WHAFS BETTER Rhan BERMUDAN? Ientific Analysis Yields Amazing Results

driven by tlii.s type of rig. To gain rccogiiilion, any competing sail configuialion should al least match the Beiniudan sail power and, preferably, surpass it on some poinis of sailing. How­ ever, the current rating aud racing rules have practically precluded development ofany other sail configuration. Even when iinorlhodox sails are not explicitly prohi­ bited, the wording of the measurement system is such that experimenls wilh un­ usual rigareeffectively discouraged. So it is that people regretfully abandon any hopeofdevelopingother types of rigunder current ocean racing rules. One may rightly ask the question: what's tho basis for the assumed superior­ ity of the Bermudan rig; can it be scientifi­ cally proven that this triangular sail is more efficient than whatever? Well, recently, 1 made wind tunnel tests of the potenliai power of a number of rigs — Bermudan, Lateen, Sprit,Gunter, Dip­ ping Lug and Crab Claw — some with modification, as shown in Pig. !*•. Hope­ fully this research, based on analysis of wind tunnel lest results, will enable the advantages and disadvantages of various sail configurations to be belter understood and predicted with reasonable accuracy. Itwill also indicate directions for improve­ ment in traditional rigs, and guide the selection of appropriate sail configura­ tions in all sort of sailing boats, including fishing and workingcraf I. A comparative assessment of merits and demerits of var­ ious rigs is made, and explanation is given why certain rigs are superior. Also, an attempt is made lo find correlation

Fip. 1. Rigs tested. 1. Bermuda Rig (with and without large and small jibs; also showing how much of the head of the mainsail was removed). 2. Lateen Rig (three different shapes of sail). 3. Sprit Rig (three different aspect ratios). 4. Gunter Rig. 5. Dipping Lug Fig. 6. Crab Claw (set at varying angles).

WHICH RIG? WHAFS BETTER rHAN BERMUDAN? ientific analysis yields amazing results. Tony Marchaj reports.

/•RILE much is luiown about high per­ cient than the gaff, sprit, or whatever. It formance sailing rigs for racing — can be scientifically proven: comparative inly the Bermudan rig — little or no curves of lift vs drag etc. would put the tematic research has been carried out triangularsail with its longer leadingedge 3 other traditional sail configurations, way ahead'*. After all, thel2Metreyacbts 3. SPRIT RIG 4. GUNTER RIG it's difficult, if not impossible, when competing for the America's Cup — an -'cting a sail plan for a boat, to deter- epitome of ultimate progress achieved in le with certainty whether a proposed the field of high-performance boats—are is more efficient (for a given sail area) n another, either anticipated or al- • The quotation laken from 'Wooden Boat' No 73. dy existing. And anyway, there's con- November 1986. arable bias in opinion as to the merits of "C-A. Marchaj: T/anform Effectofa NumberofPigson Bermuda type of rig to begin with, Sail Power'—Proceedings of Regional Conference oa lost people believe that this rig, which Sail-Motor Propulsion, (Vlanila, November 1985. The paper presented is Ihe result of a research program ni nates the contemporary sailing funded by the Overseas Development Administration lie, both for racing and cruising, must of Ihe British Government. The program carried out by 5. DIPPING LUG RIG 6. CRAB CLAW RIG lie best rig available; to quote: 'Every- MacAlister Elliot and Partners Ltd is aimed at improv­ ing the performance and the fuel economy of sailing 1 • Icnows tiie Bermuda rig is more effi- fishing craft in the developing worid. J(51 SEPTEMBER 1988 123 .!i(iii'.i;l!;j»!»ufj2iliijj;i;n:

betweenthepolentiaJdrivingpowerofthe rigs in question, and tlie speed perfor­ mance of a given standard liull driven by these different rigs. Interpretation of wind tunnel results The whole problem of wind tunnel testiug, and how tests on models are conducted, is closely allied lo what one hopes to gain from the lest. If one wishes to delermine the forces ou nn actual sail, under normal sai lingcondilions, then the logical thing to do is go on asailingboat and measure those forces in action. Although difficult and time consuming that task is not insuper­ able. But testing models in a wind tunnel allows a systematic variation of important geometric and physical factors, which can be lield under close con trol. Thus, one may rightly expect dissimilar results when sail area is keptconstantbutchanges are made in the sail plan, i.e. sail area distribution, Flg. 4. Models of sailing rips. A representative •^spect ratio, and so forth. fishing boat hull, consisting of that part nor­ mally above the water, and ata nominal angle From Fig. 2 it's evident that rigid con­ measured or controlled. That's the reason of heel ® = 10 degrees, was built for the trol is necessary over any experiment why certain factors contributing to suc­ tests.Here, models of a Bermuda rig (A) and a vhether conducted fuU size or on a model. cessful design sometimes remain obscure, Crab Claw (8) are being tested in the South­ ..t's difficult, if not impossible, to deter- misplaced or controversial. ampton University wind tunnel. Sails were minetheeffectofchangingone factor, if at It's not proposed here to enter into a Initially adjusted to each predetermined the same time, one or more other factors detailed discussionof all the factors which heading angle relative to the wind so that the set seemed good to the practical sailor's eye. alter. The wind tunnel here offers gi-eat can influence the forces developed by a advantages: good control of the tests im­ Subsequently, lift and drag measurements sail, but Fig. 2 will give an indication of were taken over selected range of headings. plies repeatable results which can be pre­ their complexity. It shows only the main Tests were conducted at constant wind sented simply, and therefore understood relationships and much has been omitted speed Vft = 15.7 knots (29.3 ft/sec). more easily. for the sake of simplicity. One such omis­ The use of a model which is not the same sion is that of 'feedback' — the way in size as the original, must inevitably intro­ which a factor affecting another is in turn tunnel test on a single sail. What we must duce limitations on the results, and the art affected by it. know first is the magnitude of the total of wind tunnel testing is largely to obtain So far, I've attempted to give some idea aerodynamic force produced by the rig model results which are compatible with ofthedifficultiesininterpretingwind tun­ at given wind velocity and incidence angle the full-size behaviour we need to compre- nel test results. The reader is invited now OC (or headingangle P-A). The magnitude of hendor predict. Even if exact quantitative lobecome familiar with basic principles of FT and its direction of action cannot easily data may not always beobtainabledue, for sail testing as shown in Figs. 3,4 and 5 — ^example, to scale effect, absence of wind which demonstrate how the driving power gradient, the unsteadiness of real wind of differen t rigs can be measured and com­ Fig. 3. This should helpanunfamlllarreaderto etc, all important trends can relatively pared in more precise terms, subject to understand the meaning of standard aerody­ easily be established. Other^vise, the de­ rational discussion as distinct from irre­ namic terms such as Lift (L) and Drag (D) and signer must rely on guessing, or on full- sponsible conjectures or armchair esoter­ how they are converted Into the driving and 'size long term observations of boat behav- ic speculations. heeling components which, in turn, are di­ /ur in conditions where everythingis real rectly responsible for a sailing boat's motion. A convenient means whereby the merits Since the leeway ang le (M Is not the same for and natural, but nothing can be precisely of different sail configuration can be esti­ every boatand its value depends not only on mated is a diagram (such as that in Fig. 6), hullshapebutalsoonthecoursesailed(p]and which indicates how the drivin g force gen­ boat speed (Vs). it became common to presentwindtunnelresultslna slightly differ­ erated by a given rigchanges with heading ent way to that shown in sketch B. This is Fig. 2. Factors Affecting Aerodynamic Forces angles relative to the apparent wind — on Sails. Some of these factors are deter­ illustrated in sketch C where the two com­ mined by design (planform, aspect ratio sail beginning from close-hauled to down­ ponents (Fy and Fy) ofthe total force (Fj) are cut, cloth properties), some depend on crew/ wind sailing attitude. parallel and perpendicular to the hull centre- expertise, and some dependen the wind (gra­ Figs. 3A and B illustrate the method of iinei.e. boat's heading (p-A) instead of related dient, velocity, turbulence). derivingsuch a diagram through the wind to the course sailed (P),

Smoolhing screen Wind tunnel wall /

U(t - L Ft - Total aorad. force

Sheeting Heading Angle o\ Wind p-A angles o„fi,^ heel velocity, Sail setting gradient, turbulence —-1 App. wind

D - Drag V, a - Angle of ^ incidence Mast Plan form Sail cloth properties. section. of sails. Cut diameter. aspect raiio Porositv. flexibilit/ sail area Roughness ® 7 Apparent wind

"xN'Sp. force . r 1 .0 Camber, Twist, magnitude variation position of incidence

A - Leeway angle Fig. 5. Polardiagramsof two Lateen Rigs No 1 Rnj No 1 and No 3. presented in Fig. 1 demonstrate the principle ot windward performance interpre­ tation. Two arrows drawn from the origin of the Lift axis and the Drag axis (marked 0) to the points marked 32.9* and 33.0° along the curves (relevant to Rig No 1 and Rig No 3) show the line of action and magnituties pro­ portional to those of the original total aerody­ namic forces. These can subsequently be resolved in the manner indicated in Fig. 3C.

1 ! ! 1 1 1 1 J Fig. 6. Diagram of driving force coefficients 1 . ; ^ Cmb Claw rig C,^ of single Crab Claw and Bermuda type of sail plotted against the apparent wind angle -! !/ (heading see Fig. 3C) provideacommon yard­ % Driving \ j , stick between differentsail forms. ^cff^ components | ƒ / j ƒ[ Bermuda rig^^_^^

-1 A), wccan find how the total aerodynamic A' 1/1^ force FJ varies over the chosen range of K. 1// angles (Fig. 5). Once the total force Fy is / known, it can be resolved into two alterna­ tive components. These are the driving \ 0^.. 0?^^ ft. 06 0» Apparcni Wind forces FR and heeling force F^ shown in \^ \. Drag coeffic. Co / Fig. SB.ThedrivingforceFK, acting in the rï iO/ ro «0 W 100 uo (TO 160 ISO direction of the course sailed, propels the Apparent wind angle (degrees) boat. The heelingorcapsizingforccFH, at a right angle to the former, causes drift of be established directly but can be deter­ ple ofaforceofthis sort is the upward force the hull and also heel. niined by nieastiring the two components which acts on the wings of aeroplanes and of FT, namely Lift L and Drag D. Lift is keeps them in the air. In spite of its name, The essential requirement of a sail is to measured at a right angle to the apparent liftdoes not necessarily act upwards — for generate a large driving force component wind y^. Drag is measured in the same example on a sail or rudder it ac(s FR.But,exceptona'deadrun',itcannotdo direction as the apparent wind. laterally. that without producing at the same time a heeling force FH. AS seen in Fig. 3B, the The first component L is traditionally By repeating theLandD measurements driving force attained is proportional to called lift because the most familiar exam­ for a numberof selected angles oc (or for p- the heeling force and in the close-hauled condition (P about 30°) Fji is roughly one- fM»97- TET-.i'Iri.CAA-rjJ T.^T.ANJ) fourth to one-third of FH- In other words, every pound (or kilogram) of driving force generated on the sail is accompanied by three to four pounds (or kilogram) heeling force that the yacht must witlistand by virtue of her stability. By analogy, the heeling force and asso­ ciated heel can be regarded ns the throttle of a motor boat; it puts a limit on the sail driving power which can be extracted from the wind. So that the results from tests on model sails can be applied to other similar sails but different in size, operating at an arbi­ trary wind speed, it's customary to ex­ press the forces measured in the form of sail coefficients. Fig. 5 shows, for exam­ ple, the so called 'polar diagrams' of sail coefficients of two Lateen rigs 1 and 3, shown In Fig. 1. The meaning of sail coefficient is quite simple; it gives the magnitude of the force which would be developed on a unit sail area (one square foot) when the apparent wind speed V/\ is one foot per second. In other words, sail coefficients can be regarded as the indicators of sail efficiency. Once the sail coefficients have been established, theactual forces generated on a fullsize sail at given apparent wind speed can be estimated by multiplying relevant sail coefficient by the actual sail area SA and the dynamic pressure of the apparen t wind VA. Thus: Driving force = Driving coefficient x SAX 0.00119 VA' (S A is given in sq.ft; VA is given in ft/sec.) Referring to Fig. 5 we may find that, at the same heading angle p-A = 33°, the driv-

Fig. 7. Some planforms of eady Crab Claw rigs called by prominent maritime historians as 'proto-lateen'or 'primitive Oceanic lateen' sails. Such a planform was characteristic of western Polynesia when Tasman and Schou - ten were exploring in the Tongo (XVII cen­ tury). As we shall see later, the Crab ClaW' type of rig is by no means primitive wher studied from an aerodynamic point of view. PRACTICAL BOAT OWNER ing force foci ncionl i\, ofllio l,alcr-n s iil .•oi)liHLii-:i(ioa.\<)1 i,sai)oulO..|7whilc(|,',( orconfiniiralion No H is about 0.3 ] As ail example, l.-t s calculate how much ch iviiif,' fore- will be developed on (lu,.se two Lateen sails differeiK in planform bul of the same area, sa.v, 100 fC and al the same wind speed V,\ = 20 knots (33.8 ( /soc), he driving force generated by sail No 1 will be: Cx X X 0.00119 X V.v" = 0.47 X too X 0. 0011 9 X 3;i.8: = 03,8 lb. While (he dris-^ mg force developed by sail No 3 will be- 0.31X 100x0.00119x33.8-' = 42 I'lb 1. e, 217 Ib less lhan thai developed bv sail The Total Ao 1. In other words, area for area, sail No 1 IS potentially about 50 per cent more efnc-ienl (hansail No3 ~ rather asurni is- iiig result! Performance Even more startling is the extraordin­ ary performance of the Crab Claw rigpre- Dinghy sented in Fig. 6. The plot of driving force coefficient versus apparen t wind angle (or heading, see Fig, 3C) clearly demonstrates Miat the Crab Claw is superior to a single Ssiiilfeiig Bermuda sail right from Ihe close-hauled condition. lis superiority increases when Tinker dingliiesare the Ofily folding sailing theboatbearsawayandon reaching, when intlatables with a built-in daggerboard. he apparent wind angle (heading) ap­ So performance - even to windward - proaches 90 degi-ees, the driving force co­ really sparkles. With a choice of models efficient of the Crab Claw rig is about 1.7 and ngs, a Tinker can be che sailing tender whereas that of the Bermuda rig is about youVe always wanted or a fast little 0.9. That is, the Crab Claw rig delivers day sailer You can scow your Tinker onboard or in che booc of the car aboutOOpercentmoredrivingpowerthaii the Bermuda rig. In the light of these results it appears that we have no reason to attribute com­ mendable performance to the Bermuda All Tinkers can be ficced wich CO-' inflation triangular sail. The practically extinct inflatable canopy and an N.M.I, design sea anchor co DOT and SOLAS scandards Rowing Tender Crab Claw type of sail — once used by How many inflacables row well' Polynesian seafarers (Fig. 7) — is much Concepc rank cesced in wind and wave conditions up co 75 knocs. Accepted for Because Tinkers are builc like a boac and superior to our safely guarded product of supplied wich real oars and rowlocks you racing and rating rules. Indeed, the Ber­ OSTAR, TWO-STAR and other trans-ocean races. The only inflatable lifeboac which can row a Tinker safely and effeccively mudan pattern is so much protected that in condicions which would be beyond the designers of 12 Metre hulls are free to can be sailed cowards your best chance of rescue. the capabilicy of many ocher solid and experiment with any form of fin-keel inflatable dinghies. They're fine load (which after all operates like a sail upside carriers too. - down — the leeway angle A being analo- jous to the headingangle p-A, Fig. 3B), but they are not allowed to do the same wi th the ng. For some incomprehensible reasons, the sail shape is considered by the rule makers as sacrosanct, and the sailing fra- •rnity is compelled to adore a sort of sa­ cred cow. How and when the peculiar Crab Claw type of rig, characteristic of Oceanic sea The unique incegral hinged marine ply craft, was originated is lost in the mists of boccom of Che Tinker dinghies, plus che antiquity. Some historians have classified underwacer shape creaced by che STAR TRAVELLER •hissail as the proto-lateen type. Similar­ buoyancy tubes mean thac, under power, 13ft X 4ft 9ins. Folded: 45in x 24in x I3in ly in planform (see Fig. 1), may account Tinkers are really fasc - a Traveller planes' Hull Weight: 75lbs. Sail Area: 65 sq ft lor such a guess. However, as we shall see at 13 knocs with one person aboard and TRAVELLER in the following parts of this series, those a 5 hp oucboard. lift X 4ft 9ins. Folded: 45in x 2lin x l2in. two rigs are very dissimilar in their per­ Hull Weight: 70lbs. Sail Area: 50 sq ft. formance, the Lateen form of type 2 in Fig. TRAMP 1 being exceptionally poor. « 9ft X 4ft 6in Folded: 42in x I8in x 12in Hull Weight: 58lbs. Sail Area: 37 sq ft

The research referred to in this article is the result of a programme funded by the Natural Resources and DIRECT FROM THE BUILDER. linvironmenial Department of Ihe Overseas Develop­ ACCESS AND ment oflhe Bntish Government. The programme was earned out by (WacAlister ElliotI and Partners Ltd BARCLAYCARD Naval Architects and Fisheries Consultants. The re­ FACILITIES. sults were presented in the Proceedings of Ihe Eight Chesapeake Sailing Yacht Symposium.US Naval Aca­ demy, Annapolis 1987: CA Marchaj Tlie Comparison • To: PETER OWEN — — — — — ol Polenlral Dnving Force of Various Rig Types Used for f^ishing Vessels. I Te7tn9M^^'^^^T^' ^^"'"8^°" "-edge, wmcanton. Somerset BA9 8BN lel: <0963) 33237. TeleX: 46123 LONGNC G Please send me full details of Tinker Inflatables I Name f Address. NEXT MONTH 1^ Tony Marchaj will continue to reveal the I «.TT-. INFLATABLES i*urprl$lng results of his wind-tunnel measurements on sails.

Mo 261 SEPTEMBER 1988