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From: Tom MacNaughton
Date: 30 Jan 2008
Time: 14:21:00 -0500
Remote Name: 66.252.38.154
We do design twin keel boats when customers either ask for them or when they look like a good solution for the particular use the client wants to put the boat to. In fact your author’s very first design when he was about 18 years old was for a small 20’ twin keel yacht. Right now my wife and I own two yachts. One is a deep draft heavy displacement vessel best adapted to offshore voyaging and long distance sailing and living aboard. The other is a 22’ twin keel vessel that we use exclusively for coastal cruising. We do think that when people suggest that we could do a twin keel series with many of the characteristics otherwise of the “Coin Collection” they are onto a really good idea. Ideally we would use a semi-flush deck which had the ports in the hull sides but used a very low “blind” trunk to keep the topsides proportionally a little lower than they otherwise could be with the somewhat higher cabin sole of the twin keel type. Then we would use a wheelhouse designed for very comfortable watch standing and arranged so that the entire handling of the vessel could be done from the wheel house. We would design these so that people could use either a Marconi sloop rig or the Chinese rig, though of course we tend to favor the Chinese rig. While we all went through a long period of rapidly expanding population after World War II, when many people were generally pretty new to sailing, we are now getting back to the point when the average client is pretty experienced. These folks tend to be much more aware that there is no one perfect boat and are much more open to ideas that will be specifically helpful for the particular type of sailing they are doing. Recently for the first time we got a commission for a twin keel semi-custom boat to be built in series by a small company. We have a feeling interest in twin keel vessels will gradually increase. Also we notice that clients building our “Silver Gull” series, two of which have the option of single or twin keels, are overwhelmingly building the twin keel version. One has been built fairly near here and I hope we’ll be able to sail with them next summer. We would be delighted to design twin keel yachts for anyone who wants one. There are only two caveats. The first is that if you came to us with a design commission deposit tomorrow we’d still probably not be able to get to starting work on it until early 2009 because of the amount of work we already have lined up. If that’s OK with anybody we’d love to do it. The second caveat is that as with any unusual feature people tend to get over enthusiastic and start creating all sorts of theories to “prove” that the particular feature is the best thing for sailors since the invention of the compass. As we see it the advantages of the twin keel vessels are roughly as follows: 1. You can ground upright. This expands your cruising options as you can anchor in shallow harbors and let the tide go out and leave you upright. Also it can greatly reduce the cost of ownership as you would normally not need to haul out if living aboard and traveling as you can run her up on a beach and let the tide go out whenever you want to work on the bottom. 2. If you touch bottom while sailing, letting the sheets run will bring the boat upright and reduce the draft so you can get off. 3. Because of the separation of the two parts of the ballast between two keels such that they are separated by nearly the beam of the boat you increase the roll moment of inertia considerably over that of a single keel vessel of the same draft. This greatly reduces the rolling, damps what rolling you do get faster, and this makes for a much more comfortable boat in a seaway. It also increases the dynamic stability in a seaway. 4. In strong winds with a good heel on so the leeward keel is nearly vertical most twin keel boats will go to windward in a quite impressive manner even when moderately shoal draft. 5. As one would expect, given a large area of longitudinally oriented fins underneath, it seems safe to say that most twin keel vessels will tend to be fairly steady on the helm. Certainly that is our experience. The disadvantages are: 1. No matter what you do it just isn’t going to be possible to get the wetted surface down to what you can have with a single keel vessel of similar proportions and stability. 2. There seems to be a narrower range of displacement to length ratios at which the twin keel vessel is efficient. So far it seems to me that while there are great advantages to higher displacement to length ratios for cruising vessels up to around 30 some odd feet, you will find that a ratio of about 300 seems to be the most that makes much sense with twin keels. In smaller sizes this can put some restrictions on the amount of gear and supplies you can carry for a live aboard or voyaging yacht. 3. In really light winds when a twin keel yacht is slower due to her somewhat higher wetted surface, she’ll tend to make quite a bit of leeway due to being upright with both keels at their least efficient. Sometimes on a small boat you can make a distinct improvement by shifting as much crew weight as possible to leeward to put some heel on her. Of course most people don’t have the tolerance for light winds that we do and either anchor or turn on an engine. Now Mr. Laughingcruiser refers us to an article by Patrick Bray on twin keels. I did take a look at this. Mr. Bray seems to be quite an enthusiast and has listed a lot of intriguing theories that people have put forth over the years. Unfortunately I’m the sort who has to work everything out and test everything and I find that seems to often put a damper on my enthusiasm for a lot of theories. As an example of this from twin keel history, Commander Raynar, an amateur designer who designed our little 22’ twin keeler in the 1960s but later turned design work for his boat building company over to J. Laurent Giles, wrote a number of ads in which he very enthusiastically claimed such things as the idea that twin keel boats had more stability than single keel boats of equal draft and equivalent proportions. Unfortunately even the most cursory study of stability theory even as a young design student told me that he couldn’t be right about this. As I remember he kept saying that since you’d moved half the ballast further to windward the boat would be stiffer. But of course he’d also moved the other half further to leeward. Somehow this didn’t count for him. It’s been a lot of years. I may remember his theories poorly but I believe that’s the general idea. I’m afraid I disagree with Mr. Bray’s interpretation of some of the history of twin keels. He implies that twin keel boats had large keels “to support them when sitting on the mud flats”. Actually there were a number of designs from this era including an intriguing series called the “Yarrows” by an amateur designer which used very small high aspect ratio keels. I have some nice pictures of one of these vessels sitting on the mud. However these had some stability limitations due to the minimal ballast that could be gotten in the keels. I believe the comparatively large keels of many of the more popular twin keel designs are simply the result of using iron ballast castings to save money. To get enough weight in an efficient shape these just had to be quite long. In Britain where the average wind strength is pretty high and people are also more interested in sailing comfortably than fast there was no great incentive to try to keep wetted surface down by spending more money in either single keel or twin keel vessels. In the United States my feeling is that we have been unusually blessed with a lot of comparatively deep water harbors and have had much less need for utilizing very shoal or tidal anchorages. This has reduced interest in twin keels except among people with special needs or a desire to minimize costs of maintenance by avoiding yard haul outs. Mr. Bray mentions the last “Bluebird of Thorne” and suggests that her tank test results were published. If they have in fact been published I have not seen them. I had understood that they were considered proprietary and were kept secret. Mr. Bray also has presented a number of the common theories that people have put forth postulating advantages that they think twin keel vessels have. I will summarize some of these briefly and then comment. 1. Twin keel boats are theorized to have a higher sailing speed in light winds because each keel can be smaller since the leeward keel is more upright when the boat is heeled and therefore more efficient. In 40 years of professional designing and a couple of years before that of amateur designing I’ve done a lot of studies for twin keel vessels along with some complete designs. At one point I hoped to have a local racing class of 20’ twin keel boats proportioned to do well offshore and intended to do day racing as well. It was obvious in all these designs that the difficulty lay with trying to get the wetted surface down. When you put two keels side by side and expect them to carry the same amount of ballast as a single keel and keep the center of gravity of the yacht as low as with the single keel it is darn hard to keep the wetted surface down. Even if you have something like an average single keel type with a fairly long thin keel such as a NACA 10% thick foil, decide to do a twin keel version, and decide to reduce wetted surface absolutely as far as possible without increasing other forms of drag, doing this through going to an NACA 15% thick foil to get more volume for ballast with the least surface, you are still going to end up with more wetted surface. But of course that isn’t fair because if you are going to optimize the foils for minimum surface as compared to ballast volume for the twin keel vessel you really should compare it with an optimized single keel version. If you compare two optimized vessels of similar proportions one with a single and the other with twin keels there just isn’t any way to get the wetted surface of the twin keel vessel down to that of the single keel vessel. 2. Since there are two keels it is theorized that they can each be shorter in length, therefore of significantly higher aspect ratio, and this will make them more efficient. If you are going to keep the stability the same you have to get the center of gravity just as low as in the single keel. Unfortunately you can’t really halve the length of the keels when you make two of them as you will lose so much in volume, through having to reduce the thickness some as well, that you will lose stability through reducing ballast, otherwise you would make them too fat to keep the drag reasonable. Therefore to get reasonable drag and the ballast as low as you need to you end up making them longer. This means that the keels will not gain as much as you’d think in aspect ratio, all things being equal. All in all most twin keels are pretty low aspect ratio. However this can be somewhat of an advantage at times in that low aspect ratio keels can be harder to stall due to the presence of vortex lift. Never mind what that is right now. The point that I’m making is that it is very difficult to get twin keels that are truly high aspect ratio in relation to a single keel vessel of the same stability though they may be somewhat higher aspect ratio individually than the single keel. So, while it is not wrong to say that the keels may be a little higher aspect ratio than the single keel, the point needs to be qualified. 3. Twin keels are theorized to have less wetted area that a full keel vessel or a longer fin keel This is another way of looking at the above theories. However, normally full keels are associated with heavy displacement vessels. Because of their relatively short girth measurement they naturally have less wetted surface than lighter more cut away types. Twin keel vessels on the other hand have relatively high girth measurements. Generally their girth measurements are much higher than a single fin keel boat and therefore even higher than a heavy displacement long keel type. You’d have to get the keels darn short to get the wetted surface down to that of a really well designed long keel boat and that’s going to be hard to do. If you could manage it, this does NOT mean that you will get a faster boat as two short keels have HIGHER frictional resistance than one long one of equal area. All this makes it pretty certain that unless you increase sail area to compensate for the increased frictional resistance the twin keel vessel is going to be noticeably slower in light winds than a similar single keel vessel. 4. Twin keel boats tend to be relatively steady on the helm. This is a fair statement. As we said above twin keel vessels tend to be steady on the helm due to a relatively high yaw resistance from the relatively high area of the keels. This should not be an excuse for lack of attention to having good skeg hung rudders of proper depth and other characteristics that tend to ease steering in tough conditions. This does seem to be quite noticeable in practice. We have noted that twin keel yachts with spade rudders seem to be the only spade rudder boats which don’t seem to have control problems off the wind. Not that we wouldn’t prefer skeg hung rudders you understand! 5. Twin keels become more effective to windward with increased heel angle, while a single keel becomes less effective. This is generally true. However this is essentially because the leeward keel is becoming more and more vertical with increased heel while a single keel would be operating at more and more of an angle. Since each individual keel is a little larger than half the single keel, the increased effectiveness of one keel makes up to one degree or another for the loss of effectiveness of the other. However in light winds with the boat nearly upright you will probably have a less effective grip on the water and higher drag. So there are trade offs here. 6. When the boat is sailing well heeled to windward it is theorized that the windward keel will be working more horizontally and will create downward lift that increases righting moment and allows more sail to be carried. We have heard this theory many times but we cannot construct a flow scenario in which it would be true. Nor can we find any evidence that anyone else has worked out any test that proves it is true. We should regard this idea as not useful until someone can actually provide some explanation of how this would work. It sounds good until you start to look at the flow lines but then it just doesn’t seem to hold up. 7. Keel tip vortex is theorized to be reduced when the leeward keel has not yet reached the vertical because the water has to flow up toward the keel root and therefore doesn’t There may be some effect on the tip vortex here. However flow over the tip is more a matter of difference in pressure from one side to the other than anything else. If there is a difference in pressure you should get a tip vortex. If there was not, you would not get lift to windward. Since there demonstrably is lift to windward there pretty much has to be cross flow over the tip and a tip vortex. All in all I think this theory should not be accepted as useful unless we can actually demonstrate and quantify a reduction in tip vortex. However tip vortices can actually produce lift under some circumstances, so a reduction in tip vortices is not necessarily a good thing in all cases. 8. The twin keels are theorized to suppress the wave making of the vessel by creating a counter wave amidships, thus increasing speed. The idea that wave making resistance can be “canceled out” by some sort of “counter” wave making is a common one. Unfortunately the idea that the energy required to produce one wave added to the energy required to produce another wave will somehow act to reduce resistance is hard to substantiate. After all resistance is the amount of energy required to get the vessel to a certain speed. Also keels, to be effective, are by definition deeply enough immersed so that they are not generally contributing to wave making. You can tell this is true because hulls have the spindle shaped water flow lines that wave making bodies need for minimum resistance and keels have the foil shaped water flow lines needed below the wave making region. Some have suggested that “bustle” sterns and bulbous bows work by reducing wave making resistance. To a certain extent this can be true for bustle sterns if their effect is to make the water flow lines effectively longer near the surface. However to be effective a bulbous bow must be well immersed where it is simply a more efficient shape for the leading edge of a fully immersed form than the sharper edge of a conventional bow would be if it was carried too deep. Thus it is normally reducing pressure resistance rather than wave making resistance. While it can reduce this a great deal, pressure resistance is a relatively modest proportion of total resistance at the speeds of displacement surface craft so the total savings in resistance are modest. Neither of these examples have any bearing on twin keels. I’m afraid again we should regard this idea of two resistances somehow canceling each other out as not useful. We have never seen any evidence in favor of this effect. 9. Twin keels are theorized to reduce pitching. This is another idea that has been put forth many times. However twin keels are pretty close to the middle of the boat and are not being moved significantly against the water when the vessel is trying to pitch. A horizontal foil well forward or well aft might well reduce pitching but it is very difficult to construct any hydrodynamic scenario in which keels in their normal position would have any measurable effect on pitching whether there are one or two. It does not appear that pitch dampening is a useful concept to associate with twin keels. 10. Twin keels reduce rolling. This is quite true. Twin keels reduce rolling, which increases comfort and somewhat reduces resistance, in two ways. First, the larger total area of twin keels entrains a higher mass of water when the vessel rolls, forcing this mass to some extent to move with the vessel, thus effectively increasing the roll moment of inertia. Second, by dividing the ballast in two and moving the halves away from each other horizontally you increase the total roll moment of inertia due to the distribution of mass further from the center of rotation. This is extremely effective in reducing rolling and it is extremely noticeable at times, such as when a boat is hit by a power boat wake. 11. Twin keels are theorized to prevent stern squatting at speed. Unfortunately this statement does not explain where such lift would come from. Inclined surfaces can certainly produce lift but the greater the angle from the horizontal the less lift they provide. Even the relatively flat angles of “deep vee” hulls generally cut lift enough so that horizontal “lifting strakes” have to be fitting to gain more lift. Twin keels will normally be at least 70 degrees from the horizontal at which angle force diagrams or other forms of calculation give vanishingly small theoretical amounts of lift. Amounts so small that it is hard to believe that they would not be lost in other flow effects and essentially immeasurable in the real world. Therefore, again, I’m afraid I think since there is not even a theoretical base for significant lift in this situation we must regard this as not a useful idea. Further squatting is not normally a problem with sailing craft, which could only achieve high enough speeds to squat off the wind when the rig tends to keep the stern up and if anything depress the bow. 12. Speed under power is theorized to be better because the propeller is not in an aperture. Of course this really has nothing to do with twin keels. In a fin keel boat the prop is often in clean water well behind the keel. In a heavy displacement long keel type there is every reason to offset the propeller to one side of the keel and no reason not to do so. 13. If twin rudders are used as well as twin keels, it is theorized that the rudder areas can be smaller than a single rudder, thus saving wetted surface. Theoretically rudder areas could be reduced for normal sailing, if twin rudders with 20 degree outboard rake are used, as they don’t carry any ballast. However to maintain good control characteristics in extreme conditions, especially running downwind in breaking seas, we find you need a certain minimum depth of rudder when sailing upright. We find we cannot reliably get good control with an upright rudder draft less than 1/7th the waterline length. Accepting that, we find that it is difficult to reduce the size of twin rudders enough to really reduce wetted surface. So far we have always ended up concluding that for the particular vessel we are designing a single rudder and skeg will make more sense and will minimize wetted surface. Most other designers seem to have concluded the same. Outside of “Bluebird of Thorne” and the “Yarrows”, I have never seen a twin keel vessel with twin rudders. I’m always happy to explore them, but so far have never found twin rudders to have enough benefit to use them on a specific concept. 14. Often people point out that theoretically both the keels and rudders can be cambered foils like an airplane wing on the theory that only the leeward foils will be effective at any given time and this will allow reduced area. While assymetrical foils can be used, it would seem that to reduce the area you would have to assume that these foils would operate at a higher angle of attack (greater leeway) all the time and reduce the amount of ballast. This can be discussed at length. But generally cambered foils have benefits under very limited conditions, and can have disadvantages if not done very carefully. So far we have concluded that without extensive, and expensive, tank testing if you are going to use cambered foils you should be very conservative about it. To date I have not actually used cambered keels, as I have not had a client willing to pay for the testing and cannot demonstrate using theory and calculations that there is a net advantage for the all around performance of the vessel. Given a large budget for tank testing I would think most designs would show that very carefully designed cambered foils would show some modest benefit. Using cambered foils without testing would force you to be conservative enough to avoid risks of increased drag so that there probably won’t be significant real world benefit, although if a client really desires them they could be done. 15. Windward ability is theorized to be as good as single keeled yachts. In a whole sail breeze, which is when you have about a 20 degree angle of heel, the twin keel yacht can be superior to windward to an equal upright draft single keel yacht. However in light winds and especially very light winds, unless you artificially heel the boat, she may well be pretty bad to windward as compared to the single keel yacht due to higher wetted surface keels operating at less efficient angles. One final comment I’d like to make is on “toe in angle”. Some have felt that in addition to the foils being cambered they should “toe in”. That is not be parallel to the centerline. By setting the leeward keel’s angle of attack at a little greater angle to water flow than the yacht’s centerline you can theoretically increase the lift. That is IF you can ignore the countervailing toe in of the windward keel. The problem is that this toe in is really only a theoretical advantage in a whole sail breeze. That is the strongest wind in which you would have full sail up, when the angle of heel is about 20 degrees. At that point you can get a good boost to windward from the leeward keel being straight down and at an increased angle of attack. If you have done very careful tank testing, you can camber the keels and toe them in slightly and gain a couple of percent more performance perhaps dead to windward in a whole sail breeze. However unfortunately a great deal of the time you will not be going dead to windward, especially if you are a live aboard and voyaging yachtsman. As soon as the yacht has less than the optimum heel angle assumed the cambered and toed in keels start to work against each other. Remember that the toe in creates extra drag. Given careful testing you may be able to be pretty sure that gains in lift at the ideal angle of heel and to windward will make up for increased drag. However the rest of the time at lesser heel angles on other courses the additional drag does not accompany any positive effect and merely slows the vessel. Therefore I tend to suggest that unless a client is willing to pay for a complete testing program to find and validate a possible optimum toe in and camber it is better to allow the designer to simply design the very best uncambered non-toed in keels that they can and forget trying to get small gains beyond those you can get quite easily by simply having an arrangement that gives more draft and a more upright keel as the boat heels. That will gain you quite a bit in windward ability over the single keel at the loss of some speed in low wind strengths unless you increase the sail area. Thus the best approach is to do the very best you can on optimized non-exotic twin keels and then make sure the vessel has plenty of sail area in a very easily handled rig so she will perform well in light airs.