Fig. 1. Crab Claw type of sail under trial in •^A one of the developing countries in Africa. THE CRAB ClAW EXPLAINED TONV MARCHAJ CONTINUES HIS INVESTIGATION Francis Bacon (1561-1626) ast month we looked at the mecha­ nism of lift generation by the high schematically in Fig. 3, is so different that I aspect-ratio Bermuda type of sail. it may seem a trifle odd to most sailors. We also considered "separation", a sort of unhealthy' flow which should be avoided Lift generation by slender foils r a sudden decrease In lift and a concur- (Crab Claw sail) ent increase in drag are to be prevented. There are two different mechanisms of A brief remark was made about a low lift generation on delta foils. One type of aspect ratio sail with unusual planform, lift, called the potential lift, is produced in and of other slender foils capable of deve­ theconventionalmannerdescribedinpart loping much larger lift. The Polynesian 2; that is at sufficiently small angles of Crab Claw rig. winglets attached to the incidence, theflowremainsattached to the keel of the American Challenger Star and low pressure (leeward) surface of the foil. Stripes (which won the 1987 America's This is shown in sketch a in Fig. 3; there's Cupseries)andthedelta-wlngof Concorde no separation and streamlines leave the — invented by different people, living in trailing edge smoothly (Ref. 1 and 2). different times, to achieve different objec- ' ives — belong to this category of slender loils. Fig. 2. Computational model of winged- keel of the American 12 Metre Stars and The question to be answered in this part Stripes which won back the America's Cup is why and bow the slender foils produce 5 Fool Extension. In 1987. Note slender winglets mounted much higher lift than conventional foils, 4.25 Foot Extension , on the inversed taper keel. They are capa­ such as Bermudasails, which operatewith ble of operating effectively at large angles of incidence induced by the hull motion in their leading edges more or less normal Standard S-87 to the oncoming airflow. Apparently, the Winglet heavy seas — that Is when the boat is heav ng, rolling and pitching. It has been character of the flow utilized by slender found that the swallow-tailed foot exten­ foils, such as the Crab Claw sail (Fig. l)or 2 sion improves the performance of wing­ delta-shaped winglets mounted on a 12 lets. The same effect was observed by the •o(re keel (Fig. 2) must be different. And author when testing various planforms of leed, this other type of flow, presented Crab Claw sails in the wind tunnel. '0 263 NOVEMBER 1988 43 Fig. 6. Every lift-generating foil, be it airplane wing or sail, spins the airflow over its tips into a kind of small tornado, called the trailing vortex. The existence of such vortices has been amply confirmed by photographs. A) Trailing vortices develop­ ing around tips of low aspect ratio square sail. B) Picture of one of those vortices as seen from behind the square sail tested in vertical position. •Tolal lift Angle of incidence a Another typeof lift, called vortex Uft, is Fig. 3. Contribution of two different me­ produced by two vortices which separate chanisms of lift generation to the total lift along the entire length of the side edges, as produced by delta wing or Crab Claw sail. shown in sketch b sa Fig. 3. These two vortices; imperceptible at low angles of creasing angle of incidence. incidence, roll up rapidly into two nearly Such fast spiralling vortices induce conical spiral-shaped coils above the large 'suction' (low pressure) over the leeward surface and grow in size with in- leeward surface of the foil (Fig. 4A). The reason is that the vortex cores (Fig. 4B) rotate like a wheel with high speed — and Fig. 4. Below left. A. Pressure distribution Flg. 5. Flow over a delta wing at two high speed implies low pressure. on the leeward surface of a delta foil different incidence angles: A) coiled vorti­ lucideatolly, in the physical sense, al­ relevant to"the Crab Claw type of sail. B. ces are attached to upper surface of foil Afr particles spinning Inside the two coiled though on small scale, the character of (small angle of Incidence). B) at large angle these vortices is similar to the waterspout vortices. The vortex may rotate with very of incidence, some distance from'apex, high circumferencial velocity thus induc­ vortices fail to reattach to the foil surface. (tornado), an intense vortex system in ing large suction over the adjacent surface From this point downstream, the vortex which air may rotate with circumferen- of the toll. The entire f lowbvar the leeward disintegrates and the reason is that as the cialspeed, rangingfrom 150 to 450 ft/sec. part ofthe slender foil, belt delta-wing or incidence angle and lift increase, the vor­ The visible funnel (vortex core), consist­ Crab Claw sail. Is dominated by these two tex core spreads in volume and ultimately ing of cloud droplets condensed due to ex- spiralling, coriical-type vortices. begins to decay from within. pansional cooling, results from markedly PRACTICAL BOAT OWNER ^^A D '°" sai's of four different aspect ratios Z,^^-,- ' ^ ^S'^ P ~ witliout mast. The leading and trailing edges (luff and leach) were kept under tension in order to maintain uniform camber (12 per cent) all along the sail height. Fig. 8 Tlie potential driving power of single Crab Claw sail cornpared with the Bermudan rig (mainsail and large jib). Hatched area represents margin of superiority of former over latter at heading angles ranging from 40* to ISO*. Fig. 9. 1. Berrnudan Rig with and without large and small libs; also showing how much of head of mainsail was removed. 2. Three different shapes of Lateen sail. 3. Sprit rigs of three different aspect ratios. 4. Gunter rig. 5. Dipping Lug. 6. The same Grab Claw sail was set at varying sweepback angles /oiver pressure within the vortex than in thesurroundiugatmospIiere(seeNote3at AR=1.0, is much higher than that for the end of the article) .Inthecaseofa large AR=1.9. The reason is as follows: as the transport aircraft the centre of the trail­ ARof the sail is decreased, the tip vortices ing vortex core from a wing-tip may re­ shown in Fig. 6A are brought nearer to­ volve at over 18,000 rpm. gether, and because they operate as a sort of pump, the boundax-y layer material It wiU be seen in Fig. 3 that the contribu­ tending to accumulate either inside the tion of the vortex lift to the total lift gradu­ separation bubble (discussed in part 2), or ally increases with the angle of incidence. over the rear portion of the leeward sur­ Those stable coiled vortices con be main­ face of the sail, is swept away. The flow tained over an incredibly wide range of continues without breakdown, remaining Incidence angles. It could be large enough attached to the foil surface to much larger to eliminate the need for high lift devices. angles of incidence, so the lift is increased. The supersonic transport aircraft Con­ Thus, as shown in Fig. 7, by reducing the corde is safely controlled in landing atti­ aspect-ratio of the sail from 1.9 to 1.0, the tude at an incidence angle of about 40 maximum lift increases by about 30 per ïrees, without the use of any lift aug- cent. This explains why the square-rigged ...enting devices such as the trailing-edge clippers were unchallenged on a reach i .e. flaps of conventional aeroplanes. in conditions in which large maximum lift Because the coiled vortices are spirall­ was beneficial, but yielded to gaff schoo­ ing downstream, the boundary layer nia- ners in close-hauled conditions when a tericd retarded by friction and tending to higher Lift/Drag ratio paid off. accumulate within the vortex cores is dragged streamwise. As a result, the re­ tarded 'lazy' mass of air is rejected down­ Comparison of sail power of stream and left behind the trailingedge of different rigs the foil where it can do no harm. Such a The maximum valueofliftobtainable, and mechanism which operates non-stop, as a so the driving force, varies considerably, sort of pump, limits the growth in size of depending on which of the two basic types the vortex core—a tendency wliich, if not of flow prevails; either the conventional tempered effectively, may lead to the flow which is characterlsticof high aspect- breakdown of the vortex structure and ratio sails (discussed in part 2); or that consequently to a rapid drop In lift. This relevant to low aspect-ratio slender foils. breakdown of an initially healthy pattern A plot of driving force coefficient is demonstrated in picture 5B. DIPPING LUG RIG 6. CRAB CLAW RIG against heading angle — ranging from closehauled (30») torunning(180'')—may One may interpret this event as a result varying with incidence angle, determines be used as a quick measure of potential of drastically reduced pumpingefficiency the degree of efficiency of the foil as a lift- driving power of different rigs tested. of a coiled vortex, no longer effective in producing device. This is shown in Fig. 8, in which the quickly removing enough of the stagnat- The most noticeable feature of ver-y low ingmaterialaccumulatingwithinitscore. Bermudan rig of mainsail and large jib is aspectratio (AR) typeof foils (squaresails) compared with single Crab Clawsail. (Fig. It will be seen from Fig. 5b that down­ with AR below 1.5 (Fig. 6) is the strong stream from the foil apex the coiled vortex 9 shows the planforms of these rigs, re­ effectof tip vortices on the maximum lift.