Which rig? What's better than Bermudan? Scientific analysis yields amazing results

WHICH RIG?

WHAFS BETTER

rHAN BERMUDAN?

ientific analysis yields amazing results. Tony Marchaj reports.

per-formance sailing rigs for racing — inly the Bermudan rig — little or no tematic research has been carried out 3 other traditional sail configurations, it's difficult, if not impossible, when -'cting a sail plan for a boat, to deter-le with certainty whether a proposed is more efficient (for a given sail area)

cient than the gaff, sprit, or whatever. It can be scientifically proven: comparative curves of lift vs drag etc. would put the triangularsail with its longer leadingedge way ahead'*. After all, thel2Metreyacbts competing for the America's Cup — an epitome of ultimate progress achieved in the field of high-performance boats—are

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).

3. S P R I T RIG 4 . G U N T E R R I G

5. DIPPING L U G RIG 6 . C R A B C L A W RIG 1

n another, either anticipated or al-dy existing. And anyway, there's con-arable bias in opinion as to the merits of

Bermuda type of rig to begin with, lost people believe that this rig, which ni nates the contemporary sailing lie, both for racing and cruising, must lie best rig available; to quote:

'Every-• Icnows tiie Bermuda rig is more

effi-J(51 S E P T E M B E R 1988

• The quotation laken from 'Wooden Boat' No 7 3 . November 1986.

" C - A . Marchaj: T/anform Effectofa NumberofPigson Sail Power'—Proceedings of Regional Conference o a Sail-Motor Propulsion, (Vlanila, November 1985. The paper presented is Ihe result of a research program funded by the Overseas Development Administration of Ihe British Government. The program carried out by MacAlister Elliot and Partners Ltd is aimed at improv-ing the performance and the fuel economy of sailimprov-ing fishing craft in the developing worid.

.!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.

I n t e r p r e t a t i o n of w i n d t u n n e l r e s u l t s

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, •^spect ratio, and so forth.

From Fig. 2 it's evident that rigid con-trol is necessary over any experiment

vhether conducted fuU size or on a model. ..t's difficult, if not impossible, to deter-minetheeffectofchangingone factor, if at the same time, one or more other factors alter. The wind tunnel here offers gi-eat advantages: good control of the tests im-plies repeatable results which can be pre-sented simply, and therefore understood more easily.

The use of a model which is not the same size as the original, must inevitably intro-duce limitations on the results, and the art of wind tunnel testing is largely to obtain model results which are compatible with the full-size behaviour we need to compre-hendor predict. Even if exact quantitative data may not always beobtainabledue, for ^example, to scale effect, absence of wind gradient, the unsteadiness of real wind etc, all important trends can relatively easily be established. Other^vise, the de-signer must rely on guessing, or on full-'size long term observations of boat behav-/ur in conditions where everythingis real and natural, but nothing can be precisely

Fig. 2. Factors Affecting Aerodynamic Forces

on Sails. Some of these factors are

deter-mined by design (planform, aspect ratio sail cut, cloth properties), some depend on crew/ expertise, and some dependen the wind

(gra-dient, velocity, turbulence).

measured or controlled. That's the reason why certain factors contributing to suc-cessful design sometimes remain obscure, misplaced or controversial.

It's not proposed here to enter into a detailed discussionof all the factors which can influence the forces developed by a sail, but Fig. 2 will give an indication of their complexity. It shows only the main relationships and much has been omitted for the sake of simplicity. One such omis-sion is that of 'feedback' — the way in which a factor affecting another is in turn affected by i t .

So far, I've attempted to give some idea ofthedifficultiesininterpretingwind tun-nel test results. The reader is invited now lobecome familiar with basic principles of sail testing as shown in Figs. 3 , 4 and 5 — which demonstrate how the driving power of dif ferent rigs can be measured and com-pared in more precise terms, subject to rational discussion as distinct from irre-sponsible conjectures or armchair esoter-ic speculations.

A convenient means whereby the merits of different sail configuration can be esti-mated is a diagram (such as that in Fig. 6), which indicates how the drivin g force gen-erated by a given rigchanges with heading angles relative to the apparent wind — beginning from close-hauled to down-wind sailing attitude.

Figs. 3A and B illustrate the method of derivingsuch a diagram through the wind

Flg. 4. Models of sailing rips. A representative fishing boat hull, consisting of that part nor-mally above the water, and ata nominal angle of heel ® = 10 degrees, was built for the tests.Here, models of a Bermuda rig (A) and a Crab Claw (8) are being tested in the South-ampton University wind tunnel. Sails were Initially adjusted to each predetermined heading angle relative to the wind so that the set seemed good to the practical sailor's eye. Subsequently, lift and drag measurements were taken over selected range of headings. Tests were conducted at constant wind

speed Vft = 15.7 knots (29.3 ft/sec).

tunnel test on a single sail. What we must know first is the magnitude of the total aerodynamic force produced by the rig at given wind velocity and incidence angle OC (or headingangle P-A). The magnitude of F T and its direction of action cannot easily Fig. 3. This should helpanunfamlllarreaderto understand the meaning of standard aerody-namic terms such as Lift (L) and Drag (D) and how they are converted Into the driving and heeling components which, in turn, are di-rectly responsible for a sailing boat's motion. Since the leeway ang le (M Is not the same for every boatand its value depends not only on hullshapebutalsoonthecoursesailed(p]and boat speed (Vs). it became common to presentwindtunnelresultslna slightly differ-ent way to that shown in sketch B. This is illustrated in sketch C where the two com-ponents (Fy and Fy) ofthe total force (Fj) are parallel and perpendicular to the hull centre-iinei.e. boat's heading (p-A) instead of related

to the course sailed (P),

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

Mast Plan form section. of sails. _Cut diameter. aspect raiio Cut flexibilit/ sail area

7

Sail cloth properties. Porositv. Roughness Apparent wind . r

1

/ Wind tunnel wall

U(t - L

V , a - Angle of

^ incidence

®

F t - Total aorad. force

D - Drag

"xN'Sp. force

Rnj No 1 % Driving \ j , ^cff^ c o m p o n e n t s | ƒ /

K .

A'

1 . ;

-! !/

1 / 1 ^

/

/

/

Apparent w i n d angle (degrees)

be established directly but can be deter-niined by nieastiring the two components of F T , namely Lift L and Drag D. L i f t is measured at a right angle to the apparent wind y^. Drag is measured in the same direction as the apparent wind.

The first component L is traditionally called lift because the most familiar

exam-ple ofaforceofthis sort is the upward force which acts on the wings of aeroplanes and keeps them in the air. In spite of its name, liftdoes not necessarily act upwards — for example on a sail or rudder it ac(s laterally.

By repeating theLandD measurements for a numberof selected angles oc (or for

p-f M » 9 7 - TET-.i'Iri.CAA-rjJ T.^T.ANJ)

Fig. 5. Polardiagramsof two Lateen Rigs 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. Fig. 6. Diagram of driving force coefficients C,^ of single Crab Claw and Bermuda type of sail plotted against the apparent wind angle (heading see Fig. 3C) provideacommon

yard-stick between differentsail forms. A), wccan find how the total aerodynamic force F J varies over the chosen range of angles (Fig. 5). Once the total force Fy is known, it can be resolved into two alterna-tive components. These are the driving forces F R and heeling force F ^ shown in Fig. SB.ThedrivingforceFK, acting in the direction of the course sailed, propels the boat. The heelingorcapsizingforccFH, at a right angle to the former, causes drift of the hull and also heel.

The essential requirement of a sail is to generate a large driving force component FR.But,exceptona'deadrun',itcannotdo that without producing at the same time a heeling force F H . A S seen in Fig. 3B, the driving force attained is proportional to the heeling force and in the close-hauled condition (P about 30°) Fji is roughly one-fourth to one-third of F H - 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. I n 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 S A and the dynamic pressure of the apparen t wind V A . Thus:

Driving force = Driving coefficient x S A X 0.00119 V A '

( S A is given in sq.ft; V A 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. P R A C T I C A L 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, 2 1 7 Ib less lhan thai developed bv sail 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!

Even more startling is the extraordin-ary performance of the Crab Claw rigpre-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 Bermuda sail right from Ihe close-hauled condition. lis superiority increases when theboatbearsawayandon reaching, when

he apparent wind angle (heading) ap-proaches 90 degi-ees, the driving force co-efficient of the Crab Claw rig is about 1.7 whereas that of the Bermuda rig is about 0.9. That is, the Crab Claw rig delivers 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 triangular sail. The practically extinct Crab Claw type of sail — once used by Polynesian seafarers (Fig. 7) — is much superior to our safely guarded product of racing and rating rules. Indeed, the Ber-mudan pattern is so much protected that the designers of 12 Metre hulls are free to experiment with any form of fin-keel (which after all operates like a sail upside - 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 craft, was originated is lost in the mists of antiquity. Some historians have classified •hissail as the proto-lateen type. Similar-ly in planform (see Fig. 1), may account lor such a guess. However, as we shall see i n the following parts o f this series, those two rigs are very dissimilar in their per-formance, the Lateen form of type 2 in Fig.

1 being exceptionally poor. « The research referred to in this article is the result of a

programme funded by the Natural Resources and linvironmenial Department of Ihe Overseas Develop-ment oflhe Bntish GovernDevelop-ment. The programme was earned out by (WacAlister ElliotI and Partners Ltd Naval Architects and Fisheries Consultants. The re-sults were presented in the Proceedings of Ihe Eight Chesapeake Sailing Yacht Symposium.US Naval

Aca-demy, Annapolis 1987: CA Marchaj Tlie Comparison

ol Polenlral Dnving Force of Various Rig Types Used for f^ishing Vessels.

NEXT MONTH

Tony Marchaj will continue to reveal the i*urprl$lng results of his wind-tunnel

measurements on sails.

The Total

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