î 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 by 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 require- ments 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 sea- keeping 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.