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From: Laughingcruiser
Date: 11 Feb 2008
Time: 23:38:24 -0500
Remote Name: 124.183.89.182
More views on twin keels! PROPER TWIN KEELS by Ian Wright Published in the Austrlian “Cruising Skipper” magazine No 15 1986 Beginning an article about the sailing performance of twin keel yachts by praising their behaviour at anchor may seem to have the cart before the horse, but no matter how well any cruising yacht sails, a greater amount of onboard time is spent at anchor than passage making. Consequently, a yacht that has great resistance to rolling at anchor, as well as other desirable qualities, has considerable advantages as a live aboard cruising yacht. I think the strongest argument in favour of using twin keels is the behaviour of the anchored yacht. closely followed by the sailing behaviour, particularly as twin keel yachts are very well-mannered downwind. The ability to take the ground upright is useful, but shouldn’t be the only reason. People who think a cruising yacht never needs to sail to windward deserve all the trouble they get into. The only times a cruising yacht needs to go upwind, it will be crucial as there will be no other choice left. The twin keels I advocate are not as easy for a builder to produce as a single fin keel (or two identical keels for that matter) and this obviously means building a proper twin keel yacht will be more expensive than an equivalent fin keel vessel. This is a definite drawback. As there are two keels to move through the water, it will need extra energy to overcome the additional drag of two keels compared with one. As a proportion of the total energy needed to keep a yacht going, an extra keel does not need much and is only significant in very light winds. No yacht that is designed specifically to win races will have twin fixed keels, though twin retractable daggerboards have been fairly successful. This is not to say a twin keel yacht will not sail in light airs, just that a racing yacht of the same size and weight will be a little quicker. In normal winds (assuming there is such a thing) a twin keel yacht (proper keels) will give nothing away in sailing ability, and will be more comfortable. The actual radius of the keel leading edge and the position of the greatest thickness of the section depend on speed of flow and angle of attack, that which is most efficient at high speed not being as effective as lower speeds. Thus it becomes a matter of opinion whether to maximise which end of performance for which yacht. A yacht keel that has to work on each tack must be able to generate lift in the appropriate direction. Asymmetric lifting sections are more efficient in that more lift is developed in proportion to the drag induced. The twin keels I recommend are each mirror image asymmetric section foils that are attached to the hull at an angle to the vertical centreline of the boat, and with a small toe-in’ angle. Each keel provides the lift to windward needed on its own tack. One way to minimise tip loss is to minimise the tip. The famous Spitfire wing has a plan- form that is very effective in this area. The ‘elliptical keels’ that are sprouting from the bottoms of IOR yachts are similarly good in the tip loss department, but there are practical problems as far as a cruising yacht is concerned with a keel that has no flat or a very small one for slipping. Another way of controlling the flow at the tip is with an end plate, or more relevant to what is happening now, with winglets which are a more sophisticated end plate set-up. An end plate simply makes it impossible for the flow to cross under the keel, at the cost of more drag. If the gain in lift is more beneficial than the drag is un-helpful, the nett effect is to the good. Flat end plates have not been very much used, but in those racing classes that allow them wings are now essential. A torpedo shape bulb on the bottom of a keel will help prevent flow crossing, but obviously not to the same extent as wings. Wings are probably too vulnerable to be a good proposition on a cruising yacht, and this also applies to an end plate. Any means of concentrating the weight of the keel towards the bottom will help the overall stability of the yacht, and this will add to the windward ability. The relevance of all this discussion of keels in general to the specific case of twin keels is that additional wetted surface (compared with a single fin keel) can be made to pay its way in terms of gains in lift to windward. The wing keels under the celebrated 12-Metre class yachts have a trailing edge that is articulated to enable the keel to be made asymmetrical to suit each tack. (Aimraha II’s trim tab was in two parts, and it obviously worked very well). Before I finish talking about end plates, let’s think about the top of the keel, the hull to keel join. If the water- flow can wander from the keel to the hull the desired pressures will not be felt in the right direction. The hull to keel join should be abrupt for the best efficiency, in effect The hull is made to act as an end plate for the top of the keel. Each keel is an asymmetric section foil, a mirror image of each other. The keels develop flare towards the bottom which finishes at a sharp edge with a Vee bottom. The join of hull and keel is a small radius. The keels are at an angle to the vertical centreline at rest, and also have a small toe-in angle as mentioned before. The thick’ side of the keels is the inboard side. In profile, the keels have a parallel leading and trailing edge with a sweep- back angle of 15 degrees except for a small aftward extension of the trailing edge near the bottom that enables the flare edge to fair cleanly. The reasoning behind this specific shape is as follows: The asymmetric shape is a more efficient lift producing shape, the lift produced by the leeward keel when (he yacht is sailing upwind resists leeway without the yacht needing to make a leeway angle. Making the keels flare out towards the bottom does a number of things that are beneficial in the overall compromise. The weight is low in the keels, in fact the centre of gravity of the two keels can be as low as a deeper draft single fin keel. The flare and its sharp edge go some way towards controlling the tip loss of these short span keels, Thirdly, the Vee bottom actually provides a flat of quite a large area to support the yacht when it is out of the water. The toe-in angle means the leeward keel is in line with the waterflow when the yacht heels. (A modern yacht, as it heels, alters trim to bring the bow down and the stern up). The sweepback is a practical answer to encouraging weed (not to say float lines etc.), to clear away by itself. The speeds that sailing craft operate makes the angle of the leading edge and hull quite uncritical (except when there is a wing across the bottom of the keel). In practice, the theory seems to work. As the designer obviously my observations will be subjective. Warlock is 11.0 m LOA and the twin keels draw 1.47 m when the yacht is upright. This is not really very shallow draft, but if the yacht is aground. I can stand on the bottom without drowning. The actual span of the keels is 1 m. There is no doubt that Warloc sails very well to windward, 30 per cent apparent wind angle is easily achieved. In very light winds, the best Vmg is about 35 degrees apparent, while in fesh breezes better than 30’per -vent can be managed. The simple act of sighting the wake over the stern confirms there is very little leeway angle, as does sailing alongside other similar sized yachts. All things being equal, the twin keel yacht will match or better the sailing performance of single keel rivals. Of course, its not often that all things are equal. Every time I have sailed past someone they are just about to scrub the bottom, or get a new No. I, and the same applies to me if we have been bested). The hull speed’ for Warlock’s waterline length is 7.3 knots and this is never exceeded to windward. Offwind and reaching, Warlock can sustain 8-9 knots for as long as the wind blows. Sailing at these speeds the wake is quite flat, with no hint of the big hollow that develops beside a conventional yacht that is traveling fast. There is plenty of turbulence visible astern which persists about three boat lengths behind, but no hint of a big wave at the quarter that the boat sits in. Even though we have given it many opportunities, the windward keel has never broken the surface, the waterflow seems to want to stay attached. There is a small wave visible at the surface about one keel length behind the windward keel. Sailing at 8-9 knots is not exceptional for 11 m racing yachts, the difference being the lack of great visible disturbance of the water that accompanies it. Sailing Warlock dead downwind is completely undramatic. The yacht does not want to develop even a hint of rhythmic rolling, and always responds to the rudder. On one memorable occasion, Warlock surfed at over 12 knots (that’s full scale on the log) and still steered properly. These observations suggest that twin keels and their effect on the water-flow helps to control the motion of the yacht and also helps the hull shape control the wave it generates. The lift of each keel is directed inwards and down wards and this stabilises the motion of the yacht. In head seas pitching damps out very quickly and there is no tendency to hobbyhorse. The comments I make about rolling in a later paragraph apply as much to the yacht underway as at rest. The downward component of the lift of the keels must cause the yacht to be immersed more sailing at speed and consequently increase the drag of the whole system, but as this happens when there is plenty of wind power it seems not to matter much. I have already admitted that compared with fin keel racing yachts, a twin keel yacht will be slightly outclassed in very light airs. I must also admit that a twin keel yacht will behave exactly like a similar displacement fin keel yacht as far as pitching motion is concerned if the boat speed is less than, say, 4-5 knots. The forces acting on asymmetric twin keels sailing upwind are shown in detail in the diagram. The centre of buoyancy is shown for the static and heeled yacht. The lift forces developed by each keel is shown as Lw for the windward keel and LL for the leeward keel. The distance from the centre of buoyancy is shown as Aw and AL respectively. As this lever arm is greater for the lift of the windward keel, its contribution to righting the yacht is greater than the leeward keel’s heeling effect. It is only with twin asymmetric keels that this happens. All other keel configurations the lift to windward contributes to heeling the yacht further. Even when the boatspeed is zero, that is when the yacht is moored or at anchor, twin keels will improve the yacht’s resistance to rolling. As a yacht rolls more or less around its centre of buoyancy and the twin keels are further away from this point than a single keel (and there are two of them) it requires more energy to make a twin keel yacht roll. The yacht will roll less, and settle down quicker, than a single fin keel yacht, or a traditional long keel yacht for that matter. As cruising yachts spend such a lot of time at anchor, possibly not in the first choice anchorage, I emphasise this quality of twin keel yachts above others. A twin keel yacht stands upright on a flat surface without any water. This quality is obviously useful in tidal areas, but can be useful for a long term cruising yacht. Being able to dry out for maintenance is helpful, particularly if it is achieved without any moving parts underwater. Even the use of slipways and lifts is easier if the yacht does not need any shores to stay in place. Road transport is another area where twin keels help. Warlock has been moved by a conventional low loader and off-loaded by forklift in a worry free move on land. Similarly, a twin keel yacht would be a straightforward deck cargo for shipping. A twin keel yacht will have less draft than its fin keel equivalent. As mentioned, Warlock draws 1.47 m with a keel span of 1000 mm. Tosca III of 11.7 m LOA being built in Melbourne will draw 1.2 m with a keel span of 800 mm. Even though very shallow draft is attractive, I don’t think the span dimension should be much less than this (on this size of yacht) if the windward performance is not to be traded off for extreme shallow draft. The new 12.0 m twin keel yacht I am going to build will draw 1.5 m and have a keel span of 1100 mm. The mechanical construction of a twin keel yacht is not really any more difficult than any other, each keel weighs half what the equivalent single fin would weigh. Despite some famous failures, mostly, single fin keel yachts manage to stay together. The same techniques used in a twin keel yacht would be loaded half as much. Warlock’s keels were built as shells of fibreglass, which were bonded into the hull to a system of webs, also of solid fibreglass. These athwartship members between the keels were boxed and from the integral water tanks. This whole structure is very strong, and its own weight and the weight of the tankage is postiotied in the most useful place in the yacht. The loadings from the weight of the keels, sailing forces, and grounding forces are distributed through a large area of bonding in this structure. As the volume in the keel shells available for ballast is nearly double that of a single fin, the density of the ballast material is not a crucial. This method of attaching keels avoids the high loadings that occur at keel bolts, and also any corrosion problems from having dissimilar metals out there in the seawater. The bottoms of the keels are made thick enough so that grinding off about half the glass in repeated groundings will not weaken the keel shell. For the unpremeditated sort of grounding, a twin keel yacht is no more or less at risk than any other sailing vessel. The problems are slightly different. If a twin keel yacht is stopped by touching the bottom while sailing, it will draw less water when the sails are freed off and the yacht comes upright. There is a good chance of being able to motor off back into deeper water. (It would be possible to sail off if the sheets are kept as free as is needed to reduce heeling). I know from personal embarrassment that motoring off with sails flapping works very well, if a twin keel yacht grounds while motoring perfectly upright, and applying full power astern won’t produce any movement then you are stuck until the tide comes in. A point worth mentioning is that the propeller on a twin keel yacht is not hidden behind the keel, or deadwood, so the available thrust astern is very good for this kind of situation. The best advice for the unfortunate skipper of a twin keel yacht grounded on a soft bottom with a rope wound around his exposed propeller would be the same as recommended for single keel yachts in the same predicament, i.e., to deliberately heel the yacht towards the shallow side, then the propeller will be easy to get at. Ian Wright