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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 dif ferent 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.

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