Some thoughts about Twin Keels

DELFTship forum Hydrostatics and stability Some thoughts about Twin Keels

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    • #35581
      John R. Coil


      I’ve been reading about twin keels — my interest stemming from their reputed benefits for moderating rocking motion given a family full of non-sailors (so many relations, so little railing… :whistle:).

      Recently I actually applied some of the design ratios in Sponberg’s The Design Ratios to some of the hull shapes I’d been toying with. One in particular, Motion Comfort Ratio, grabbed my attention because of how it dealt with a boat’s beam and what was said about why.

      For anyone unfamiliar with MCR, this was devised by Ted Brewer and included in what he considered a tongue-in-cheek article in September 1990 Cruising World. Even so that Mr.Brewer had actually applied himself in coming up with MCR was attested to by Sponberg and it apparently “does provide a reasonable comparison”.

      MCR = Displacement / (0.65 * (0.7LWL + 0.3LOA)*BEAM^1.333)

      … where Displacement is in pounds, “0.65” is a generic waterplane coefficient, and the rest are self explanatory … or are they?

      “BEAM” and not “BWL” makes me wonder if the equation doesn’t actually call for overall beam that just the beam at waterline because of how Brewer describes it in his book, which Sponberg quotes:

      Beam enters into it also as wider beam will generate a faster reaction, particularly in beam seas. In effect, the comfort ratio measures the displacement of the vessel against its waterline area, adds a factor for beam, and thus is a means to compare motion comfort for boats of various sizes and types.

      At one level I suppose it really doesn’t matter which is actually used because this got me thinking about why having a lot of extra width at the deck level (never mind situations like this: Matteo Polli’s Final Year Dissertation) could “generate a faster reaction, particularly in beam seas” and from there I passed on to thinking about the reverse being true for twin keels.

      Thinking of a sail boat as a cross between a metronome and a pendulum the affect of weight up high or down low would alter not only the perceived speed of its rocking motion but also the amplitude of same. Old news to be sure. But a boat carrying a relatively wide deck seems like it should have a bad effect on this motion because that is weight, no matter how well balanced, moved away from the centerline and also above the waterline. I suppose the same could probably be said of having a heavy galley with stone countertops on one side of the boat and needing to add ballast to the other to bring it back into trim. Or, for that matter, a certain twin mast set up used on some catamarans. Which is to say these are all the opposite of any twin keel set up (which would also be weight moved away from the centerline but under the waterline, where their contribution to balancing out our “pendulum” is good rather than bad).

      Something like a real playground teeter-totter vs some sort of “anti-teeter-totter”?

      Anyone know of any papers dealing with this aspect of twin keels and how it may compare to the more normal approach of hanging the same amount of weight from a single point in the boat’s centerline?

      EDIT: after some poking around it seems as if the Motion Comfort Ratio does use the widest point of the hull rather than at the water line. There’s also a variation that assigns somewhat more significance to LOA (1/3rd rather than 30%) and uses a slightly higher generic waterplane coefficient too.

    • #35582
      John R. Coil

      Then there’s something I just read elsewhere where a forumite was relating his problems with a twin keeled yacht he brought. Seems his 11.5m boat has brakes on after about 5kt and he thinks it may be because of “turbulence” between his 2m long keels that are only 0.8m apart.

      In the past I’d read about how having a catamaran’s hulls far enough apart was necessary to avoid increased drag from their combined wave systems … the ratio of the distance between hull centerlines and LWL being recommended at something like 2.2 (i.e. a 22′ LWL cat should have around 10′ between keels). Now, waves systems at water’s surface are one thing but there’s still gonna be pressure built up between twin keels in motion, right?

      Do you think that, not entirely unlike a cat with hulls too close together, there may be a distance that’s too close for twin keels so that the water, past a certain point, just has trouble moving between the keels because of built up pressure?

      If so this may be WHY having a bit of toe in is good, since that would help to relieve pressure farther aft. As for the issue of closeness: 2m / 2.2 = 0.9m … which is more than the 0.8m given … maybe the same desirable ratio applies in this case too?

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