So how does the Crab Claw work?

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20' 25' 30- 35' 40" 45" 50' 55'

F I G . l

So how does the

Crab Claw work?

Tony Marchaj continues his investigations

All men, by viriue, desire lo know.'

Arisiolle (384-322 BC)

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superior-y of the Bermuda rig. All of which raises an bvious question: why is the driving force of aeCrabCIawrig—a practicallyextincttype f sail once used by Polynesian seafarers — ubstantially higher than thatof the triangu-ir Bermuda alternative?

The plot of lift coefficient ( C L ) in Fig 1

h ows that the Crab Claw prod uces a greater

I at similar heading angles, and of course s the lift coefficient that primarily

contrib-utes to the driving force. Its superiority is particulariy impressive in terms of maxi-vium CL- Thus, beyond heading angle of about 40 degrees the lift generated by the Bermuda sail gradually decreases, while the lift produced by the Crab Claw still goes up. Such an extraordinary behaviour of the Po-lynesiansail sostartled the author, when the initial test data were recorded, that the wind tunnel was switched of f for a while to inspect the balance system which measures the aero-dynamic forces. But everything was in order.

The only sensible conclusion from the ex-peri mental evidence would be that the mech-anism whereby li ft is generated by these two types of sail must be radically different.

To assist the understanding and interpre-tation of the test results relevant to the sail configurations shown in Fig 1 (and other types of rigs too) it's worthwhile reminding

Fig 1 Lift produced by two types of sail. Crab Claw and Bermuda, in presence of the same hull. Note that, when bearing away, the lift produced by the Bermuda sail gradually increases to a maximum value of 1.35, approximately at a heading angle of 40 degrees. At this angle the sail is said to stall and the flow round is considerably modified

(see Figs 4 and 6).

ourselves of some principles of lift genera-tion by the Bermuda sail.

The concept is easily explained in qualita-tive terms although the quantitaqualita-tive esti-mate of lif t is by no means easy witliou t wind tunnel measurements. Suppose a uniform stream of air blows steadily with velocity V,\ over a sail set at an angle of incidence oc

relative to the wind direction as indicated in Fig 2a. The air flow in the immediate vicinity of the sail will be deflected, so that in some regions the flow is speeded up and, in others, is slowed down. Fortunately, a variety of visualisation techniques has been developed for obtaining a physical picture of the flow patterns, such as those plotted in Fig 2a or 3. Figure 2a which illustrates a sail section (as seen when looking down from above the boat) facilitates better understanding of what is going on round the sail. In such a picture, the actual flow pattern is depicted by means of so-called streamlines which in-dicate the flow direction. Moreover, the pic-ture of flow given by streamlines indicates, although not directly, the local velocities V, around the sail. These can be quite easily read, at least in a qualitative sense, and the rule is: where the streamlines are widely separated, as they are along the windward side of the sail in Fig 2a (or a foil in Fig 3) the local air velocities V] are lower than that in front of the sail, i.e. V ^ . Conversely, where the streamlines are close together (as they are along the leeward side), the local veloci-ties V | are higher than V ^ . At a sufficiently great distance upstream of the sail, the streamlines are straight parallel lines with equal spacing between them.

The totaleffectof differences in local velo-cities results in changes in the local air pres-sures over the windward and leeward sur-faces of the sail. Thus, a decrease in the local velocity, giving V , less than V ^ , leads to a local increase in pressure above the atmos-phere pressure. Conversely, an increase in V | to a value greater than leads to a decrease in the local pressure below the atmospheric pressure. The result is that over some parts of the sail, generally on the windward sur-face, there's a local increase of pressure, whilstoverthe leewardsurface, particularly near the leading edge, there's a suction ef-fect. ('Suction' isjust a quick way of designat-ing a pressure difference which tends to move the foil from the higher pressure, or windward, side towards the low-pressure, leeward side).

In the wind tunnel it's relatively easy to determine these pressure changes around a foil, and a typical result in shown in sketch b of Fig 2,

From this it can be inferred that the total aerodynamic force F T can be estimated by summing all these pressures, which are indi-cated by thin arrows pointing toward the leeward side of the sail. In other words, the lift force which is the main component of the

Fig 2 a) Flow pattern around single sail (without mast) set at an ideal angle of incidence. Such a flow can be observed aiound the sail section somewhere in the middle part of the sail where the (low is not affected by so-called tip vortices.

VA=apparent wind velocity measured some cfistance ahead of sail.

V|=local flow velocity (different from VA) due to influence of the sail on local flow velocities.

b) Pressure distribution over the wind-ward and leewind-ward surfaces of the sail. Note: the suction intensity shown by curves b and c is reduced due to increase in size of the separation bubble when

incidence angle increases.

Wind

Total aerodynamic force

Low prtssure

r ' ^ Higtx pressure region

high spetd,low pressure

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total aerodynamic force appears primarily in the form of pressure forces distributed over the surface of the sail. It is also clear that the

major portion ofthe lift comes from the suc-tion forces concentrated close lo the leading edge (LE) of the sail. As we shall see, the

character of the flow close behind the L E

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largely determines the magnitude of suction over this inost important part of the sail.

Visual study of the flow pattern, frequent-ly used in aeronautical experiments, has be-come quite common in yachting, asevidenced by all sorts of tell-tales attached to sails. These have made sailing people acutely aware of the relation between the flow over that part of the sail adjacent to the L E , and the forces produced. These flow indicators may play a signi f icant role in detecting unde-sirable flow patterns, thus assisting the helmsmaninkeepingthesailsatproperangle to the wind. Figures 2a and 4 illustrate the point.

Observations, made by the author, of tufts attached to a cambered rigid sail (at first withoutmast),showedthatsmoothattached flow along the whole leeward side of the sail occurs only at one unique angle of incidence which might be called the ideal angle of Fig 3 Flow pattern round thin aerofoil with different curvature of its front part (nose).

a) Separation commences right from the leading edge — a bubble Is formed and then the flow reattaches to the foil surface some distance downstream.

b) The same aerofoil with 'drooped nose', the flow enters the leading edge smoothly without separation. This exam-ple shows that in order to avoid an early separation the curvature of the front part of the foil must correspond to the inci-dence angle of the local oncoming flow.

As you can imagine, such a picture of streamlines (or flowlines] around the foil is of great help in understanding the character of the flow and associated phenomena which determine the foil

efficiency as a lift-producing device.

T[io ruSF:.irch r e f e r r e d to ni this article is the result ot a program (unded by the Natural Resources and Environnietital Department of the Overseas Devel-opment of tho British Government and was carried out by M.icAlister Elliott and Partners U d Nnval Architects and Fisheries Consultants. Results were presented in Proceedings o f the Eighth Chesa¬ peake Sailing Yachi Symposium. US Naval Aca-demy. Annapolis 1987 :CA tv/larchaj — Tlie Com-parison oi Polential Driving force ol Various Rig

Types Used lor Fisiiing Vessels.

incidence. Only al that one angle did tlic

u'ool-tufts fly straight aft, on both sides of the sail, and right from the leading to the trailing edge. This is sliown in Fig 2a by the streamlines marked 1 (leewaid) and w (windward). .M higher angle of incidence than tlie ideal, a region of flow separation near the leading edge appeared and wiis clearly detected by fluttering yarns.

Figure 4a shows, in some exaggeration for thesake of clarity, what was then happening over the leeward side of tlie sail. It will be seen that tlie flow has separated at the lead-ing edge, and tli is abrupt change is accompa-nied by a local reversed flow vortex, rotating inside a bubble underneath the separated streamline. Observation of tufts indicated that the flow re-attached to the sail some distance behind the bubble, and then fol-lowed the leeward surface closely. As the angle of incidence was further increased, the separation bubblegrew in both thicknessand streamwise extent until it covered the entire leeward surface of the sail model, at about which stage maximum lift was reached (Fig I). At that moment, the flow over thé whole leeward side of the sail was totally separated in the manner shown schematically in Fig 4b. This was full stall.

Observations of flow pattern were then made on the same cambered sail but with a streamline head-foil attached to its leading edge. Attention is now drawn to the wool tufts in the photographs in Fig 5. The picture of recorded flow patterns is similar to that in Fig4, but with thesignificantdifference that separation at the leading edge occurred less readily when the incidence angle was in-creased. Inother words, the character of the flow at the L E became somewhat less sensi-tive to the variation in the incidence angle.

The peculiar behaviour of the air flow in photograph 5a (and the explanatory sketch below) is connected with an inability of the streamlines to remain closely attached to the surface while passing from the stagnation pointSi, situated on the windward side, then around the L E to the leeward surface. Whetherornotthe flow willsmoothly negoti-ate the L E without separation, and then fol-low the leeward surface up to the trailing edge, depends on two factors:

1) The incidence angle at which the sail is set relative to the wind

2) The curvature of the sail behind the L E The influence of the incidence angle ha; been already discussed when referring tt Figs 4 and 5. The complementary Fig 6 illus trates schematically the two distinctly dif ferent flow patterns — before the stall am after — and their effect on lift and drag. I will be seen that the flow over the whol leeward side of the sail is dominated by ran dom, large-scale vortices. The fact that ai aerofoils (including sails) experience dra far greater than the unavoidable drag due t P R A C T I C A L BOAT OWNEF

Fig 4 Two types ol flow separation. a Separated flow with re-attachment. b) Separated flow without re-attach-ment (sharp separation) S, indicates so-called stagnation point where the flow is brought to rest.

It appears that, by building up the bubble, kind of an artificial thickness at the leading edge LE, nature makes the flow round a sharp LE easier. Without a bubble to ease the flow round a sharp LE, the flow would separate and never re-attach to the foil surface (sketch b). In other words, the bubble may be regarded as an agent mitigating consequences of total separation. However, the bubble has to be paid something for services rendered. In three dimensions a reversed flow vortex inside the bubble (sketch a) can be imagined as a rotating cylinder of air all along the lee side of the sail. Such a rotating vortex has certain mass and is kept m rotational motion (like a sort of flywheel) at the expense of the kinetic energy which is extracted from the on-coming external stream of air (wind). The additional drag experienced by the sail-like foils (much higher than that pro-duced by thick streamline foil such as wings) is a measure of wind energy lost. F'-- 5 Flow over the lee side of the sail 1 head foil attached to the leading .age.

a) At small incidence angle oc.flow reattaches to sail surface just behind the separation bubble in place marked R. Downstream from R, tufts are lying flat along the remaining leeward side of the sail.

b) Larger incidence. Fully separated flow is indicated by unstable tufts point-ing towards the leadpoint-ing edge i.e. full stall. The main factor limiting increase of lift is flow separation; if the growth of separation bubble with the incidence angle could be somewhat arrested, say by continuous removal of the boundary layer — stagnant material accumulating inside the bubble — the lift would in-crease well beyond that normally deter-mined by separation of type b see Fig 6. Fig 6 a) Relatively high lift and low drag are characteristic of so-called attached flow i.e. before stall occurs,

b) The Bermuda type of sail cannot

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iv-ment, lift suddenly decreases and the drag increases. The reason for the growth ofthe bubble size with Incidence angle is that more and more boundary layer material, peeling off from the front

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artificial means such as, a pump. friction is explained by the presence of those vortices; they are to be regarded as the real seat of drag. They prevent the leeward and windward streamlines from closing up at the trailing edge, and thus cause continual waste of wind energy to maintain such an un-healthy flow.

The influence of sail cui-vature right be-hind the leading edge, or the nose curvature of a thin aerofoil for that matter, is shown in Fig 3a and b. It will be seen that attached flow around the leading edge of a thin foil can be assisted by bending the section 'nose' intothe oncoming flow. In Fig 3a the separation bub-ble is present, while in 3b there is no separa-tion and the flow over the leeward side is attached. In other words, the occurrence of No 262 O C T O B E R 1988

Transition lo lurlniloni Dl Separnlion bubble {reversed How vortex) Flow Laminar separation nilhoul reallachmeni Large lurbuleni \vol<e F I G . 4

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Flow direclion indicated by the Leading edge woot-streamers attached to the (Head foil) tee - side ol Ihe salt Separation

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S - Separation R - Reattachment

Total aerod. force

' r ^ - ^ \w.Dellection \ of the original flow

Flmvclireclion behind ihe sail

F I G . 6 '^Drag „--"^ ' ^^^^^^ '^^^Meon flow direction _{/"^^behind the sail}

separation can be precipitated if the curva-ture of the sail immediately after the leading edge does not match the angle of incidence. However, if the curvatureof the frontpart of the foil (be it wing or sail) is properly adjust-ed to the incidence angle, itis possible todefer separation to considerably larger angles of incidence and to achieve much higher lift.

In view of what has been said so far, one may rightly infer that the sail efficiency is not primarily determined by the streamwise position of the maximum camber (usually expressed in termsoflengthofthesailchord, say 1 or { from the L E ) but by the sail

curva-ture close behind the leading edge, where most of the suction force (and hence lift) comes from.

At this point some readers may ask why so much attention should be paid to all those streamlines and flow patterns? The reason is that any abrupt change in flow, leading to the formation of a separation bubble, is accom-panied by part ial collapse of the suction peak near the leading edge, shown in Fig 2b by the broken lines. Such a re-distribution of suc-tion is followed by an increase in drag, and hence a drastic reduction of sail efficiency in terms of driving force. Sails are not rigid •11

WO SPECIAL PEOPLE

J o h n D r i s c o l l l o o k s b a c k

slnictmes like wings biil lloxiblc niom-lu ancs. Their shape can ell'cctiveiy be con-trolled, to the best advantage, by means ot" luning and trimming devices, provided Ihe crew know how to modify Lhe sailsliape and why. In acomple.\ sport likesailing che effec-tive u.se of luning gadgets becomes itself an additional skill which can hardly be perfect-ed by those who do not understand the corre-lation between the flow character around sails and the driving power.

There can be liltledoubt that the larger the separation bubble the heavier are the in-curred losses in suction. The collapse in suc-tion becomes deeper, and is characterized by rc-distribu tion of suction along the sail chord into the more or less flattened plateau (curve b in Fig 2b). Such a re-distribution of suction demonstrates how critically the lift and sail efficiency depend on the nature of the flow over the leeward side of a sail.

Thesuctioncollapses because windenergy is absorbed by the reversed flow vertex which is kept in rotation inside the separa-tion bubble (Fig 4a). To maintain such a motion against internal friction (due to air viscosity), a certain amount of wind energy must be used. Because the circulation veloci-ty of the reversed flow vortex is low, the suction over the bubble is generally nearly constant, and for this reason the suction distribution is almost flat (curves b and c in the Fig 2), except in the region of re-attach-mentof the flow where the bubble terminates (Fig 4).

Il might be added that the stall observed during tests was usually so violent that, coin-cident with it, a sudden drop in the wind tunnel speed was recorded — evidence of wind energy being wasted. Most yachtsmen will be well aware of a similar effect in the phenomenon of reduced wind energy which can be experienced in the region of the 'wind shadow' left behind the boat — a situation carefully avoided by racing helmsmen.

Ref erring to Figs 4 and 5 one may plausibly assume that the flow separation, with its consequences of premature stall and rapid decrease in lift, could be prevented if the retarded air accumulating inside the separa-tion bubble were continually removed. Con-sequently, the bubble would not grow, and the lift might continue to increase with in-creasing incidence angle. Unfortunately, in thecaseofsailsor foils of higheraspectratio, that is above 2, operating with their leading edge more or less normal to the oncoming airstream, there is no natural mechanism available to remove this 'dead' air. That could only be done by artificial means which, so far, are notpractical from the viewpoint of ordinary sailor or fisherman, though extrac-tor pumps have been used experimentally in aeroplanes.

Flowever, there is another type of flow, associated with sails of different planform, and with other types of lift-generating foil, where that possibility of continuous removal of the boundary layer tending to stagnate inside the separation bubble, can practically be accomplished. Delta wings or Crab Claw type of sail or winglets applied to the keel of the victorious America's Cup contender

Slats and Slnpes belong to this category.

They are known as slender fails and will be analysed in greater depth in my next article

which you can read next month. •

RYA News & Views

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The first came immediately before the naming ceremony for our new RYA/Lom-bard Southern Region Optimist fleet. There cannot be many PBO readers now unaware of the RYA Young Opportunity Scheme, which introduces youngsters to sailingby providing travelling fleets of din-ghies in every region.

As the first of the fleets to start operating when the Scheme was launched in 1986, the original Lombard boats were showing signs of wear after two busy seasons and so the sponsors very kindly agreed to fund a replacement fleet.

The old boats were sold to Gurnard Sail-ing Club, thanks largely to the generosity of two local businessmen, and have now started a second life, introducingyounglsle of Wightchildren to oursport. Meanwhile, an official naming ceremony was planned for the new boats, scheduled to coincide with a Try-a-Boat event run by the British Marine Industries Federation.

Everyone turned up on time for the cei mony, which was to be performed a t Oce.-Village, Southampton, by Mr Ron Youi Director of Lombard North Central. Qu apart from the guests, we had correspc dents from the local papers, a reporter frc local radio and a happy bunch of lo' schoolchildren ready to sail the boats aw after their naming.

W e l l . . . almost everyone, for the pro ised local T V crew didn't show up andso I natural inclination was to wait for them, we all stood around talking while somet went off to phone the studio to find i what had happened. Now it's all very v for adults to stand and wait, but the bo dom threshold of a group of lively l l y olds is not high.

Suddenly, Mr Young disappeared i the pavilion, only to reappear a few i ments later clutching a large paper bag of king-sized Mars Bars. He was immi ately mobbed by the children, not to ir, tion one or two ofthe adults.

The T V crew never did show up, but ceremony went ahead anyway with a si libation of champagnepoured over the of each of the new Lombard boats, children sailed them happily away. Young left for his next meeting in a 1 schedule and the guests all dispersed.

The boats are now in use at clubs