Join Date: Feb 2017
Location: Spokane, WA
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Re: Home Made Keel Design Questions
Regarding heeling and the sides/chines/skegs/twin-keels "losing efficiency": That's a good consideration, and one that I have fully considered. First, all keels lose efficiency by tilting away from vertical when heeled (except twin or bilge keels). In my model these show up as vertical and they would slightly shorten in lateral resistance. The windward side/chine would come out of the water but the leeward would go deeper. Also, the "real" version will have 20 degree outward tilt and therefore they'd be getting both deeper and more vertical when heeled. Second, while they are "short" and "shoal draft" style keels or twin keels, they have extra lateral resistance due to "area". Interestingly, calculations of keel efficiency show that the leading edges are more effective than trailing, so a long keel would be inefficient by some unknown (unless you're a hydrodynamics engineer) percent. Yet sheer quantity can compensate. Third, that "quantity" compensation actually adds to wetted surface, true, yet calculations of drag from wetted surface are "inconsistent" between engineers. So I opt to just try it out in the real world. Fourth, "fast tacking" is impinged upon by all long keels and these are probably extra "good" for tracking (and therefore bad for tacking). But enlarged rudders have been documented as ways to force tacking in such "good tracking" hulls configurations. Again, trials in the real world will provide answers. I actually expect to trim and shape the length and depth of these "sides" until they permit fast tacking.
My goal for this small boat, besides being an experiment in hull design, is to "support" large sails that need plenty of "work" to handle, thus creating a "training vessel" to teach crews how to coordinate their trimming tasks with a skipper who is calculating course and wind direction. I have a narrow stretch of river behind a dam near my house and only a very small boat can be used--so I'm packing as much training into a single dinghy/skiff sized shoal-draft, non-ballasted sailboat as possible. Uh, I'm afraid to admit that this will also be tested in a schooner variant, with two masts and both jib and staysail up front on a bowsprit. Yeah, it's a crazy experiment--but it might actually work. At least it will be educational.
And, I'm lying about the ballast (sort of). I'll have concrete ballast distributed at the 4 corners and the bow to create "inertial resistance to roll", but only 125 lbs. total in movable lugs. Obviously, too many sleepless nights have added too many variables to this hull to seriously try to experiment with--but it's a cheap hobby in the size I'm talking about...