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From: Tom MacNaughton
Date: 19 Feb 2008
Time: 11:16:59 -0500
Remote Name: 66.252.35.241
As I’ve said before it is true that resistance to rolling is one of the great advantages of twin keels. Mr. Wright certainly has that … er … correct. It is a great advantage whether in harbor or off the wind at sea. He is also quite correct that their good steering qualities off the wind are a great plus as well. I’m not so sure that the twin keel yacht will be more expensive than a single keel yacht. To clarify what he is saying the additional wetted surface which seems to always result from having twin keels is what makes them a bit slower, which is especially noticeable in light winds where frictional resistance is much more important than wave making resistance. Where we get into some differences is in the use of cambered foils combined with toe in. This is frequently recommended on the grounds that it will increase windward efficiency. However twin keels boats tend to be quite efficient to windward anyway if reasonably well designed, while cambered toed in keels just have to increase drag if the course is anything but dead to windward as they are working against each other. I would suggest that it would take very careful tank testing in a very expensive program to be establish a combination of camber and toe in that would show any all around benefit on any particular boat. If a client isn’t willing to pay for that, I’d say you’d be a lot better off just sticking to non-toed in keels. Personally I probably wouldn’t bother cambering them either. There is one area where I can say that Mr. Wright doesn’t have the whole story. He feels that the keel should join the hull ‘abruptly’ without much fillet. However for this to be true the hull would have to be very straight in section near the keel root. Then the end plate effect of the hull and the lack of induced turbulence will be efficient. However if the bottom of the hull is curved in section where the keel root is attached as is very often the case you will have to be very careful to use a generous and carefully shaped fillet or you will get “humps and hollows” in the water flow which will generate sudden separation of flow and a great deal of additional drag. This is very easily demonstrated by quite simple flow analysis methods. I note that he suggests that the toe in of the leeward keel will be in line with waterflow when heeled “[because] A modern yacht, as it heels, alters trim to bring the bow down and the stern up.” While I would be the last to say you won’t see boats today which trim down by the bow when heeled there is nothing “modern” about this. It is just poor design! As a well designed boat heels the bow and stern will both pick up roughly the same amount of buoyancy to leeward and changes in trim should be quite minor. If they aren’t, it is the hull which needs work not the angle of the keel! He also suggests that the windward keel will work to right the vessel because its thrust would be downward due to the toe-in. I have heard many people say this, generally almost word for word quoting Arthur Robb who I think said this first but without giving a theory to justify the statement, and it sounds good, but so far I have not been able to construct any flow scenario which would appear to support this. Also so far I have not seen any studies which would support this. I think we need more than a verbal statement before we can accept these types of assertions as useful models for the real world. Twin keels are very useful and have many advantages but I don’t think we need new theories to explain their performance. The standard naval architectural theories of resistance and flow plus mass dynamics seem perfectly adequate to me to explain the performance of twin keeled vessels.