Denny Desoutter looks at a shallow draft form
of keel w h i c h can make deep fins
just a bad memory
P
"^ in keels, spawned in IOR racing waters, h^ve few attractions for cruising owners, other than a promise of higher sailing speeds, and latterly they have even begun to lose their charm for racing owners too.By contrast with a modem fin, the unconventional Tandem Keel shown on these pages (Fig.l) has a reassuring look about it. Shallow and sturdy, i t looks as i f it would stand up to the realities of seagoing — and of ditch-crawling. But what about perfor-mance?
By all accounts the short answer seems to be that i t wiU out-perform a fin keel i n every respect. That may sound unlikely, but there is by now plenty of evidence from owners and skippers that where a standard fin keel is replaced by a tandem keel the boat's general behaviour is much improved, and that speed made good to weather is either the same or better.
Warwick Collins, who has spent more than three years in developing his tandem keel, with tests on tank models and on full-sized bDats, sums it up by saying that there is now no 'special
sailing requirement for a deep fin'. I t means that a cruising
owner, who is free from the artificial constraints of rating rules, can have a keel which gives the same speed as a 'radngfin', with only two-thirds the draft. and with a valuable gain in seaworthi-ness and seakindliseaworthi-ness.
The heavy end-plate makee an excellent foot for thet>oat to stand on. This Is the Sadler 34, Talnul, during acrulse of the Channel Isles and Brittany, during which her owner confirmed the claims made for seakindliness and
handling.
In fact the main claims made for the Taridem Keel are:
1. A lower centre of gravity.
2. Better directional stability and resistance to broaching. 3. Better damping of pitch and roll,
4. Smarter tackijig.
5. Improved weatherliness in rough water
Su6h alist ofbenefits seems hard to beheve especially i f one is accustomed to the idea that tn steady fluid flow a single foil usually performs best, because i t is free from turbulence and interference. Here we have two foils, one behind the other, and a massive end-plate bridging their tips. The opportunities for creating interference and drag seem to be built-in.
In practice there is 'interference', but it proves to be beneficial. And that, in part, is because a boat's keel is so often subject to unsteady flow. But of that, more later. For the moment let us look at the simplest claim, the lowered centre of gravity.
In effect a hole has been punched i n the centre of the keel, and the metal has been re-sited in a lump at the bottom. I t is a hefty lump, amounting to more than half the total keel weight. With such a low centre of gravity, this cast-iron keel actually gives a better righting moment than a lead fin of the same weight but of forty per cent more draft.
Fair enough, but we are accustomed to the idea that a reduced draft means a worsened windward performance; tiiat a deeper keel of higher aspect-ratio is more efficient because it suffers less drag for a given amount of lift. I t is 'induced drag' which people have in mind in that connection, the drag which is an inevitable consequence of hydrodynamic lift, and which is associated with tip loss. I t is text-book stuff that a shallow keel must incur a substantial drag penalty, and the reason is to be foimd i n that escape of water around the tip of the keel from the higher pressure on the leeward side to the lower pressure on the windward side. Since the whole action ofthefoildepends,likethatofawing, on creating and maintaining a pressure difference between its two faces, any leakage must be a loss which has to be made good. I n effect that is done by making more leeway, whichmeansthatthe foü is working at a higher angle of incidence with a higher l i f t coefficient and a higher drag coefficient.
Something of a surprise
That is what one would expect of a shallow keel of low aspect-ratio, but tank tests, and full scale trials, some of them i n races, have shown tha t in smooth water the shallow tandem keel has a Uft-to-drag ratio as good as that of a conventional fin. I n rough water and gusty winds the tandem keel actually does better.
Some explanation is called for!
One can see that the large end-plate, shaped like a delta wing must restrain some of the flow around the lower ends ofthe two keel foils. (And see No.236, p39.) But that is not the whole story. Warwick Collins says that the key to the special windward advantages of the design is i n the shape ofthe gap between the fonvard and the main keel. There is a flow through this gap from the high pressure side to the low, and as indicated in the diagram CFig.2).this has the opposite rotation to the escaping flow round the tip of the keel.
He has designed the gap so that it is wider in the upper part and narrower at the bottom and this tends to direct theflowupwards, countering the tendency to 'escape'. A t the same time the flow over the low pressure side of the main foil is accelerated by the energy of the stream through the gap, so that the pressure is lowered yet more. That's to say, the l i f t is augmented.
As I said earlier, some explanation is called for, and in a long discussion with Warwick Collins I felt that the thing remained a bit of a mystery. Nor do I think I am being unfair i f I say that he himself has more or less the same feeling. Nevertheless, onemust attempt some explanation of the success of the Collins keel, i f only for the benefit of anybody who feels that letting water escape through a hole in the middle of a keel is not Ukely to be much better than letting i t escape round the end.
Unsteadyflow
When it comes to sailing in rough water the picture is clearer. The underwater flow approaching the keels will vary in direction and speed, not merely because the sea surface is disturbed, but
F A R E W E L L TO H N S ,
COUNTER FLOW
R O U .
1 — 77» Tandem Keei has two foils with a carefully-shaped gap between. The end-piate which bridges the tips ot bath foils accounts for at least 55 per cent of the total keel weight, and gives a
totter
righting moment than an equivalent fin keef. 2 — This simplified dlagramatlc representation suggests the pattern of flow through the slot. The streemlln&s move upward toward the hull and thus run counter to the normal outflow toward tha tips of the foils. At thesamettmetheflowoveftheweathef (low-pressure) fac«ofthemalnfoltlsacceleratedarydsteBdled.3 — When the boat rolls, lateral movement of the keef gives a relative angle of Incidence, creating lift which worksagainst the lateral motion.
because the boat's rolling and pitching and yawing will modify the relative flow between water and keel
In such a case the function of the forward foil, and of the gap between the two foils, is to smooth and direct the flow over the rear foü. One can see that there may then be less drag and amore continuous lift. A disturbance i n a fluid flow, or stalling in an extreme case, is followed by a time interval which is necessary for the flow to settle down again. I f a boat is rolling, say, a new disturbance may occur before the necessary time has elapsed, so that the flow never gets properly settled
Last summer two identical X-Yacht X-79 production boats were tested against each other. Onehadthe conventional fin keel drawing 4ft. 6in., and the other a Tandem Keel drawing 3fL 2in. Both keels weighed the same and were of cast iron.
In the lower and middle speed ranges there was no difference between the po^ormances of the two yachts. But in gusty condi-tions i n a chop the Tandem Keel boat was able to sail higher and faster. She also showed greater stiffness,better directional stabil-ity, better resistance to broaching, and less pitching. Similar results have been found in other boats fitted w i t i i the Collins keel.
Hiis rough water behaviour does suggest that the two keels with the carefuUy-shaped gap can create a steadier flow regime. Marchaj (No.202 p.36) has explained how a high aspect-ratio fin with short chord can suffer from disturbances of the flow patt-ern, disturbances which are less likely with the 'old-fashioned' long-chord keel. I n the Tandem Keel i t is not length which steadies the flow, but the forward foü. The principle of directing the flow toward the main foü is used i n some taü-first aircraft, aUowing them to reach higher ahgles of incidence than would atherwise be possible without stalling.
Stalling
Stalling is very significant in this story. To setthescene it may be as weü to repeat that a hydrofoü, such as a keel or rudder, can develop Uft i n the same way as a wing, provided it meets the water flowatanangleof incidence. A rudder is angled to the flow by the movement of the tiller. A keel on the other hand enjoys an angle of incidence only because the whole boat is mo.ving sUghÜy sideways — making leeway as we say.
In passing it may be remarked that leeway is usuaUy thought Df as arising from thelateral windpressureonthesails,butitalso arises when the rudder is turned. The function of the rudder is to alter the attitude of the boat i n relation to the line of fravel: momentum keeps the boat moving i n the same direction even when the stem has been swung out of line. Then i t is the l i f t developed by the keel as i t meets the water obUquely that tums die boat. ' •
To get back to the staU. L i f t generated by a foü grows directiy in Droportion to the angle of incidence — imtü the angle reaches something in the region of 15 to 20 degrees. Then it stalls, andUft
suddenly vanishes. Some force is stUl generated, rearward drag in fact, but the sudden coUapse of Uft combined with a shift i n the cenfre of pressure can render a boat uncontroUable, whether i t is the rudder or the keel which has staUed, or both.
Tank tests have shown that the main foü of the Tandem Keel does not stall untü a leeway angle (incidence) of some 30 degrees is reached, whereas a typical fin will stall somewhere between 10 and 12 deg. A t high angles the forward foü of the Tandem wiU probably be staUed, but it wiU stLU have a beneficial effect on the main foü and the total effect is obviously much better than having the whole of a single keel stalled.
Resistance to staU is important when tacking, where the leeway angle (incidence) can easüy reach stalling point with a conventional fin. The boat is then slowed by the increased drag, and the helmsman has to bear away sUghtiy to gain speed and to re-estabUsh steady flow. i
Quick tacking has its value to both racing and cruising owners, but directional stabiUty and sea-kindUness are more important to most of us, and a keel's resistance to stalling is very important to those quaUties too. Once again, I have to remind readers that Marchaj has explained the underlying phenomena. Broaching, for example, can occur when a foUowing sea swings the boat's stem out of Une. The keel is then subject to a rapid increase i n its angle of incidence, and may staU. I f i t does there wül be a great increment of drag, making the situation much worse: the boat may 'trip over' her own keel
RoU damping is also improved by resistance to stalling. When a sea or a gust of wind causes the boat to heel, the keel is simiütaneously moved in one direction, givingthe relative water flow a component in the opposite dfrection. (Fig.3) Momentarily th,e angle of incidence may tiiereby be increased or decreased, but on many occasions i t wiU increase to stalling point w i t h the consequence that the lateral l i f t ' of the keel wiU be eliminated. But i f steady Uft can be maintained, the damping force against
Mr. Mike Gear, Joint owner of a Sigma 362 with the Tandem Keel. Amortg other ber}eflts the owners reported
I ^ R E W E L L TORENS
rolling is preserved: there is by now much experience that this is the case with the Tandem Keel.
Similarly, directional stability is better when roll damping is better, and when there is steadier flow over the keel. That can be valuable to a cruising owner who may want to make quite long passages with a rather short-handed crew.
The End-Plate
So far, I have been writing about the gap between the keels, and the way i n which the downwash firom the front foil improves the flow regime over the after foil. Some attention must now be given to the end-plate, which is more than just a lump of ballast. Nevertheless, i t is heavy, accounting for at least half the total weight of the keel, and i n some boats as much as 60 per cent I n either case that is a much higher profwrtion than would be found in the wings of other keels.
The end-plate also has quite a large surface area, around forty per cent ofthe total keel area, hence its hydrodynamic effects can be significant. Evidently, i t serves to block the escape bf water around the lower tips of the two foils. But it also serves to damp pitching and heaving motions. Thus, i f the boat is moving forward and pitches head down, the delta-wing end-plate expe-riences an upward angle of incidence i n the water flow, and so generates l i f t in the maimer of a delta-winged aeroplane, The Hft in this case is upward, and in opposition to the downward nod of the boat herself.
I f the boat pitches upward, the end-plate Uft acts downward. That sounds fine, but one suspects that i n practice timing, or harmonisation, may be all-important. Some time must elapse before the f u l l l i f t force is developed, and onp can conceive that this might come just at the wrong moment, when the boat is about to reverse her 'nod' in any case. Whether or not that possibility is a real one, i t seems that the inertial characteristics of boats fitted with the Collins keel have matched well with his design of end-plate so that their pitching i n turbulent water has been noticeably smoothed.
I n principle there is nothing out of the ordinary about this damping effect Any foil i n a fluid flow develops a resistive force i f i t is moved laterally, or to be precise while i t is being moved laterally. I t is the movement across the flow which changes the relative angle of incidence so as to create Uft, which of course may be upward, downward or lateraUy. But i n any case, the force is
Warwick Collin's own boat Fighter on a fast reach. Lift from the end-plate holds fier ftead up (note themanonthe
foredeck), and also gives her a flattened wash.
generated in opposition to movement which is causing it — i n a' word, damping.
There are two factors to be remembered. The first is that to be effective the foil must not staU, K the lateral movement is too rapid the angle of incidence may go beyond the stalUng angle, and then there will be no Uft and lots of drag. A foil which has a high stalling angle can be valuable, and that is a characteristic of low aspect-ratio foüs, slotted foüs like the Tandem, and the delta planform. (Consider the steep attitude of the Concordes at take-off and landing).
The second factor concerns time, which I have already touched on. The change i n l i f t does not occur simiütaneously with the change in incidence. There is a time lag, known i n the jargon as "hysteresis' from a Greek word meaning delay, so its a time-lag just the same. One consequence is that i n a roUing or pitching boat, say, the resistive force may stiU be effective as the boat begins to s-wing back the other way. I n other words the 'correc-tive' force may be i n or out of phase with the huU's natural osciUation.
I must leave i t there. I t is a subject which is interesting but elusive. Should you want to know more, t u m to Marchaj's article i n PBO No.202, or look i n his book Seaworthiness from p.295 onward. Suffice to say that the phase effect seems to ha ve worked out very nicely in tandem-keeled boats so far—perhaps a facet of the 'good luck' which Warwick Collins modestly says has been with him in this work.
There is another damping influence — that of the entrained mass of water which has to be accelerated when the keel moves. Old-fashioned long keels of large area benefit i n that way by comparison with a fin, The Tandem Keel also entrains more water than a fin by reason of the scoop formed by the angle ofthe end-plate, This kmd of damping makes a notable difference to the roUing of the boat at her anchor, as weU as when underway.
Elevating the boat
Words have to work hard. I have been writing of l i f t ' i n the specific sense of a hydrodynamic force generated by afoü. Using the word in that same sense, the delta end-plate can generate quite a lot of Uft, and so lift the whole boat above her normal waterline. That happens particularly when a boat is running at speed, straight and level. A t such atimeshetends tosquataUttie, giving the necessary incidence to the end-plate. A ten knots the end-plate on CoUins's own boat Fighter generates about 8001b upward force, which is a very valuable amount i n a racing boat whose owner has probably been trying to save the odd couple of pounds here and tliere.
NaturaUy, the end-plate creates drag when itUfts, butthereis a large net saving i n tiie drag of the whole assembly, boat-plus-keel, and a positive acceleration is reported by owners.
Indeed, although I have dabbled here i n a good deal of theory, the reaUty is that a growing number of owners and helmsmen are now able to say that this keel design does work: that it results i n a stiffer arid more sea-kindly boat; more controUable; steadier in aU three axes; more resistant to broaching; and better to windward i n rough water than a comparable plain fin.
Although the first appUcations have been mainly in racing boats, aU the qualities claimed for the keel would be attoactive to a cruising owner who wants fin speed with long-keel seaworthi-ness, yet with minimum draft.
Where a wing-keel would not be welcome on account of its vulnerability, and a tendency to coUect weed and flotsam, the ColUns end-plate is massively strong, and the 70 degree sweep-back of its leading edges would not hold weed or anything else. The end-plate provides a good foot for taking the ground, and for laying up, with the important proviso that a skipper should be very careful not to settle the boat at the side of a creek or on a beach which is very steep-to.
With that one caveat, i t seems to me that the Tandem Keel offers great benefits to both racing and cruising boats, imless you want a totaUy retractable keel — or untü somebody comes along
