the steady and theboundary speed, and to examine their validity at higher speeds fluctuating forces on the rotor; the duct may reduce the non-a further series ofexperiments and calculations are being made.14) uniformities in the inflow to the rotor, thus further reducingThese are for the fine forms (CB 050, CB 0525) previously the fluctuating forces which can cause shaft and hull vibration. mentioned(7); experiments are being carried out in irregular To determine whether these advantages can be achieved, and head seas reproducing sea states Beaufort 5 and 7 (as defined whether the furtheradvantage of improved propulsive efficiencyby the British Towing Tank Panel) for ships about 550 ft. in can also be obtained, experiments have been made at N.P.L. tolength at speeds up to 35 knots, while motion calculations have develop a ducted propeller system for typical single screw fullbeen made for a range of ship lengths and speeds in the same forms.These began with a relatively simple axisymmetric ductsea states. added to a hull form for a 750 ft. bulk carrier (CBO8O), and The results of the motion calculations for CB 0525 are given were intended to give basic flow and performance data for usein Fig. 17.These have been derived entirely from theoretical in subsequent improved systems designed as an integrated unit.considerations without using any empirical data, and agree well Although thisinitial ducted propeller arrangement did notwith the values directly derived from the model experiments demonstrate any clear advantage over a conventional open where direct comparisons are possible.These calculations thus screw, it indicated that a gain in propulsive efficiency should beprobably give a good indication of the way in which ship length possible for a larger ship in which the propeller loading coeffi-and speed influence motions for the range of length and speed cient would be greater, partly owing to the effect of diameter appropriate to present and future high speed cargo liners. restriction discussed earlier. Generally, variations in speed from Tq o9 to 11 have little Further experiments were then made with a model of a typicaleffect, except possibly on pitch and relative bow motion, and mammoth tanker about 1,000 ft. in length, again with blockthe most important factor is ship length.In all cases increase coefficient 080, fitted with an improved ducted propellerin ship length decreases motions and acceleration forward, in system,'2 the overall duct diameter being about 275 ft. andsome instances very markedly.The broad conclusion from that of the ducted rotor 23 ft.The propulsive coefficientsthese calculations is that increasing the size of present cargo obtained in these experiments are shown in Fig. 16; the valuesliners will improve rather than worsen their seakeeping qualities are about 10 per cent higher than those obtained with a modelwhile increasing their speed may not adversely affect these of a conventional open propeller of diameter about 25 ft.In qualities.Similarly, increases in the speeds of large full form the loaded condition about 30 per cent of the total thrust istanicers will not tend to affect their behaviour in most sea carried by the duct, and thus it should be possible to reduceconditions. the size of the propeller shaft as well as that of the rotor compared Some present high speed cargo liners have experienced steering with a conventional screw.These experiment results suggestdifficulties in strong following and quartering seas.The need that serious consideration should be given to installing ductedto keep the longitudinal centre of buoyancy aft of midships to propellers on very large tankers, particularly if their speeds arereduce calm water resistance and powering leads to afterbody raised above those usual now. sections which induce marked changes in the effective transverse stability when in waves coming from astern or on the quarter. Seakeeping Qualities These stability changes can cause violent rolling and yawing which make course-keeping difficult, and these effects may be Performance in Waves accentuated by attempts to maintain higher service speeds.It The likely performance of high speed ships in waves could be is possible that the introduction of a transom stern may be a critical factor in their development, particularly since at helpful in such circumstances, though increasing the initial 177 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

30 os RELATIVE BOW MOTION ACCELERATION FORWARD O4 SEAUFORT 20 03 S173 y3 02 to BEAUFORT S (ci) EEAUFORT 5 0.1

09 F, 027 o T 0.9 F 027 o 10 030 10 - 030 I.------033

7.5 - 25 HEAVE 20 50 - 3EAUF0T 7 15 2 2I3 IO 25 - (ii) EAUF0T 5 (de3)

o e Oo 500 600 700 800 300 500 600 700 800 L(ce) L (Çi) FIG. 17.EFFECTS OF SHIP LENGTH AND SPEED ON MOTTONS IN IRREGULAR HEAD SEAS metacentric height above the value of about 1 ft. customaryjected to critical examination, including carefully conducted today may also be beneficial. dispassionate model experiments, some novel ideas have undoub- The proper power allowance to enable service schedules to be tedly proved very successful in laboratory conditions and on maintained in the weather and sea conditions anticipated onmeasured mile trials, although their performance in normal normal routes is important in any ship design.It is particularlyservice conditions is generally difficult to assess. A hull feature important in high speed ships because of the large powerswhich should have increasing value as ship speeds increase is involved.In the power estimates made here the allowance for the bulbous bow, the general principles of which have long been average service conditions above the power needed for measured understood. A more recent innovation is a particular form of mile trials has been arbitrarily taken as 25 per cent for cargobulbous bow more correctly described as a ram bow; although liners and 20 per cent for large tankers at all speeds.Some the flow mechanism by which it operates is not yet clearly recent cargo liners built abroadtt5 have installed powers whichunderstood, it also may have increasing use as speeds rise. only allow a much smaller margin, but it is believed that these have not always been able to maintain service schedules. Clearly Bulbous Bows there is a strong need for information which will enable power Bulbous bows have been used for many years in high speed margins to be assessed more accurately, and model experimentsships in attempts to reduce resistance in the deep load condition, and measurements at sea are now being made to help satisfy thisand recentlytheirpopularity has increasedconsiderably. need. During the past twelve years about 60 sets of resistance eperi- ments have been carried out at N.P.L. for hull forms (other than Freeboard Requirements trawler forms) with and without bulbous bows where the direct Calculations and measurements of relative bow motion, sucheffect of adding a bulb can be readily determined.These bulbs as those shown in Fig. 17, are valuable in assessing the prob-were seldom simple additions to the parent form, but were ability of occurrence of wetness at the fore end of a ship.Pre- usually associated either with a reduced waterline angle of dictions of this kind have been made at N.P.L.t16 for ships ofentrance or a finer forward shoulder, or with both of these varying fullness and length in typical irregular head waves changes.Although the bulbs varied somewhat in size, shape, representing sea states Beaufort 5 to 9.These showed that, forand position, the majority had a bulb area ratio about 5 per cent a given probability of wetness, the necessary freeboardratio atwith a ram area ratio usually 7 per cent to 74 per cent. the fore end decreases steadily as ship length increases; indeed, The results of these experiments have been examined0' to for ship length above about 600 ft. the decrease in freeboardderive a broad indication from N.P.L. experience of the likely ratio is equivalent to a constant freeboard.Although theseeffect on calm water resistance of fitting a bulbous bow to a calculations were made for a relatively slow speed (Tq 060), itnormal ship form. The principal purposes of this examination is believed that the general trend of the results will apply towere to determine the conditions under which gains or losses higher speeds.If so, then the adoption of higher service speedsare to be expected, the way in which these are related to the for either large tankers or large cargo liners need not involve acharacteristics of the parent form, and whether these effects significant change in the proportions of the above-water formconfirm the findings of theoretical analyses, particularly the forward. suggestion that greater gains are possible when the wave resistance of the parent form is high. The first analysis con- Special Bow Shapes centrated on the effect of the bulbous bow on total resistance. Many unusual hull forms have been proposed during the past For each form the change in resistance after fitting .bulb was few years. Although most suggestions for special details in hullrelated to the total resistance of the normal form at a series of features have not justified the claims made for them when sub- speeds below and above the boundary speed.Fig. 18 is a 178 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

-o-00 Unorthodox High Speed Ships Previous sections of this paper have been concerned with some -005 of the hydrodynamic problems involved in raising the speeds of typical present day ships by 20 or 30 per cent.It is clear -004 that even such relatively unspectacular increases in speed will o only be achieved if shipowners consider it justifiable to install -003- propelling machinery with much higher powers than those fitted s today to almost all merchant ships other than passenger liners. -o02 GAIN Engineering developments may make such machinery both o available and economically practical, and itis not impossible -00l that within twenty years or so there will be marine propulsion 5' machinery with very much higher outputs than feasible today, and with such improved power-weight and fuel consumption c5 0-6 LOSS characteristics as to make much higher ship speeds commercially +0-Ql - o, attractive.Indeed, some owners have already made tentative 0-5 0.1 0.70-6 0.5 e LOSS 1 enquiries about ships with such high speeds that only completely

0-50 0-70 0-30 0-30 -00 IO novel machinery could provide the necessary power outputs- -V/,, and only completely novel propulsion devices could transform these powers into propulsive thrust effectively. FIG. 18.EFFEcT OF BULBOUS BOWS ON CALM WATER RESISTANCE Ships for Supercritical Operation Apart from hovercraft, hydrofoil ships and other relatively composite plot giving the average values derived iE this way;novel high speed marine craft, there are several possible forms as anticipated, it shows that, in general, a bulbous bow gives a reduction in resistance at speeds above the boundary speed,of very high speed surface and sub-surface ships which could and an increase in resistance at speeds below this speed.Next,utilize extremely high powers effectively to reach speeds of the resistance change due to a bulbous bow at the boundary40 knots and above.In a general survey of such ships,t18) speed was compared with the total and wave resistances of the Lewis compares the performance characteristics of long, slender parent form at that speed and with the "good" values inships (with very low displacement-length ratios), ships with Fig. 8; this indicated that appreciable gains at the boundaryvery large bulbs at bow and stern, semi-submarines in which the speed due to a bulbous bow occurred for forms for which themain hull runs just below the surface and carries a small super- wave resistance coefficient was higher than the "good" values.structure above water on hydrofoil struts, and submarines running either shallow or deeply submerged. A further analysis was based on calculated values of the wave Lewis stresses that seakeeping qualities are a vital factor in resistance over the whole speed range for each form; this confirmed that a bulbous bow is likely to be most effective if theassessing the possibilities of any surface or near-surface high wave resistance of the parent normal bow form is high, eitherspeed ship.One way of reducing motions in rough seas at because the ship is "overdriven" or because the wave resistancespeeds above 40 knots is to design for "supercritical" operation is greater than the lowest value attainable for its designedin which the period of encounter with the longest important operating condition.This clearly suggests that bulbous bows wave is shorter than the natural pitching period of the ship. will be of increasing value in all classes of ship as speeds areThe slender hull with a large bulb at both bow and stern is a increased above the boundary speeds.However, the problempotential supercritical ship, and it may well be that such unusual of deciding whether to incorporate a bulbous bow in any parti- hull forms may be essential when sustained sea speeds of 40 knots cular hull form should not be considered in isolation, but asand above become realistic for ships about 600 ft. in length. part of the more general problem of designing a low resistanceIn the meantime, power estimates for relatively conventional hull form to suit the specified design conditions. high speed submarines and surface ships may provide useful approximate standards of comparison.

Ram Bows Submarine Tankers and Cargo S/lips A detailed comparison of the power requirements for surface Recently ram or projecting bows have been incorporated inships and submarine tankers and cargo ships was made about many full form tankers and bulk carriers to obtain powersix years ago by Todd,t19 and its conclusions are broadly con- reductions in the ballast condition, and ships fitted with themfirmed in a more recent study by Watts.t20) These indicate that, have achieved excellent performances on measured mile trials,for equal deadweight and speeds up to about 25 knots, closely confirming the predictions of gains based on the results submarines of circular cross-section could be designed to have of model experiments.However, although these ram bowssubstantially the same power requirements in average service show clear advantages in the ballast condition, this is not so inconditions as surface ships, largely because submarines could the deep load condition, nor are the gains achieved in calm operate immune from the effects of bad weather and would thus water maintained in heavy seas.Indeed, recent model experi-need much lower service power allowances.However, such ments at N.P.L. with models of full form ships in waves havesubmarine ships would have excessive draughts, and if these are shown no difference between normal hull forms and those fittedavoided by using elliptical sections, then the superiority of the with ram bows.Consequently, the decision whether a ram bowsubmarine disappears.In addition, submarine merchant ships should be fitted to a ship depends on the proportion of time at have obvious handling and rnanoeuvring difficulties. sea likely to be spent in the ballast condition, and also on the distribution of weather conditions likely to be met over a fairly Very High Speed Displacement Ships long period in service.It is therefore not easy to decide whether Although it is extremely unlikely that large, single hull dis- to recommend fitting a ram bow or even a more conventionalplacement ships will be built to operate at very high speeds, it bulbous bow, and efforts are being made to establish criteria may be of some interest to estimate the general characteristics to determine the design characteristics giving the highest hydro-which such ships would have.As a starting point, tentative dynamic efficiency for a ship throughout her service life. power and weight estimates have been made for a series of very 179 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION fine form ships (CB 040) designed for service speeds from 30all consumption of about 05 lb./hp/hour for a range of 1,000 to 60 knots, and the results of simple calculations for one ofmiles, and machinery power-weight ratios of 400 and 200 hp/ton this series are given in Fig. 19.For a ship 400 ft. in length athave been assumed; about 400 hp/ton has been achieved for 50 knots the speed-length ratio is 2 5; this value is considerablymarine gas turbine plants of 20,000 shp, but this high ratio may higher than any likely boundary speed of the kind considerednot be possible for much higher powers. On the basis of these previously, and would require a hull form with very differentassumed figures, Fig. 19 shows that a useful payload between characteristics from those used as a basis for the high-speed500 tons and 1,000 tons might be possible at speeds about cargo liners for which estimates are given in Figs. 10 and Il. 50 knots.Although these deadweight and power figures are A round bilge form with high prismatic coefficient and a transomdifferent from those for equivalent hovercraft and hydrofoil ships,t21they are sufficiently similar to provide a basis for coo 25 practical comparisons between these forms of high-speed marine craft. Acknowledgment The work described in this paper forms part of the research ,40O 2.0 programme of the National Physical Laboratory and is published n z by permission of the Director of the Laboratory. 'I) d hp n Symbols and Nomenclature 300 t.5 = Amplitude of significant acceleration. a, b= Constants in formula for boundary speed. 2 (1 + b)= Appendage resistance coefficient. 200 1.0 B = Breadth of ship at load waterline. Q- CB = Block coefficient. -U 1,000 r 2 =Circle resistance constant, non-chmen- 00 3000 sional if in consistent units. WEl'!T ©sN = Circle resistance constant for naked ship at 15° C.(590 F.). 2000 = Circle wave resistance constant. r (tons) = Resistance coefficient, non-dimensional if Py2 in consistent units. f000 dhp = Delivered horse power. y Speed-length constant, non-dimensional 04 o F - if in consistent units. (Froude number) " - Speed-displacement constant,non-dimen- \/g V " sional if in consistent units. 0.2 g = Gravitational acceleration. H = Hydrodynamic efficiency factor.

o = 05834 = Speed-displacement constant 30 40 SO Go y (knos L = Length of ship (generally in feet). FIG. 19.-VERY HIGH SPEED DISPLACEMENT sHIPS = Length between perpendiculars 400 ft. x 64 ft. B x 22 ft. T x 040 CB N = Ship propeller rate of rotation. 6,400 tons SW. P = Horse power, in general. r= Resistance, in general. s113 = Amplitude of significant bowmotion. stern would probably be suitable, and a systematic series of S = Wetted surface area. experiments with such forms has been in progress at N.P.L. for t = Thrust deduction fraction. some time; data from this H.S.D. (high speed displacement) T = Draught of ship. series has been used for the power estimates in Fig. 19. Although only approximate, these estimates show that extremely high Tg = Taylor quotient (as defined by Saunders) powers would be needed to reach high speeds, even in calm or speed-length ratio. water; for 50 knots the power exceeds 350,000 dhpa quite unrealistic value by present standardsand the hydrodynamic y = Speed, in general. transport efficiency ¿ V/dhp is less than I, compared with values V = Ship speed in knots. of over 70 for large tankers and between 15 and 20 for cargo = Boundary speed in knots. liners at their boundary speeds. w = Taylor wake fraction. To give an indication of the possible useful payload or dead- x = Overload fraction. weight of such a ship, weights of hull, machinery and fuel have y = Service power allowance fraction. been estimated from information for smaller high-speed ships z113 = Amplitude of significant heave. and from other sources.The hull weight has been taken at = Displacement of naked form (generally in just over 2,500 tons, the fuel weight has been based on an over- tons S W). 180 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

= Change in ship resistance constant. LUKE, W. J.: "Further Experiments upon Wake and Thrust 6'10 = Correction factor for open-water efficiency. Deduction," TRANs. I.N.A., 1914. = Quasi-propulsive coefficient. HADLER, J. B., MORGAN, W. B., MEYERS, K. A.: "Advanced - Propeller Propulsion for High-Powered Single-Screw '1H -w= Hull efficiency. Ships," Trans. S.N.A.M.E., 1964. = Propeller open water efficiency. ENGLISH, J. W., GRANT, S., POULTON, K.: "Mammoth = Relative rotative efficiency. Tanker Propulsion with a Ducted Propeller System: p = Mass density of water. Experiment Results," N.P.L. Ship Division Tech. Memo. 121, 1966. 6/3 = Amplitude of significant pitch. EWING, J. A., GOODRICH, G. J.: "The Influence on Ship y = Volume of displacement. Motions of Different Wave Spectra and of Ship Length," TRANS. R.I.N.A., 1966. References GOODRICH, G.J.:"Comparisonof Calculatedand AYRE, A. L.: "Essential Aspects of Form and Proportions Measured Motions in Waves for Single Screw Hull as Affecting Merchant Ship Resistance and a Method of Forms with Block Coefficient 050 and 0525," N.P.L. Approximating E.H.P.," Discussion by F. H. Alexander, Ship Division Tech. Memo. No. 132, 1966. Trans. N.E.C.1.E.S., 1927-28, Vol. 44, p. 186. HUGHES, G.: "An Analysis of Ship Model Resistance into MooR, D. I., SILVERLEAF, A.:"A Comparison of the Viscous and Wave Components," TRANS. R.I.N.A., 1966. Hydrodynamic Performance of Some Recent High Speed BAKER, G. S.:"Some Considerations in the Design of Cargo Liners," N.P.L. Ship Division Tech. Memo. 57, High Speed Cargo Vessels," Trans. N.E. C.I.E.S., 1942-43, 1964.Also Shipping World and Shipbuilder, September Vol. 59, p. 23. 1964. SAUNDERS, H.: Hydrodynamics in Ship Design, I, 517. GOODRICH, G. J.: "The Influence of Freeboard on Wetness," Standard Procedure for Resistance and Propulsion Experi- Fifth O.N.R. Symposium on Naval Hydrodynamics, ments with Ship Models, N.P.L. Ship Division Report 1964; also N.P.L. Ship Division Report No. 60, 1964. No. 10 (Revised), 1960. SILVERLEAF, A., DAwsoN, J.: "A Preliminary Assessment of MooR, D. I.:"Resistance, Propulsion and Motions of Bulbous Bows for Ships," N.P.L. Ship Division Tech. High SpeedSingleScrewCargoLiners,"Trans. Memo. No. 50, 1964. N.E.C.I.E.S., 1966. LEWIS, E. V.:"High-Speed Ships," Intnl. Science and DAWSON, J.: "Performance Data in Calm Water for Single Technology, April 1963, 38. Screw Hull Forms with Block Coefficient 050 and TODD, F. H.: "Submarine Cargo Ships and Tankers," 0525," N.P.L. Ship Division Tech. Memo. No. 130, Third O.N.R. Symposium on Naval Hydrodynamics, 1966. 1960; also N.P.L. Ship Division Report No. 20, 1961. Wmm, G. P.: "Preliminary Power Calculations for Some High Speed Cargo Liners," N.P.L. Ship Division Tech. WATTS, B. R.:"The Commercial Operation of Cargo Memo. No. 131, 1966. Submarines is Technically Feasible," Naval Engineers SILVERLEAF, A., and ENGLISH, J. W.: "A Note on the Journal, 1966, Vol. 78, 107. Hydrodynamic Efficiency of Propulsion Devices Suitable SILVERLEAF, A.: "A Comparison of High Speed Craft," for Use with Geared Diesel Engines," N.P.L. Ship New Scientist, Feb. 1965, Vol. 25, 277. Division Tech. Memo. No. 129, 1965.

DISCUSSION Professor H. B. Benford, B.S.E. (Member): The title greatlytransport capability and all other factors leading to some understates the paper's contents; but I interpret this as a reflectionmeasure of profitability.The important thing to note here is on the authors' modesty rather than any imperialistic ideas they that, as far as bulk carriers are concerned, optimal speed and may hold concerning the realm of hydrodynamics.In additionfullness of form bear no simple relationship such as that expressed to touching on several non-hydrodynamic matters, the authorsin equation (1).Studies here show that optimal speed decreases give us much useful information on basically slow speed shipsslightly as length of voyage increases and, as indicated by the such as tankers.Their paper forms a compendium of useful authors, becomes gradually higher with increases in size.The hull form and powering data, which can be combined with otherfollowing table shows our estimate of the most economic speeds technical and economic factors in seeking an optimum design. for ocean ore carriers.These are designed around the specified In this regard, I am particularly pleased that they make nooperating draughts inthe manner outlined above.Block claim that the most economic design can be determined bycoefficients of 080 are assumed throughout: hydrodynamic considerations alone. With regards to bulk carriers, I should like to suggest a simple approach to finding the most economic hull form and Optimal sea speed in knots speed.These ships generally find their cargo in unlimited Operating Deadweight Draught (long tons) supplies and should therefore be made as large as possible.If (ft.) 4,000 miles round 14,000 miles we assume that operating draught is the most severe restriction, trip round trip we should extend the hull proportions as far as practical based on that constraint.This may produce a beam-draught ratio of 20 8,000 l28 l25 30 and a length-depth ratio of 14.We then make the block 25 18,000 l40 l38 coefficient as high as practical, usually in the range of 080 to 30 31,000 l50 l49 083.The final step is to treat designed sea speed parametrically, 35 52,000 l58 157 seeking the most profitable ship by iteration.This is most 40 80,000 166 165 conveniently done by assuming arbitrary values of shaft horse- 45 118,000 l725 l72 power and estimating, for each, the speed, weights, costs, 181 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

Our studies also show that as long as the designed speed isguiding lines or principles for the design engineer by means of within one knot of the optimal, the profitability of operationsimplified equations and diagrams.In view of the complexity will be within I per cent of the maximum. and width of this field of work, the task as such, and the manner In the liner trades ship size is primarily limited by cargo in which it has been presented, deserve our full admiration. availability.Hull form and proportions may therefore be Nevertheless I take the liberty of "crabbing" a little on the combined in many variations.This then is the sort of vessel intendency revealed in the paper towards simplification and which the authors' work should prove most useful.But evengeneralization. here I find myself out of step with my contemporaries in that I To characterize the features of present and future ships, the fail to see the fundamental virtue in the trend to higher seaauthors use the term "maximum economic speed of the ship" speeds.This, of course, implies no criticism of the paper oror, in short, "boundary speed."For a given ship, from the the symposium. But I am concerned because the marinepoint of view of hydrodynamics, that speed is termed "economic fraternity appears bent on competing with aircraft on the latter'sspeed," beyond which the propulsive power required increases own terms: high speed.This rather hopeless endeavour can beabruptly. blamed, in my opinion, on the artificial structure of the ocean To determine the economic speed or "boundary speed," an conferences.These rate agreements are made to prevent ruinous equation is established in this paper which contains a relationship competition; yet, since they set rates that are the same regardlessbetween the block coefficient and Froude number, that is the of speed, they have triggered not a rate war but a speed war. speed of the ship. Which is more ruinous, low rates or high speeds?Unless the By means of a short example I should like to show that this conferences introduce speed-adjusted rates, or are dissolved, theequation can be bypassed, i.e. that even with block coefficients current craze for ship speed may well have its final result in awhich do not comply with this equation, the propulsive power net gain for the airlines. required can be kept low by other means. Let me close on a more cheerful note.If merchants are The overall attraction of this method is not only that we can indeed willing to pay for higher sea speeds, continuing refine-do something "forbidden," but also that we can get out of the ments in displacement hulls seem much more likely to satisfyship more deadweight and loading capacity, or speed. that demand than any credible development in hydrofoil craft, The upper part of Fig. 20 gives the equation, with the Froude air cushion vehicles or submarines. numberwhich in the paper is called "Taylor Quotient Tq" and the block coefficient CB. The example is based upon a Dipl. Ing. C Gaffin: The authors of this paper have given us an up-to-date and interesting survey of the field of hydrodynamics of ships.Moreover they have tried to establish

(1)Tq = l7l4CB V i Tq = Talor quotient V = Maximum economic speed "Boundary speed" L =LWL I IA Example B + V models 17-5 Knots V Length between perp. 400-26 ft.(12200m.) Lpp = 12200m. 40026 ft. Breadth 6266ft. (19-10m.) LWL =12340m.=404.86ft. Draught mouided.. 2388ft. 7-28m.) Volume of displ. 375,808 cbf.(10,641 m.3) Tq 0 870 Displacement 10,787 ts (10,960 ts) \/40486 Block coefficient 0627 Recommendedblock 1,00 1-7 -0-870 wps coefficient: CB - 0-593 5 HP Provided blockcoefficient: CB = O626 1070 -005 900 A -0-05- 'r 800 A' -0-04-0-037 IA -°° 700 ______-0-02 tIIP! POWER REDUCTIoN: 4-9 /o -0-0I 600

0- 500 + 0-01- 400 +002 loo 70 180 90 20-0 060 0-70 0-80 0'0 l-00 KN V Tq Results according to Hamburg Model Basin. Fic. 21 .CorP.RJsoN TESTS 0E MODELS WITH AND WITHOUT BULBOUS Fio. 20._EIrrEcT OF BULBOUS Bows ON RESISTANCE IN CALM WATER BOWS 182 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION normal cargo liner with a length between the perpendiculars of But, as we all know, in such cases we do not feel satisfied; l22O0 m. = 40O26 ft., which is required to run at a speed oftherefore we investigated another bulbous bow (Fig. 22). 175 knots. The ship without the bulb is almost the same as the former one; The resulting Froude number, or Taylor quotient, is 0870.some slight alterations only were made in the breadth and If this value is used, the result of the equation will be a recom- moulded draught for reasons of stability.The block coefficient mended block coefficient CB of O593.However, the ship wasis a little lower, but still far above the normal value.The given a greater block coefficient, for the purpose of gaining morecorresponding model is referred to as Modelli in this illustration. deadweight and loading capacity, namely 0-626. In this case the bulbous bow was set on the ship without Comparative calculations showed that this increase in theconsideration of its fullness.The contour of the ship with this block coefficient, that is in the volume of displacement, with the new bnlb is shown in the upper part of Fig. 22, whilst the results remaining dimensions of the ship being the same, would, for aof the model tests are plotted in the lower part.The ship with speed of l75 knots, result in an increase in the power requiredthe bulbous bow is referred to as Model lIA.As will be seen of 10 to 11 per cent. from this diagram, the new bulbous bow resulted, according to In order to nullify this increase in power required, we could the model tests, in a power reduction of 15-8 per cent.So this provide a bulbous bow. considerable power reduction is clearly greater than the loss in If we were to consider for this the mean values given in thepower due to greater fullness.Consequently, if this were paper, in Fig. 18, for bulbous bows, we could, with Tq = O87Odesired, we could permit ourselves an even greater block co- and CB 0626, expect a power reduction of 3-7 per cent. efficient, without having to run the risk of a loss in propulsive We tried to clear up this point in the shipyard of Blohm &power.The additional volume of the bulb has not been gained Voss with the help of model tests.Fig. 21 shows the principalat the cost of capacity, nevertheless the power reduction is high. dimensions of the ship and its displacement.As the curve in That the results of these model tests may well be correct is the upper part of the figure shows, Model I has no bulbous bow, also borne out by a comparison of the wave photos of the two whilst Model IA has been fitted with a bulbous bow.Care wasmodels II and lIA (Fig. 23).Model lIA, the ship with the taken to keep the displacement constant, so that the great blockbulbous bow, has practically no bow wave, compared with coefficient was not increased further.The diagram in the lower Model II, the ship without a bulbous bow. part gives the results of the model tests.The power reduction caused by the bulbous bow was 4-9 per cent, which corresponds Model II roughly to the mean values quoted in the paper. (Without bulbous bows) Lpp =12200 ni. 40026 ft. V 17-50 knots D = 11,100 t = lO,925ts ò = O-618

II lIA B + V models Length between perp. 400-26 ft.(122-00 ni.) Breadth .. 6332ft. (19-30m.) Draught moulded 24-31 ft. (7-41 m.) II lIA Volume of Dispi. 380,611 cbf.385,803 cbf. Model lIA 10,777 rn.3)(10,924 rn.3) (With bulbous bows) Displacement 10,925 ts 11,075 ts (ll,lOOt) (ll,252t) Lpp = 122-00 ni. = 400-26 ft. 0618 0626 V = 17-SOknots Block coefficient D = 11,250 t = 1l,075ts 11,000 ô =0626 wPs / 5$ P / io,000 / /1 / / / 9000 // IA 8000 /'4' / II ITA FIG. 23 .CosIr'ARIsoN OF WAVE PHOTOGRAPHS OF MODELS WITH AND 7000 / / WITHOUT BULBOUS BOWS 'POWER PEDUcTION: 5.8 01e Summarizing. I should like to explain that it has not been 6000 ----y my intention to advertise here any particular bulb orship; the data are not sufficient, considering the short time available, and 5000 - there are advantages as well as drawbacks, for example when the ship goes on ballast.What I have been trying to do is to show that even if we follow guiding principleswhose justifica- 4000 I 6-0 7-0 180 19-0 20-O tion I do not wish to denythe results obtained can be very different. We should therefore use the guiding principles, Results according to Hamburg Model Basin. equations, etc., applicable in each particular case, with great Fru. 22.CoMPAusoN TESTS OF MODELS WITH ANT) WITHOUT BULBOUS BOWS care and caution. 183 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

The main point I want to make is that in spite of many newthan CB = 040 is permissible since at the Froude numbers in efforts made to establish standards and guiding principles, therequestion a prismatic up to 070 may be to the point. is still much room left in naval architecture for creative art. We are indebted to the authors that they have considered all problems connected with the hydrodynamic design, especially Professor Dr. Ing. G. WeiibIum (Member) : The purpose ofthe matter of seaworthiness.In the light of our present know- research in naval architecture, especially in ship hydrodynamics ledge on the irregular seaway E. Lewis' original idea concerning is to promote the building of better ships and related vessels.supercritical operation appears to be applicable under excep- Our profession is satisfied that more funds are invested today in tional conditions only. ship research than before ; even if they are extremely modest compared with those granted to more spectacular fields of Mr. H. Lackenby, M.Sc. (Member of Council) : This question modern technology. of high speed operation of ships is very much to the fore just It is a common experience, and perhaps to some extent anow and I have been intrigued to see how the authors have necessary prerequisite for scientific progress that ideas gain an analysed and crystallized the vast store of data at N.P.L. to independent life; research people, being deeply concerned with show how propulsive performance varies with changes in speed their problems, forget the truism mentioned above (i.e. theand other parameters.I would like to comment on one or technological purpose) and even results slightly outside the field two of these. of their immediate interest.Scholars are a little out of date In the first place the tremendous penalty on power for pushing today. on into the overdriven range comes out very clearly in Table II, Therefore we welcome the fact that the superintendent of one namely about 24 times the power for a 20 per cent increase of the greatest establishments in the field of ship hydrodynamics above the boundary speed. summarizes results of the current work from the designer's point The advantages to be gained by increasing ship size are also of view, thus giving an impressive picture of modern activity in very striking.I suppose this is rather surprising when one gets our classical discipline.As theoretician, or at my age moredown to figures for particular cases, but there are of course correctly, promoter of theoretical methods, I would have likedquite a number of favourable influences involved here and I to have heard some encouraging words on the beneficial influence would just like to refer to them briefly. of theory on progress. To quote some examples from the domain of my personal activities: the idea of the "ram bow" First of all we have Froude's law which favours the has actually been put forward by Mr. Wigley.The idea of larger ship and for a given block coefficient the authors' fitting a bulb at the bow and the stern mentioned on page 179, boundary speed in knots increases pro rata with the has been proposed and investigated by the present speaker. We square root of the length.For example, if you had admit that theory so far has served more as a stimulus than as 16 knots as the service speed for a 65,000 tonner the an independent method of solving problems. corresponding speed for a 100,000 tonner would be Designation and nomenclature are an important topic.The i 7knots and so on.As the authors show, however, old wisdom about the prophet and his country is corroborated it is not usual to take full advantage of this as tanker by the fact that Froude's name is eliminated in the definition of sizes increase. the Froude number (page 180), thus two different expressions are Secondly, you also have a bonus on resistance for the designated by the same name.There is much wisdom in the large ship due to the well known reduction of the skin well established concept "quasi propulsive coefficient" which friction per unit area with increasing length. expresses the fact that this coefficient represents a figure of merit, There is perhaps more in this reduction of skin friction with not an efficiency.It is a pity that the I.T.T.C. has spoiled thelength than meets the eye however, and I would like to refer here nomenclature by denoting by such concepts as hull effi-to Fig. 4 of the paper, which shows a growth in the authors' ciency and relative rotative efficiency, which are figures of merit. Hydrodynamic Efficiency Factor with increasing length.This Turning to the more substantial facts, it is noteworthy that arefers to conditions at the boundary speed and I can only assume simple formula like that by Alexander can represent a usefulthat the growth in H of 9 per cent between 400 ft. and 1200 ft. relation between boundary speed (a useful notationafteris largely due to the skin friction correction applied to the model "economic speed" has been somewhat "overdriven") and block results.From the order of this growth, I think the authors must coefficient as far as normal ships are concerned.Presumablyhave used R. E. Froude's "O" valuesI cannot see any definite different results will be obtained when more sophisticated formsreference to this in the paper and I would be glad if the authors (e.g. with strong bulbs) are admitted, and the formulae may losewould comment on this.If it is Froude, then I would like to their value.Again, it is probable that the prismatic coefficientmention that if the I.T.T.C. line had been used the increase in is a more essential parameter than the block coefficient althoughH would have been 12 per cent instead of 9 per cent in gong beneath the speed barrier F,, < 035 say, they are roughly pro- from 400 ft. to 1,200 ft. portional to each other.In investigations, however, as re- I think it is important to mention however that these figures presented in Fig. Il it becomes advisable to use the prismaticrefer essentially to model results corrected for length by more and the midship section coefficients together.In the light ofor less conventional means without reference to ship trials D. W. Taylor's findings results dealing with the small (400 ft.) results.If you bring in the reduction of ship-model correlation and the large (650 ft.) ship are not very surprising.It wouldfactors with increasing length then the advantages of increasing be interesting to know the principles following which the parent size are all the more impressive. forms for CB = 050 and CB = 0525 have been designed since These factors can only come from the correlation of carefully a lot of useful basic information can already be derived fromconducted ship trial results with model tests, and B.S.R.A. and the Standard Series. N.P.L. have been working on a joint programme on this for a Finally I would like to ask the authors if a metacentric heightnumber of yearsB.S.R.A. being responsible for the ship trials of about one foot only is still customary for the type of cargoand N.P.L. the model tests. ships considered (page 178). Taking the trends in ship prediction factors as agreed last Of especial interest is the information on very high speedyear by the British Towing Tank Panel one would expect an displacement ships.Obviously, a destroyer-like form with aadditional improvement in performance of no less than 17 per comparatively high volume coefficient C =¡L3 and strongcent in going from 400 ft. to 1,000 ft.If we add this to the bulb appears promising; but probably a higher block coefficient corresponding length improvement of 8 per cent shown in Fig. 4, 184 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION which, as I see it, is based on model results only, we would then This feature is a bulbous stem arrangement of a special type, expect a total improvement of about 25 per cent for the ship inpatented for the writer's company (British patent 878 345, going from 400 ft. to 1000 ft.It is clear therefore that natureGerman Patent 1.034 060).Numerous bulbous stern vessels certainly favours the larger ships as far as powering is concerned. with this feature were built or are now under construction. Of course these correlation factors as presently definedTheir total capacity represents more than one million tons include propulsion scale effects as well as hull roughness ordeadweight. frictional effects, but I have no doubt that roughness plays a Former publications of the writer, as well as that mentioned very significant part.One could explain this by the fact thatat this session by the Chairman of Schiffbautechiiische Gesell- whereas the hull roughness or surface finish remains much theschaft, Professor Lerbs, concerns tankers or bulk-carriers, i.e. same for any size ship, the relative roughness, i.e. the ratio ofships of comparatively low speed, but having high block co- roughness to length steadily declines with increasing size so thatefficients (=C8 08). These ships are operated at high hydrodynamically speaking the hull surface becomes relativelyspeeds as specified in this paper by Messrs. Silverleaf and Dawson. smoother and less resistant. Experience shows very good results for this class of vessel, I realize of course that appropriate ship prediction factorseven for comparatively high loaded screws, i.e. for propellers have been used in the examples given in Figs. 10 and 11, but Ihaving, at normal output, a thrust load coefficient thought it pertinent to mention how these varied over the whole range of length considered in the paper. T CT The last item I want to comment on concerns unorthodox Ir V D high speed ships referred to on p. 179.I think there are pos- 4 sibilities here for multi-hull ships.As the authors are aware, B.S.R.A. has been doing some work on this and a particularwhere CT=thrust load coefficient; three hull configuration has shown some promise from the T=thrust; resistance point of view. Finally I would like to congratulate the authors on this p=density of water; informative and stimulating paper which is so topical at this V,=entrance velocity at the propeller: D=propeller diameter. time. The rotation velocity at the blade tips reaches 43 rn/sec. in Dipl. Ing. Leopold Nitzki: As a supplement to this com-special cases (motordriven vessels).Comparatively high QPC prehensive and interesting paper there may be mentioned thewere also reached.Propeller excited vibrations were limited to utilization of a special feature for the advanced hydrodynamica negligible extent and cavitation was avoided.At the same design of ships, especially of those classified by the criteriontime the vessels have excellent resistance qualities even without "High Speed Operation"according to the authors for ships"ram bows." the Froude number of which is V//L> 1'7 - l'4 CB. After we succeeded in applying bulbous sterns to some fast TABLE III

Model number

1780 1870-1 1855 C

Length between perpendicular.. .. 15400 m. 15500 rn. 133'OO m. Breadth moulded ...... 2200 m. 22 60 m. - Design draught ...... 8'50m. 855m. - Block coefficient ...... 060 0'56 052 Deadweight ...... 10,000 t. (metric) 10,000 t. (metric) - Design performance ...... 20,000 bhp 20,000 bhp 14,000 shp, metric Design speed ...... 21 knots* 23 knots* 23,6 knots*

Revolutions per minute .. .. 115 115 130 Propellerdiameter ...... 620m. 620m. 530m. Disc, area ratio ...... 064 064 070 Number of blades ...... 4 4 4 resp. 5 Tank test results (metric units) dhp According to Froude...... 14,320 17,800 - ©design speed ...... 0'973 0941 - dhp according I.T.T.C. + 00002 .. 13,300 16,533 - ©design speed ...... 0'905 0874 0'945t Quasi propulsive efficiency .. .. not less than not less than not less than 0'78 0'75 0'76

Bulbous bow ...... Taylor type Ram bow Ram bow

*far beyond the critical point according to the statistical data of H.S.V.A. where ecrit. H.S.V.A. > vcjj. Silverleaf and Dawson. t critical point according to statistical data of H.S.V.A. range between critical point and relevant overcritical speed according installed rating. 185 V 180° 770 760 150 1800 5io 70° L60° '500 140 130 75 7400 o 0% 1300 720° z 100 E /,çI. VimÖ 0 1100 zC).I1 1000 C)t'io lIIuL1 900 Z Vi o'-11Vi 50° t'iZo 300 o FIG. 24.-WAKE PATrERN TN THE PROPELLER NO. 1870-1 DISC. H.S.V,A. MODEL FIG. 25.WAKE PA'ITERN00 IN PROPELLER DISC. H.S.V.A. MODEL /0 20° NO. 1855 FIG. 26.WAI PATTERN IN THE PROPELLER DISC, H.S.V.A. MODEL:j00 70° 20° NO. 1855 c HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

dry cargo vessels with smaller block coefficients and higherTherefore model tests leading to a better knowledge of bending speeds we carried out model tests with= CB = 06 to obtainmoments would be welcome. relevant experience.The results are shown in Table III.The In Fig. 12 the authors give typical dimensions and speeds. model i 855C shows the result of collaboration between theIt is true that at present 16 knots is a typical service speed for a writer's company and a yard with which we are on friendlyship 720 ft. in length and the figure confirms that this service terms. speed is the boundary speed for a block coefficient 080.The present writer is in a position to confirm that this ship is capable The expression "critical point" in this Table relies on statisticalof maintaining her power in a head sea Beaufort 6, which means data of the Hamburg Tank (H.S.V.A.), based on the phenomenonthat the ship is not overpowered. explained by the authors, but resulting in the equation Estimating power in normal service, the authors take an allowance for average service conditions of 25 per cent for cargo (V/i/i:) H.S.V.A. > (V/i/i: = 17 - 14 CB) liners and 20 per cent for large tankers, and they add there is a due to the fact that the H.S.V.A. statistics include in theircertain doubt about these figures.These figures are correct as consideration also models of highest hydrodynamic qualities. far as the large ships are concerned.For the large ore carrier The hydrodynamic merits of the development are shown inon the Atlantic trade the allowance is 18 per cent for weather. Figs. 24, 25 and 26, Fig. 24 shows the wake patterns of modelAdding 6 per cent for fouling, the total allowance is as high as 18701, Fig. 25 the wake in the original stage of model 185525 per cent.It must be said that the block coefficient of the before application of the bulbous stern, and Fig. 26 verifies theore carrier referred to above is 080, and it is suggested that improvement of this model using a bulbous stern. faster ships with block coefficients of the order of 0525 have It may be mentioned that in case of model 1855 an improve-smaller allowances for weather and for fouling.Large tankers ment in the performance was not expected, i.e. the equation of the Persian Gulf trade will have smaller allowances than 25 per cent. shPnormai stern= shp1b.stern As to the controversial subject of the bulbous bow, the present was kept, due to the excellent qualities of the basic design,writer has no information on the behaviour in waves of a ship where QPC> 76 per cent was reached.But comparisonfitted with a ram bow. He may say however that the bulbous between Figs. 25 and 26 will show that the danger of vibrationbow is no worse in waves than a usual shape.The ore carrier and cavitation was eliminated. referred to above, fitted with such a bow, maintained her power in a head sea Beaufort 6, no solid water coming on deck in this sea state at normal speed.This invalidates the opinion often Written Discussion put forward that a bulbous bow augments wetness.The authors are to be congratulated on an excellent paper. Professor G. Aertssen (Member): This paper from the staff of Ship Division N.P.L. is to be examined in the context of the actual explosive development of the size of bulkcarriers and Reference tankers and the less conspicuous though also spectacular rise of speed of some types of cargo liners. (22) MURRAY, J. M.: "Development of Basis of Longitudinal It is gratifying that the study has been conducted so far that Strength Standards for Merchant Ships," TRANS. R.I.N.A. the behaviour of these marginal ships has been examined in 1966, Vol. 108, p. 217. irregular waves.The results of calculations are given in Fig. 17 and it is hoped that these results will be correlated to model Mr. E. P. Lover, R.C.N.C. (Member): This paper is an data.Therefore it is necessary that motion measurements beunusually interesting one because it discusses the question that carried out on board very large ships in a seaway.At theis of vital importance to all of us who are concerned with trans- moment Ceberena has in hand a programme of full scale measure- port by sea.The paper is a Quo Vadis; the authors are wise ments on board a very large ore carrier, the largest of the Belgianenough not tt give their guess as to where we go from here but fleet.Some data are now available and they correlate satis-present the evidence in a most readable way. factorily with the motion data described in Fig. 17.The ship The focus of the paper is the boundary speed.At the British has a length of 720 ft., a draught of 37 ft. and its block coefficientAdmiralty Experiment Works at Haslar, we work for the Royal is 080. The service speed is 16 knots.In a head sea Beaufort 7,Navy and the Royal Navy like other naviesor indeed because the Froude number being less than 01, the pitch angle doubleof other navieshas to overdrive its ships.The boundary speed amplitude 2 Oj/3 was about 5 deg. compared with the 4 deg. ofis therefore one we wistfully glance at as we proceed up to full Fig. 17; the bow accelerationl/3 was about o3 g comparedpower and to the full speed required by the Staff Requirements. with the 027 g of the same figure; the heave double amplitudeIf at all possible we try to determine a form which gives good 221/3 was 12 ft. compared with the 13 ft. of Fig. 17.Thisperformance (i.e. a high endurance) at a lower cruising speed as means that Fig. 17 shows a realistic behaviour of the ships andwell as demanding the minimum power at the maximum speed, it is therefore suggested that perhaps over the full range ofbut the latter usually takes priority.For the record however lengths and speeds considered the ships are capable of maintain-I have plotted the boundary speeds of some of our post war ing their speed and developing their power in these rough head forms using block coefficient as the significant variable and find seas, at least as far as the motions are concerned. A doubtthat although formula (I) fits the data reasonably well at block arises when stresses are considered.There is in this paper acoefficients down to about 06, at Ø45 to 050 it gives Taylor lack of information regarding bending moments in waves. quotients which are about 10 per cent low. This is important because so little information is available on One fact stands out most clearly from the paperthat the the wave bending moments of large ships.J. Murray was wiselypenalties incurred by overdriving are really dreadful.The advised when in his last R.I.N.A. paper22, considering waveresistance of the hull increases disproportionately, propeller bending moments, he said that "there is still an element ofefficiency is penalized and the efficiency of the propellerhull uncertainty about the behaviour of very large ships."Thecombination falls off.Indeed as a warship designer I wonder present writer suggests, on a basis of the few full scale data he why the merchant fleet requires these higher speeds and whether possesses on the subject, that the wave bending moments ofthey are really justified economically.However, the authors very large ships are perhaps larger than is generally thought.have given us the facts, the trend is towards higher speed, so 187 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION there must be some facet of sea transport economics which Development tests were started with the model of a 25,000 dwt makes it profitable. bulk-carrier.Three vessels of this type have been built, the lines If the need for high speed is accepted as a fact then the evidence of which were carefully developed some years ago.These lines indicates that the very fast ship will have to be a lot larger thanwere taken for the basic model. The basic model was then the 400 ft. example taken in the authors' concluding remarks.modified according to the SV-bow conception, until the best If the ship owner wants50knots he would be advised to take aresult was obtained.Fig. 27 shows the power curves of the hard look at the economics of the 1,600 ft. vessel. A monster of 1,600 ft. and 130,000 tons would need something in the order 25000 TDW BULK-CARRIER of one and a half million shaft horse power to give50knots. LOP 600-0 . B r,,ld 76'-O d Od An unrealistic figure perhaps, but offering a hydrodynaniic BLOCK COEFFICIENT 0,806 transport efficiency some five times that of the 400 ft. vessel. BlIP SPEED POWER DIAGRAM, LOADED CONDITION Increased length offers a further bonus.The sea is never as 18000 / calm as the experiment tank and, for instance, the resistance of / the 3-400 ft. ship in the North Atlantic exceeds the smooth 17000 / water figure by at least 30 per cent for something like50per cent of the year.Ingenuity in a given design can keep these losses / to a minimum, but in general the long ship will maintain50knots for longer than the shorter one in the face of worsening sea conditions. 4000 I would like to make one further point. We are all aware of / the absurdities of air travel when the time gained from higher 3000 air speeds is more than lost by poor airport handling facilities. i This danger is also with us in sea transport, and although special 12000 / 1) KNOTS terminals would be required to handle the monster high speed ship, new terminals and new concepts of cargo packaging and 1000 / handling will be necessary in any case if turn round is to be / INOENTICAL HULLS 10.000 speeded sufficiently for the high ship speeds to be fully exploited. / DIFFERING N BOW FORM ONLY / SAME PROPELLER USED N BOTH CASES Dr.-Ing. D. D. A. Csupor (Member): Articles in daily news- 000 ONOTO papers and reports in technical reviews confirm the statement / BUCO made by the authors that a large number of tankers and bulk- 15 6 17 18 SPEED IN KNOTS carriers have been fitted lately with bulbous ram bows in order to REDUCTION IN REQUIRED POWER obtain higher speeds in the ballast condition.In general, these PER CENT BHF bow bulbs have a considerable volume by which the deadweight of the ship is increased, but which, on the other hand, does not 4Q contribute to an increase in the cargo capacity.In addition, the roundings of these bow bulbs in their main waterlines have often a very big radius, and therefore in a seaway, when the bow bulb rises to the water surface because of the pitching motions of the 2000 ship, a considerable decline of the advantages must be expected. Another drawback is the flat fore bottom required in the design 15 1000 of these big-volume bow bulbs, which in bad weather is especially exposed to damage due to slamming, and such slamming occurs 16 IB SPEED IN KNOTS again and again in spite of the higher damping effect these bow Fio. 27 bulbs generally show. In order to eliminate these disadvantages, Maierform S. A.,basic model and of the model with SV-bow, indicating also the Geneva, developed within the scope of their research programme power gains respectively, and the speed increases obtained.The a new fore-body construction called the "Maierform SV-bow."results indicated refer to the fully-loaded condition.In the The name "SV-bow" was chosen, since the lateral view of themedium and high power range a speed increase of Ii knots ram-bow-like stem is S- and the frames of the fore-bottom arewas reached, whereas at lower powers the speed increased by V-shaped. A patenthabeen applied for for several character- Ø.9 knots.In the speed range from 15 to l75 knots the power istics of this fore-body design. savings obtained were between 1 ,400 and 4,900 bhp, which means The idea was to elongate the wetted part of the stem of a ship 17 to 27 per cent relative to the powers required for the basic having a fore body with moderate V-shaped frames, withoutmodel.Hence, the economic speed range of this type of ship was creatingtrue bulbouslines,especially bulbous waterlines.considerably extended by the application of the new SY-bow. Naturally, this is only possible in the lower part of the wettedApart from the magnitude of the gains the power curve of the stem, because the projecting part of the stem must be led backSV-bow model shows a smaller slope than that of the basic to the forward perpendicular, this zone being designed like themodel.As to the absolute speed values, it should be mentioned forward part of a very moderate bulb, but without constrictionsthat the diagram shows the direct tank test results, whilst the in the waterlines. ships built according to the basic model ran Ø2 knots faster than The results obtained with models with this fore-body con-the model during tank tests. A similar shifting can also be struction were striking.Speeds in the ballast condition increased; expected for the same vessel fitted with the SV-bow. they had to increase in this condition, due to the lengthened Encouraged by these results a whole series of models with floating waterline.Speeds in the loaded condition, however,medium block coefficients was tested with the SV-bow. At not only increased in the same measure, as was to be expectedpresent a test programme is being carried out for the develop- according to the underwater lengthening of the fore body, butment of SV-bows for finer hulls.Fig. 28 shows the front view considerably more, which proves that a strong bulb effect couldof the model of a fast refrigerated cargo ship, the lines of which be reached. are actually being developed.

188 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

contrary, it was just at these trim conditions that the gain, as against the basic model, further increased.This means that in the case of this ship there is no reason for any apprehension as to the rising of the bulbous part to the water surface when the ship is sailing in bad weather. These tests were run with the model of a passenger/cargo vessel to be fitted with SV-bow which is under construction at the Burntisland Shipbuilding Co. Ltd., Burntisland, on account of Messrs. East & West Steamship Co. Ltd., Karachi, West Pakistan. The curve for the block coefficient of 0685 is valid for a 8,800 dwt cargo vessel which was to be improved by the later fitting of a SV-bow on the ship already built.For manufacturing reasons the SV-bow was not faired into the lines of the ship.As a consequence, the gain was not as high as in the case of the above-mentioned models; according to the speeds it ranged between 85 and 15-5 per cent.However, these figures do not refer to the final bow design, but only to an intermediate one, since development tests for this ship have yet to be concluded. A comparison of the curves published by the authors, which represent mean improvements obtained in about 60 tests with conventional bulbous bows, with the results obtained with SV-bows proves that results obtained with the latter are far better than those reached with conventional bulbous bows. FiG. 28 This may be due partly to the fact that the results obtained with SV-bow were in propulsion tests, whereas the results shown in Fig. 29 has been prepared to compare with Fig. 18 of the paper. Fig. 18 are based on resistance tests.However, as the differences The power gains due to known bulbous bows fitted on ships ofbetween the results represented in Fig. 18 and those obtained various block coefficients have been entered in accordance withwith the SV-bow are very big, this fact might not be of much Fig. 18.The full-line curves show the results obtained with importance. Maierform SV-bows.The curve for 0-806 block coefficient With a block coefficient of Ø8 itis remarkable that the corresponds to the results of the 25,000 dwt bulk-carrier model already shown in Fig. 27. 30 The power gains pertaining to 0-635 block coefficient, which increase with the Froude numbers from 18 to 22 per cent, are the result of tests made with the model of a 6,500/8,400 dwt motor cargo vessel which is being builtand equipped with SV-bow- J. at The Caledon Shipbuilding & Engineering Co., Dundee, on PORE SHIP OF W1TH OPTIP-tIM ØASIC MCDLI. L C.B account of Messrs. Robert Bornhofen, Hamburg. ACCORDING SERE 60 The curves corresponding to a block coefficient of 0693 are n of special interest.Two curves are shown: the lower one gives the results of tests carried out on even keel and ranges between WITH L CaFWD 20 L ML 17-5 and 21 per cent.According to the design requirements the 00 centre of buoyancy of this ship lies ahead of the theoretical ' CRQß93 optimum.Giving the basic model and the model fitted with MAIERFORM SV- BOWS SY-bow a higher trim to the aft, a situation of the centre of ACCORD-4G TO COMPARATIVE buoyancy was obtained with which both models improved +15 PHOPULN T$IH considerably.During these tests with trim the most striking V?ENNA MODEL BASIÑ. phenomenon that could be observed was that the gain obtained due to the SV-bow further increased.In general, one would imagine that in the case of low-resistance ships bulbous bows give less gain than in the case of high-resistance vessels.This may be true for the statistical mean, but in this particular case just the contrary occurred.During the runs with optimum position of the centre of buoyancy the power improvements BULBOUBOWS ACCORDING TO obtained, due to the SV-bow, ranged between 22 and 24 per cent, FIG. 180F THE PAPR. as against 17-5 to 21 per cent with non-optimum position of the +5 ¿_ // / centre of buoyancy. ,_ // I // For these tests the fore body of the model with SV-bow was / 1/ / 3_// ,// built according to the "Series 60" model family developed by the _, ,' r r David Taylor Model Basin, whilst the after body was of identical r 0 ('1Wz r. design in both models.Thus the improvements mentioned are / // // valid in comparison with a model having the fore body of a ¡ "Series 60" model. The most striking phenomenon observed during tests with the -4 SPE LENGTH R-110 SV-bow model with trim to the aft was that no loss was stated 05 Q7 08 09 -- 10 at trim conditions at which the most forward point of the stem, i.e.its bulbous part, appeared at the water surface. On the Fio. 29 189 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION corresponding curve showing the improvements ends at aboutweighing between 50 and 60 tons and the capacity exists in this 2 per cent, whereas with the SV-bow a slightly fuller ship showedcountry for propellers considerably in excess of this.This power gains up to 27 per cent. reference is of course to solid single piece propellers but there is According to the diagram published in the paper no improve-no reason why if circumstances demand built up or loose bladed ment could be obtained with a 0806 block coefficient model atpropellers could not be used for sizes considerably greater. 0595 speed-length coefficient.With the SV-bow, on the contrary, an improvement of more than 15 per cent was still Professor O. Grim: With the introduction of a new factor, the obtained at this speed-length coefficient.This leads one tohydrodynamic efficiency factor H, the authors wish to obtain a suppose that all ships with a speed-length coefficient of morebetter measure of the hydrodynamic efficiency of the hull form than about 047which corresponds to a Froude number ofand its appendages.The factor H comprises the resistance about 0 14can be improved by the SV-bow. coefficient, the appendage resistance coefficient, the hull efficiency Thus it seems that the SV-bow is a means especially appro- and the relative rotative efficiency; however, the propeller open priate to improve the resistance and to extend correspondinglywater efficiency is eliminated.This gives the impression that the economic speed limits of any kind of displacement ship. this factor is independent of the propeller and that only the hydrodynamic properties of the hull and appendages are involved. Mr. L. Sinclair (Member) : The authors are to be complimentedIn fact, the paper does not mention any dependence of the factor on providing a comprehensive summary of the state of the art asupon the diameter or other data of the propeller. it stands, but unfortunately they give less assistance than one It appears right to me that the factor H depends also upon would expect in the design of vessels above the so-called criticalthe propeller.At least the hull efficiency and the relative speed which are of pressing interest at the moment. The firstrotative efficiency depend upon the propeller diameter.This part of the paper deals with well-known facts expressed in a ratherdependence upon the propeller diameter is mentioned later in different way with slight changes of nomenclature and adapta-the paper where it is shown that hull efficiency decreases sig- tions such as for example the Alexander formula (a usefulnificantly as the propeller diameter increases.Fig. 15 also criteria used for so many years) but does not push forward theshows that factor H depends considerably upon the propeller. design frontier very far.It is also rather difficult to understand Otherwise the ratios (110) andD/D (110) would be the use of approximate curves for such values as open propeller identical in this figure. efficiency when this in particular, can be so easily and more Therefore the questions arise: What is the physical meaning accurately lifted from propeller curves such as Troost systematicof factor H, for which propeller the presented diagrams are series data. valid, and what is the dependence of factor H upon the propeller? It is of course true that higher speeds are being called for on the dry cargo vessel, and also the size of the tanker continues to Dipl.-Ing. Hans Brehme : This very interesting paper contains increase.Both these factors emphasize the need for more powerin Figs. 10 and li a comparison between a high-speed single- on a single screw as the single screw vessel continues to be the screw ship and a twin-screw ship from which, unfortunately, it is most desirable economic solution in most cases.The majorpossible to draw a false conclusion, namely that the power problem is therefore one of developing a propeller-stern con-required for the twin-screw ship is only slightly higher than that figuration which will permit the safe absorption of powers aboverequired for the single-screw ship; the reason for this is to be and possibly well above, 30,000 shp on a single screw ship.It isfound in the particular choice of propeller dimensions. disappointing that no indicatioh was given of the work which the As an example, let us consider Fig. 10 and a service speed of N.P.L. must surely be pursuing on this major aspect of design. 26 knots in the centre of the plotted range.For this speed one Twin screw vessels such as Canberra and Oriana are working could well choose either a single-screw or a twin-screw arrange- very successfully with powers between 40 and 50,000 shp whereasment, and from this diagram it would appear that the question on single screw vessels powers exceeding 20,000 horsepower areof the power required is of only secondary importance compared almost invariably attended by difficulties sometimes of a seriouswith the other problems that arise, and so could initially be nature such as propeller cavitation, vibration, tailshaft failures,ignored.In fact, however, experience shows that a twin-screw etc.These have nothing to do with the propeller as such butship requires 10 to 20 per cent more power for the same speed. stem from the fact that the conventional single screw stern givesIn the table that accompanies Fig. 10 it is stated that the hull a much less favourable velocity field than does the twin screwresistance of the twin-screw ship is about 10 per cent the higher vessel thus varying the blade loading enormously during eachdue to the appendages, but that this is compensated for by an revolution of the propeller.Experimental work is thereforeimprovement in the propulsive efficiency of about the sanie called for as a matter of urgency, to devise a hull form giving a magnitude.This is accounted for by the fact that in the twin- more uniform velocity field thus permitting quite conventionalscrew ship the propeller area and hence the area of the propeller propellers to be designed for very much increased power. stream is doubled, so that the thrust loading coefficient 5, of The other possibilities mentioned such as contra-rotatingthe propeller is halved.The pitch ratio has been appreciably propellers and ducted sterns are in fact primarily inspired byincreased to suit a lower propeller speed.Both these factors the same problems, and the possible use of larger propellers lead to a marked increase in the propeller efficiency,. running at lower revolutions is another step calculated to push With a twin-screw ship it is, however, seldom possible (unless the single screw a little further up the scale of power.It wouldthe BIT ratio is very high) to fit side propellers that are as large appear to the writer that these are only palliative measures andas would be fitted to a single-screw ship having the same principal more fundamental changes in the design of stern are needed. dimensions.This at the same time makes it necessary for the In passing it is of interest to note that on pages 171, col. 1, twin-screw propeller speeds to be higher than the optimum page 174,col.2;and page 175,col. l,reference is made to limitationvalue for a single-screw propeller, and not lower as indicated in imposed by propeller manufacture.The writer knows of noFigs. lO and 11. such limitations either by the manufacturer in U.K. or for that If the various factors for these conditions are calculated the matter manufacturers overseas.The British manufacturer hasfigures shown in Table IV willbe obtained.Thefirst in fact strongly advocated the use of large propellers on thecolumn applies to a propeller corresponding roughly to the single screw tanker as being one measure of halting the continuous single-screw propeller referred to in Fig. 10, the missing figures diminution of propulsive coefficient with increasing size.Pro-for the wake fraction and thrust deduction factor being replaced peller designs were prepared three years ago 30 ft. diameterby values suitable for a ship of these dimensions.The same 190 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION applies to the twin-screw ship in column 2.Column 3 finally Professor Philip Mandel, B.S. (Member) : Not only is it true, shows the results obtained for a twin-screw ship in which theas the authors state, that the old concept of "maximum economic propeller dimensions bear the proportions relative to those forspeed from hydrodynaniic considerations" defies precise defini- the single-screw ship in column i which are almost inevitablytion, but the advance into the computer age has rendered the encountered in practice.The basis for these calculations wasuse of this concept obsolete.In the days of manual design, that the total propeller stream area for the twin screws shouldsome means had to be found to reduce the number of indepen- equal the stream area for the single screw and that the thrustdent variables that had to be dealt with in the ship design process, loading coefficient should therefore be about the same.Thein order to make that process amenable to hand calculations. power was calculated so as to produce approximately the sameIf authorities like Alexander, Posdunine, Telfer, Troost, Dawson, thrust (and hence the same ship speed) as for the data in column 2.Hughes and now Silverleaf stated that a relationship, no matter The slight reduction in the hull resistance in column 3 is accountedhow obscure and empirical, existed between two of the inde- for by the fact that owing to the lower torque and the lowerpendent variables, that is Tq and CB, then ship designers were propeller weight the dimensions of the appendages are smaller certain to use this relationship because it reduced the number of than in the case of column 2. independent variables that had to be treated by one.With the A comparison of the figures, such as is given at the end ofadvent of the computer, we need no longer be so concerned with the table, shows that a twin-screw vessel with propellers designedlimiting the number of independent variables. We can now in accordance with column 3, which more nearly represents aaddress ourselves to the real question which is applicable to all practical solution than does column 2, requires about 18 per centships, not just those operating at a Tq value less than i0. more power for the same speed than does a single-screw ship ofThis question is:- the same dimensions. "For any given set of owner's requirements, i.e. payload At the same time, these results show the importance of weight, payload volume, speed and endurance, what are the choosing the lowest possible propeller speed and the largest possible propeller diameter, without, of course, reducing the dimensions and coefficients of the 'least cost' * ship satisfy- clearance between the propeller and the hull to too low a value, ing these requirements as well as regulatory body constraints and also on propulsive grounds for retaining a single-screw concerning safety and structural integrity, and also navigas installation for as long as possible while attempting to solve the tional constraints establishing maximum permissible value- for length, beam or draught 7" vibration and cavitation difficulties, which become increasingly acute with increasing speed and power, by a suitable design of Within the context of this question, optimum CB emerges the afterbody (as, for example, proposed by Herr Nitzki in hisalong with optimum values of all the independent variables contribution to the discussion on this paper). describing the ship including Tq.The latest approach to Experience to date indicates that a twin-screw ship willproviding answers to the preceding question is given in Ref. (23). require a power increment of at least 10 per cent over that of theAn earlier approach is described in Ref. (24). corresponding single-screw ship. For this reason, I look upon the authors' suggestion of yet another empirical Tg - CB relationship under a new name, with TABLE IV concern.Of what use is the concept of an overdriven or under- driven ship if the concept depends on an ill-defined boundary? Single-screw Twin-screw ship If, for good and sufficient economic reasons, an owner wishes to ship carry only a small payload at very high speed, as for instance in the cross channel passenger trade, then a least cost ship exists 1 2 3 for his specification, even though its T value far exceeds 10 and its speed is far above that associated with the authors' V, knots .. 26 26 26 formula (i).The design question in this case is no different w .. .. 026 016 0i6 .. 44,200 45,600 52,200 than if the owner wished a very large payload to be carried at Ne togal ehp low speed. Thn .. Ø.9 098 098 n,rptn. .. 1290 976 1900 The authors obviously did not wish to delve into the entire D,m. .. .. 686 686 485 design problem of high speed ships.Unfortunately, with the F0im2 3696 7392 3694 knowledge available today, papers that approach the design 0952 0977 0957 question from a narrow perspective (hydrodynamics is a narrow D/DOPt perspective) inevitably raise more questions than they answer. Fa/F.. .. 085 058 085 Their H/D.. .. 0924 i242 0979 The authors touch on a very wide range of topics. 062i 073l 0637 paper is useful as a summary progress report on N.P.L.'s work J .. .. 1007 073l in these various areas. KQ .. .. 00329 00392 00343 KT .. .. 0i913 01785 0l28O References M, mkg. .. 241,000 164,000 96,800 MANDEL, P., and LEOPOLD, R.: "Optimization Methods S, kg..- .. 204,105 109,034 108,710 Applied to Ship Design," Trans. S.N.A.M.E., 1966. -. .. 1081 0449 Ø9O2 MURPHy, R. D., SABAT, D. J., and TAYLOR, R. J.: "Least 0818 0909 084l Cost Ship Characteristics by Computer Techniques," t .. Ø.9 017 017 S.N.A.M.E. Marine Tech., Vol. 2, No. 2, April 1965. 1093 0987 0987

1a .. i000 1000 1000 Mr. G. Hunter, B.Sc. (Member): It is valuable to have co- O679 0722 0641 efficients based on modern hull forms for use in preparing W, kg. .. 165,300 181,900 180,500 preliminary estimates for new designs, and the diagrammatic Comparison: presentation of the authors' data is very convenient for easy W/W0 .. I 1100 1092 reference.Towing tank superintendents have an immense fund N/N0.. .. i l03 il8 * Where least cost may be defined not only in terms of building costs but also in terms of annual operating fuel costs. 191 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION of information at their disposal.It would be helpful if theyformance.The propeller open water efficiency is the obvious published more of it in a form which would be of immediate use choice for propeller performance and the propulsive efficiency to designers in the early stages of new projects. 7D and overall propulsive factor ¿,, (ratio of iD to) are useful The cost and weight of very high powered machinery in cargo criteria for hull and propeller performance and interaction liners such as the suggested 530 ft. ship, together with the cost effects.The hull performance and interaction effects can be and weight of additional fuel, would be considerable, and incorporated in the Hydrodynamic Efficiency Factor [equation (3)] knowledge of the financial rewards obtained from the operationwhich can be re-stated in the form of cargo liners of more moderate speeds confirms that 25 knots would not be a commercially profitable operating speed.This H= 530 ft. ship, although representative of an increasingly common (1 + b)©SN type of cargo liner, is somewhat larger than the average.The progress of port improvements to accommodate larger ships is For a good quality hull form the product (1 + b)©SN would such that it will be some time before the majority of new cargohave a low value.For minimum losses due to interaction liners are of this size.Many cargo ships will still be limited toeffects, the overall propulsive factor,would have a high value. between 450 ft. and 500 ft. in length between perpendiculars, andConsequently, for the optimum combination of hull form and they will be even more remote from economy at 25 knots thanpropeller the hydrodynamic efficiency factor H would have its the examples quoted in the paper. maximum value.From the data given in this paper it is possible The authors recognize that speed increases of 20 to 30 per cent to obtain for a given speed and displacement, a value of the are unspectacular.They are indeed, but the difficulties encoun-hydrodynamic efficiency factor, but by how much is the value tered in trying to achieve them might well be considerable. affected by variation in critical parameters, for example, screw Small increases in speed are not very significant except on verydiameter?With the object of obtaining some additional data, long passages, and it seems unlikely that there would be muchestimates were made for two tanker hulls, both of the same demand for such limited increases in speed at so great a price.displacenient and operating at the same speed, one having a There would appear to be more economical ways of saving timemoderate diameter screw (D = 2325 ft.) and the other having in the cargo liner trade.These are to be found in more rapida large diameter screw (D = 295 ft.).The propulsion estimates cargo handling and more efficient port services, thus reducing were made using data obtained at N.P.L. [(ref. 25)].The results inport periods of cargo ships, rather than in resorting to the of the propulsion estimates as given in the report [(ref. 26)] are inefficient use of power by overdriving.In the case of tankers,summarized in Tables V and VI, and the derived values of however, this kind of quick turnround has already been achieved,hydrodynamic efficiency factor are as follows: and further time saving will not be readily attainable. Hydrodynamic improvements of orthodox displacement hulls }lydrodynamic Efficiency are of real value in relation to the performance of the ship and Factor H Screw dia. the economical use of fuel, but they are unlikely to be of great Hull Screw advantage in the saving of time unless they are of a more (2) fundamental nature than those examined in this paper. The (1) overdriving of displacement hulls would be an unprofitable i I 2325 20 20 practice and we should look to the desi of relatively novel 2 5 295 2O 185 high speed craft in our search for time saving hydrodynamic developments. Percentage reduction below 8 Mr. T. P. O'Brien, C.G.I.A. (Member): In assessing the value for screw i aspects of propulsion discussed in this paper, it is desirable to consider three criteria relating to (1) hull performance alone, Value obtained from Fig. 4. (2) propeller performance alone and (3) hull and propeller Values estimated from data given in the paper (ref. 25). performance together.The results obtained can be applied in making estimates of resistance of hull, efficiency of propeller From the above comparison it would appear that in making and propulsive efficiencyD or"quasi propulsive coefficient" ofpowering estimates for large tankers the hydrodynaniic efficiency hull and propeller.They can also be applied in making poweringfactor should be reduced if a large diameter screw is to be estimates where the components due to hull, propeller andadopted. interaction effects are combined. The data given in this paper and those summarized in Table V The hull resistance coefficient © and appendage resistanceshow that significant improvements in propeller open water coefficient (1 + b) together provide the criterion for hull per-efficiency can be achieved by the adoption of large diameter

TABLE V HULL 1 AND 2PRopuL.sIoN FACTORS

Propulsion factors

Screw Tria! speed Wake Hull factor Relative flow Overal! pro- Hull series Dia. D V (knots) fraction factor pulsive factor

WT ta ta

A 2325 17 043 142 l02 l45 2 B 3000 17 038 l28 105 l34 2 C 3000 174 0'38 l28 l05 l34

192 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

TABLE VI

Scaws ITO5GEor,&riuc FEATURES AND PERFORMANCE DATA Delivered horsepower, 22,000 dhp

Geometric features No. of Rateof Trial blades rotation speed Blade area Hull Screw Pitch ratio Thick ratio Dia.D (ft.) Np (rpm) cknoBts) PT r

1 1 4 110 17 2325 0625 0770 0055 1 2 5 110 17 2325 0700 0750 0052 3 6 110 17 2325 0780 0740 0050 1 4 6 100 17 2325 0800 0840 0047 2 5 4 75 174 2950 0530 0820 0049

Performance data

Screw Propulsive Per cent increase above efficiency efficiency screw

'ti, 'to 'Ip

1 4 110 17 0485 0703 -- 1 2 5 110 17 0485 0703 O O 3 6 110 17 Ø475 0689 14 14 1 4 6 100 17 Ø49Ø 0710 4 4 2 5 4 75 174 0575 0770 18 10 screws running at low rates of rotation in lieu of moderateinvolved in increasing the speeds of merchant ships.It is good diameter screws running at moderate rates of rotation.Someto see certain of the usual tacit assumptions questioned.For comparative values are as follows: example, why should the fact that "30 ft. is presently regarded as the maximum diameter of propeller which can conveniently Propulsive be manufactured" be considered an absolute barrier?It seems Propeller open Water efficiency Rateof efficiency , to this writer that the increase in diameter for larger ships is a Hull Screw Screw rotation lip natural development to achieve sizable gains in propulsive dia. (ft.) (rpm) efficiency.Surely, modern technology can find a solution, (1) (2) (2) presumably with built-up design of propeller, combined with I I 2325 110 044 0485 0703 the use of suilable reduction gearing.Hence, the suggestion 2 5 295 75 058 Ø575 0770 regarding "special efforts to develop methods of manufacturing and handling larger propellers and of enabling then to run more slowly" is particularly timely. Percentage increase above 32 18 lO Referring to the interesting section on Seakeeping Qualities, value for screw 1 the advantages of increasing ship size are again confirmed, as well as the relatively minor significance of variation in fullness. Values derived using equation (5) and (6). The general comments on the effects of speed on motions of Values given in Table V. fine, high-speed ships call for some discussion, however.In the first place, the range of speeds covered (Froude No. 027 to 033) However, the above comparisons also show that the corres-is believed to be so small as to prevent a clear picture of trends ponding gain in propulsive efficiency is less than the gain inwith speed to be obtained.Secondly, the qualification, "except open water efficiency.In particular, an increase in open water possibly on pitch and relative bow motion," is a serious one. efficiency of 18 per cent is related to an increase in propulsive Generally the relative bow motion is the most important factor efficiency of only 10 per cent. governing attainable speed in rough seas, since it determines the probability of shipping water andto a large extentof slam- References ming.Fig. 17 shows appreciable differences in this quantity within the narrow speed range covered. PARKER, M. N.: "The B.S.R.A. Methodical SeriesAn Furthermore, it should be noted that finding a small effect of overallPresentation.PropulsionFactors," TRANS. speed on relative bow motion does not imply the converse that R.I.N.A., 1965, Vol. 108. relative bow motion does not affect speed.On the contrary, if O'BRIEN, T. P.: "Optimum Performance Screws for Largean acceptable level of relative bow motion (as indicated by Tankers."London, Shipbuilding and Shipping Recordfrequent shipping of water or slamming) is exceeded in a par- May 1966, 107, 587 (reprinted in Ship Division report 88). ticular sea, then a very large reduction in speed may be required to reduce the relative bow motion sufficiently.For example, Professor E. V. V. Lewis, M.S. (Member): This paper providesconsidering a 600 ft. ship at 275 knots (Fr = 033) in Beaufort 7 a valuable survey of the problems and design considerationshead seas, Fig. 17 indicates S113 = 242.If freeboard at the 193 HYDRODYNAMIC DESIGN OF MERCHANT SHOPS FOR IilGH SPEED OPERATION bow were 30'8 ft., the probability of shipping water would beperformance because of particular design requirements.How- ØO4 (assuming a Rayleigh distribution),i.e. 4 out of 100ever, these improvements tend to disappear in light ballast pitching cycles.If this were considered excessive by the master, conditions.Incidentally, Mr. Gallin appears to take Froude a reduction of speed to 225 knots would reduce the shipping ofnumber and Taylor quotient as equal ; itshould be made clear water to only about 3 out of 100 cycles.Hence, the ship wouldthat although the Taylor quotient (V/-/L) is a measure of the either have to further reduce speed or change course to bringFroude number (v/gL), it is not in fact equal to it. the shipping of water to an acceptable level. The authors agree that the design criteria introduced in the Finally, it is suggested that perhaps the greatest value of this paper should be used with care as guiding principles because, paper lies in the data it gives on the effects of variations in speed,as Mr. Gallin states, there is still much room left in naval fullness, length, propeller diameter, etc. rather than on optimum architecture for creative art and also, to reassure Professor values.Because of the increasing attention given to theWeinblum, because thereis continuous progress under the economics of ship operation, ship designers are more and morestimulus of theory ; the work of both Mr. Wigley and Professor interestedinthe"tradeoffs" among designfactors.ForWeinbium on bulbous bows is a striking example ofthis beneficial example, how much extra fuel consumption could one afford ininfluence.The authors can only apologize for omitting Froude's return for the ability to carry several additional containers ofname in defining the non-dimensional speed-length constant in cargo? To answer this one must know how much powerthe advance text; Professor Weinbium's strictures are well requirements increase as block coefficient, length of parallelmerited. middle body, position of LCB, etc., are varied from the optimum Although prismatic coefficientis a better hydrodynamic values.The optimum hydrodynamic solution will seldom be the parameter than block coefficient, the latter has been used because optimum economic solution. of its value in the early stages of a new ship design when little Authors' Reply more than displacement, speed and approximate dimensions may be known.The parent forms for CB = O . 5Ø and O . 525 were Professor Benford's outline of a procedure for determiningdesigned to satisfy requirements for cargo liners of the type in the main design characteristics of bulk carriers for maximumservice between Europe and the Far East, where increasing speed profitability is very similar in principle to techno-economicappears to be an essential requirement.For this type of vessel, studies for other types of ships which have been carried out ata metacentric height of about one foot is still customary as a N.P.L. for some time.It is not suggested that a simple relationminimum, but this value is often exceeded.Professor Weinblum like that in equation (1) of the paper can link optimal speed andwill be interested to know that the high-speed displacement ship fullness of form; indeed, the whole emphasis in equation (1) is of CB O 40 had a prismatic coefficient of O 69. on a hydrodynamic boundary speed which may be quite different Mr. Lackenby is right in assuming that the correction for from the optimal operating speed.As Professor Benford pointslength given in Fig. 4 is based on skin friction correction according out, the trend to higher speeds, especially for cargo liners, may well be unjustified but it does seem to be one of the facts ofto R. E. Froude.Where values of © are given in the paper, commercial life and it is hoped that the paper helps to put it they are also on the Froude basis.The authors agree that if the into perspective. I.T.T.C. line had been used, the increase in hydrodynamic The authors agree with Mr. Gallin that simplification and efficiency factor H in going from 400 ft. to 1200 ft. length of generalization in ship design methods can be dangerous, butship would have been 12 per cent instead of 9 per cent.How- believe that good designers naturally take account of suchever, as stated in the paper, although the absolute values of H pitfalls.However, they are puzzled by Mr. Gallin's suggestiondepend on the method of extrapolation used to derive the ship that the term boundary speed is short for "maximum economic resistance coefficient, power estimates will not be affected because speed of the ship"; indeed, the principal reason for introducingthe performance prediction factor (1 + x) will also change the concept of hydrodynamic boundary speed is to separatecorrespondingly. hydrodynamic effects from overall economic considerations, as Mr. Lackenby raises a very interesting and important point emphasized by Professor Benford.Mr.Gallin's example,when he refers to the effect of size on ship-model correlation showing the advantages which can be gained by using a largeand the ultimate ship trial performance.The figure of 25 per bulbous bow, is of considerable interest.Fig. 18 of the paper, cent total improvement for ship in going from 400 ft. to 1000 ft., which gives average values showing the effect of bulbous bowswhich he gives, refers of course only to (1 + x)/H in equation (7). on calm water resistance, was based on results for forms withThe effect of propeller efficiency and the increased values of bulbous bows of about 5 per cent section area ratio and 7 perdisplacement and speed would also have to be taken into account cent ram area ratio. The results given by Mr. Gallin in Fig. 21 before the powers for the 400 ft. and 1000 ft. vessels could be provide a good check on these average values, as Model IA hascompared. a similar type of bulbous bow and the size appears to be about Regarding unorthodox high-speed ships, the authors agree the same as those from which Fig. 18 was derived.Model ITA with Mr. Lackenby that there appear to be possibilities for has an appreciably larger bulb, both in section and profile, and multi-hull vessels, and there is an active programme of work at a different result would be expected, as indicated in Fig. 22. N.P.L. to study their performance characteristics in calm waler However, it must be admitted that in the authors' experience a and in waves. power reduction of 158 per cent is unusually large for a block The bulbous stern arrangement referred to by Mr. Nitzki is, coefficient of Ø626 at a Taylor quotient of o87s.For a blockof course, well known and the wake patterns shown in Figs. 24 coefficient of 0626, the boundary speed is given by a Taylorto 26 confIrm the authors' experience for the effect of such quotient of 0824, so the 400 ft. ship represented by Model lIAchanges in stern design.It is interesting to note that the is being over-driven by only one knot.Conversely, for theH.S.V.A. analysis of data gives a "critical point" speed higher speed required, equation (1) indicates that the block coefficient than the boundary speed given by equation (1), but it is difficult should not exceed 059 if excessive *ave-making is to be avoided. to comment on this without knowing itg basis and also something Recent work at N.P.L. has shown that bows of the type in-about the value of resistance coefficient associated with it.Any corporated in Model ITA give appreciable improvements for hull form may be designed for a speed higher than that indicated vessels having block coefficients of 060 to 065 in a loaded condition, the greater improvements being attained where theby equation (1), but the authors would then expect the © original normal bow form had a relatively poor hydrodynamic values at the boundary speed to be greater than 07l on the 194 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION

Froude basis for a 400 ft. ship.The values of QPC given inbody to give favourable propeller-hull interaction effects is Table III are good, but they are more than offset by the extremely mentioned in the paper, as well as contra-rotating and ducted high resistance coefficients.For example, for Model 1780propellers, and much effort is being devoted to finding a solution (CB ° 60) the authors would expect a QPC of only a little over to these problems.It is encouraging that there is apparently no limitation on propeller dìameter due to manufacturing o . 7Ø but a © value on Froude basis of less than O 80, for acapacity, as appreciable gains in propulsive efficiency are likely normal stern design. for the very large ships, but only if the propeller rate of rotation The authors are grateful to Professor Aertssen for his con-can be relatively low. firmation from ship service data of the values given in Fig. 17 As Professor Grim notes, the hydrodynamic efficiency factor for ship motions and in Fig. 12 for dimensions and speeds, alsoH is not entirely independent of the propeller, especially the for his confirmation of the service power allowances used in thediameter, as the hull efficiency and relative rotative efficiency are paper.Bending moments in waves were not considered to beincluded.Because of the variations in type of machinery and within the scope of the paper but it is agreed that more information propeller rate of rotation, a better measure of the hydrodynamic is required, particularly for the very large ships now being built. efficiency of a hull form and its appendages is given by eliminating lt is interesting to note Professor Aertssen's comments on thepropeller open-water efficiency.Hull efficiency and relative behaviour of bulbous bow forms in rough water conditions.rotative efficiency are influenced by the propeller, but to a lesser Reports of performance of ships at sea are often conflicting andextent than the open-water efficiency.For H as given in this is one of the reasons for the bulbous bow being a con-equations (4) the propeller diameters were close to the optimum troversial topic. values for rates of rotation from i 10 to 140 per minute at ship Mr. Lover has examined data for forms with block coefficientslength L 400 ft.Of course, these equations oñly apply for the as low as 045 and finds that the boundary speed given byship boundary speed in terms of block coefficient. equation (I) is about 10 per cent low for block coefficients Mr. Brehme refers to the power estimates given in Fig. 10 below 050. The lowest block coefficient in the data used forand doubts whether 225 ft.diameter propellers would be the determination of equation (1) was 050, and although it ispractical for the twin-screw ship.The estimates given in the reasonable to expect that the boundary speed for lower blockpaper indicate that for the 530 ft. cargo liner at 26 knots in coefficients would be somewhat higher than that given by theservice, the twin-screw ship would require only 5 per cent more formula, it is likely that the resistance coefficient also would bepower than the single-screw ship, and Mr. Brehme considers higher than 071 on the Froude basis for a 400 ft. ship.Mr. this a false conclusion because the twin-screw ship would have Lover draws attention to the penalties of overdriving and somesmaller propellers arid require about 18 per cent more power. of the factors which must be taken into account if the trendThe authors feel that this illustrates the possible danger of towards bigger and faster ships continues, and sea transport isgeneralization because, for this particular ship, it was possible to be economic.His hypothetical example of the 1600 ft.,to accommodate 22 5 ft. diameter propellers with satisfactory 50 knots vessel with very high hydrodynamic transport efficiencyhull-tip clearance and immersion. confirms the advantage of increased size for high-speed ships. The calculations summarized in Table IV confirm that slow- The authors agree that port facilities and methods of cargorunning, large diameter propellers have higher efficiencies than packaging and handling are extremely important, for if increased the faster, smaller screws and this was the reason for basing the productivity is to be achieved it is essential to have a quickestimates given in Figs. 10 and 11 on the largest practical twin turn-round in port. screws.For the change in diameter considered in Table IV Dr. Csupor has given some extremely interesting results forthe authors would expect an even greater difference in power forms with the "Maierform SV-bow." Of course, this design ofrequired because the smaller screws would have lower hull bow is quite different from the bulbous bows referred to in efficiencies. Fig. 18, which had a section area ratio of about 5 per cent and The authors are familiar with the "least cost" ship approach a ram area ratio of about 7 per cent.Some quite startlingand they agree with Professor Mandel that hydrodynamic improvements have been achieved with large ram bows, whichconsiderations alone are not sufficient to achieve the most have a relatively slender section, the best results being obtainedeconomic design.Hydrodynamics may be a "narrow pers- in a ballast condition for very full forms and in a loaded conditionpective" but it is an important part of the ship design process, for very fine forms.Flow studies using a hot film probe havemaking an increasing use of computers, and it is as well to know demonstrated that this is not a laminar flow phenomenon, andthe penalties for departure from good hydrodynamic design. both the character of the resistance curve and the lower wave Incidentally, Professor Mandell's definition of "least cost" is not profile suggest that this is due to a reduction in wavemakingas broad as the concept of maximum profitability adopted for the resistance.However, this may not be due to the ram bow actingN.P.L. techno-economic studies already mentioned. as a wave-cancelling device like a bulbous bow, but to the Mr. Hunter stresses the need for data published in a form increased length and changed fullness of the immersed hull form. which is of immediate use to designers in the early stages of new In some instances it has been noted that the greatest improve- projects.The authors had this in mind when preparing the ments were attained when the original normal bow form hadpaper and they hope that the criteria introduced and the method design features which resulted in a relatively poor hydrodynamicfor producing quick power estimates will prove useful. performance. For the cargo liner trade, the authors agree that more eco- The authors regret that Mr. Sinclair is disappointed that thenomical ways of saving time may be fourd in improved cargo paper is not more helpful in the design of higher speed ships. handling and port services, rather than in resorting to the They had hoped that the criteria which they have introducedinefficient use of power by overdriving.The paper only deals would be useful for preliminary design purposes when only thewith single hull displacement ships but it is clear that for very displacement, speed and approximate dimensions may be known. high speeds, and relatively low deadweights, hovercraft and A method for obtaining a first estimate of propeller open-waterhydrofoil ships should be considered. efficiency was included, as it can be used before sufficient data Mr. O'Brien has considered the hydrodynamic efficiency are available for the use of propeller design charts. factor H and the effect on it of increasing propeller diameter on The authors agree with Mr. Sinclair that the safe absorptionlarge tankers.In the data used to derive values of H, the of powers well above 30,000 shp on a single-screw ship presents largest single screw had a diameter of 255 ft. for a ship length problems, such as cavitation and vibration.Shaping the after- of 1,010 ft.The authors agree that for very large propellers H 195 HYDRODYNAMIC DESIGN OF MERCHANT SHIPS FOR HIGH SPEED OPERATION should be reduced to allow for two effects; first, the tendency forconsider wetness without regard to freeboard, and it would not hull resistance to increase and second, the fall in propeller-hullrequire a large increase in the freeboard of the 600 ft. ship interaction factors.Gains in propulsive efficiency with large, considered by Professor Lewis to reduce the probability of slow-running propellers are sometimes appreciably less than thewetness to a very low order. gains in open-water efficiency because of the second effect. The final remarks made by Professor Lewis are appropriate These general conclusions are confirmed by Mr. O'Brien'sas a general conclusion to the discussion.The optimum hydro- estimates as summarized in Tables V and VI. dynamic solution will seldom be the optimum economic solution, The comments by Professor Lewis on the effects of speed onbut the latter will not be achieved unless the penalties for relative bow motion and thus on ship wetness draw attentiondepartures from the best hydrodynamic design are known. to an important point.It is agreed that decreasing ship speedOnly then can such penalties be "traded" against other factors will generally reduce the frequency of occurrence of wetness,to ensure the most efficient design of ship. and that the speed reduction would have to be greater than the Finally, the authors wish to express their thanks to all who range shown to have an appreciable effect.However, we cannot have contributed to the discussion of the paper.

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