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From: Tom MacNaughton
Date: 04 Feb 2008
Time: 14:50:05 -0500
Remote Name: 66.252.35.175
Presuming there really is a French designer named Erik LeRouge, sort of a Gallic Viking (?), this is a good little piece on twin keels. I note a couple of places where the translation makes the English a little misleading and a couple of things that some readers might want a little clarification on but in general this pretty much agrees with my experience and studies. In reading Mr. LeRouge’s piece on twin keels, remember that you have to read it from the point of view that he is mainly comparing them with some of the very shoal French centerboard designs. These are much shoaler than most American or British centerboarders and some statements must be read in the context of this comparison. He is certainly correct that lifting keels are complicated and expensive, etc. I would add that in my experience you can spend an enormous amount of time designing them to be strong enough and as foolproof as possible and then the builder will just change things and end up with a dangerously weak mechanism. All in all lifting keels are to be avoided when you can. Note that the original French word “biquille” or “twin keel” is used through out but in some cases has been mistranslated as “bilge keels”. In English bilge keels refer to unballasted foils on either side of a central ballast keel, which is not what is actually being discussed here. Mr. LeRouge is clearly talking about true twin keels. I do have one caution that I mention frequently when people speak of “high aspect ratio” keels as more efficient. Remember that we are trying to minimize wetted surface and therefore there is a desire to reduce the length of the keels as much has getting sufficient ballast on them allows. This raises the aspect ratio, but remember a high aspect ratio foil is only more efficient in terms of a given area. In other words more efficient means more lift per square foot not more total lift. Therefore it is important to remember that a larger area keel even if technically of lower aspect ratio may be more effective to windward than a fin which has less area because it has been shortened and therefore has a higher aspect ratio. Also we know more about the possible effects of tip vortices than we did a few years ago and we now know that not all effects are necessarily negative. So whether a keel is high aspect ratio or low aspect ratio is of less importance than that the total design is very carefully worked out. Chasing one aspect of performance at the expense of other aspects is a game that can get you in trouble fast. I’m certainly not saying that Mr. LeRouge has done this. So far as I can see from the illustration with his article he is doing a great job with his design work. The results of his conversion of a light single keel vessel to a shoaler twin keel vessel seem to bear out my findings very closely in that he finds that after going to great lengths to reduce the wetted surface he still ends up with a little more than the single keel version. He also notes, as I do, that windward performance in a good sailing breeze may be similar to a single keel boat of deeper draft, though I put it differently. I would say that given the same draft, hull, and height of center of gravity the twin keel boat will be superior to the single keel boat to windward in a breeze. We also both note less speed off the wind in light weather due to the slightly higher wetted surface and note that this is easily cured by being generous with the sail area. When he states that the twin keel vessel “benefits form its greater stability upwind in strong winds” but note that he is comparing with a very shoal centerboarder, not with a single keel boat of equal draft. We note that Mr. LeRouge has stated that he proposes twin keels as “standard” for his cruising and voyaging designs. Although many twin keel vessels, even quite small ones, have voyaged and proved very successful at it I believe he is the first designer to propose to make it his standard option. We own a 22’ twin keel vessel which many years ago crossed the Atlantic and we know of circumnavigations in twin keel vessels as small as 26’. However there is one restriction. So far we have found that it appears difficult to design efficient twin keel vessels with a displacement to length ratio over about 300. While this is a good ratio for all around sailing ability, in vessels around 30’ or less we usually find that in order to have generous carrying capacity we need to go to a heavier vessel. Therefore the twin keel vessel will be more preferable as the vessel gets larger in size and given fairly minimalist equipment and “gear” storage requirements in vessels under about a 25’ waterline. I definitely agree that getting the ballast just as low as you can is important. I am somewhat less inclined to use distinct bulbs and winglets. This is because my resistance studies show that actual wetted surface is less important than several other factors in figuring the total non-wave making resistance. This allows me to use slightly longer keels and avoid some of the flow problems you can get with bulbs and winglets if your client will not pay for a complete tank testing program and computational fluid dynamics studies. Also for cruising and voyaging yachts avoidance of bulbs and winglets can prevent fouling lines on your keels which can be a real pain. Reading and commenting on people’s differing views I think helps all of us. Thinking about these questions in terms of other people’s thoughts and assertions helps me a great deal in anticipating what I should add to our lessons which touch on twin keels and thus helps improve our Yacht Design School curriculum.