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Home Made Keel Design Questions

I've read many "keel" design related threads in various forums. Yet I persist in dreaming up a revolutionary new hull design for a homemade plywood sailboat. I can't find anyone with experience in testing my "keel-less" hull concept; so I thought I'd ask specific questions:

1. The flat sides of a plywood scow or barge sailboat could simply extend straight down a foot or three and function as fixed or permanent "leeboards". Sure there's the debate about wetted surface and length and depth and effectiveness and drag... But has anyone done this?

2. "Fins" or horizontal skegs could be mounted on the above mentioned side/keels in such a way as to "lift" the lee/heeled down side of the boat. That is, a fixed horizontal "trim plane" could project along the side of the flat sided boat with little or now portion under water when "level" but all of it under water when heeled over, creating a downward deadrise (? term) on the lee side when heeled over and thus lifting the boat--instead of using ballast on the windward side to press down and creating a righting moment.

I thought of these items by comparing outriggers and "M" shaped hulls and speculating about using hydrodynamics to lift or push up on the lee side. Size and shape and angle of these horizontal stabilizers would need to be determined by trial and error, I believe.

I'm trying to create the least displacement 14 ft sailing dinghy or scow or skiff possible that will still support a maximum amount of sail area without capsizing. I figure I could trade the drag from extra displacement for the drag of this crazy hull.

Note: My "actual" hull shape encapsulates these two elements (twin keel-like vertical sides plus fixed trim plane/fins) into a space-age "body" by enclosed some "space" above the fins with flotation voids and other skins just like Star Wars spaceships all have aerodynamic shells even though they are useless in space.

But if the whole thing just flips upside down and embarrasses me, I might continue to cut out paper models of alternatives until everyone stops mocking me here...

Thanks,
Phil
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Re: Home Made Keel Design Questions

The simple formulation is what's called a bilge keel, basically where the chine I sail there is a runner that stretches for and aft to prevent slippage. They have lots of wetted surface, add lots of drag, but allow for very shallow water operation. They are fine for slow boats that don the mind pointing low.

The more complicated option where you suggest using lifting foils is pretty much what the Quant 23 has done. But the foils are short (bow to stern), deep, and very high aspect to maximize lift and minimize drag.

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post #3 of 26 Old 3 Weeks Ago
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Re: Home Made Keel Design Questions

Many multihulls use the ideas you've presented here. The flat straight outer sides of the sponsons of some trimarans allow them to go better to windward than the more rounded ones.
Piver used fins on the sponsons of his first generation tris, but added multiple hard chines on his later AA series.
But what works on a multihull probably won't on the heavier monohull. A well designed multihull rides primarily on the surface of the water, unlike a mono which must move through it. This allows for more radical designs as there is so much less strength required to skim on the surface.
There are a lot of hard chine monos out there which do use the chines to help prevent leeway. However, adding a fin to a chine would probably act more as a stabilizer than against leeway. Certainly, the more the boat heels, the less efficient those fins will be.
I think you are searching for something similar to foils, which are in their infancy at this point. I would love to see the advancements in that tech in 20 or 30 years, but alas that will never happen.

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Re: Home Made Keel Design Questions

It is hard to "see" what I am really talking about, I'm sure. But bilge keels are totally different in that they are ballasted and fairly short and aren't attached at the chine. And the hydrofoil Quant boats aren't displacement boats at all.

I'm just thinking of vertical sides that extend deep below the water while the flat bottom floats only inches deep. And the lift fins aren't foils, just a fixed "trim tab" that runs down the length of the boat's side, only touching water when heeled over.

I'll finish building my tiny test vessel, run some tests, and report as to whether or not this method works for dinghy construction out of plywood.

As I said, this scheme is just to allow lightweight dinghies or scows to carry much more sail area (and rigging).

A few hundred dollars on a prototype can only be a "learning experience". I can always throw a bunch of sand bags into the boat to hold it down in a gale...
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Re: Home Made Keel Design Questions

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Originally Posted by CaptainFaris View Post
It is hard to "see" what I am really talking about, I'm sure. But bilge keels are totally different in that they are ballasted and fairly short and aren't attached at the chine. And the hydrofoil Quant boats aren't displacement boats at all.

I'm just thinking of vertical sides that extend deep below the water while the flat bottom floats only inches deep. And the lift fins aren't foils, just a fixed "trim tab" that runs down the length of the boat's side, only touching water when heeled over.

I'll finish building my tiny test vessel, run some tests, and report as to whether or not this method works for dinghy construction out of plywood.

As I said, this scheme is just to allow lightweight dinghies or scows to carry much more sail area (and rigging).

A few hundred dollars on a prototype can only be a "learning experience". I can always throw a bunch of sand bags into the boat to hold it down in a gale...
For some weird reason it's still called a bilge keel even if it is unballasted. Also google chine-runner.
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post #6 of 26 Old 3 Weeks Ago
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Re: Home Made Keel Design Questions

Is a drawing of your keel possible to port?

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Re: Home Made Keel Design Questions

Yes, I'm sure cavitation, drag, leeway or adequate lateral resistance, etc. are all things that must be tweaked by making the depth and shape and toe-in and design correct instead of my total guess-work current scheme. But I'm intrigued by the lack of testing of flat sailboats...
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Re: Home Made Keel Design Questions

What direction am I supposed to look at that from? Is that a side view, bottom veiw?
Not trying to be negative but I can't figure out what I'm looking at.
Maybe you should try some of your ideas out as an RC sailboat. Seen some pretty wild sailboats in the "open" classes where length and sail area are the only constraints. Since you wouldn't be racing it you could do anything you want.

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Re: Home Made Keel Design Questions

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Originally Posted by CaptainFaris View Post
Yes, I'm sure cavitation, drag, leeway or adequate lateral resistance, etc. are all things that must be tweaked by making the depth and shape and toe-in and design correct instead of my total guess-work current scheme. But I'm intrigued by the lack of testing of flat sailboats...
As soon as you heel, these Sides" as you refer to them will drastically lose their efficiency, if I understand you correctly. They will also seriously hamper the boat's turning ability, I believe. A centerboard or keel is a point around which a boat can turn, but I don't see that in your design.

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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...
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