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post #10 of Old 04-22-2019
Jeff_H
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Re: Me & My Collection of Project Boats

Quote:
Originally Posted by theluckyone17 View Post
As to the Tylercraft... I spent all winter researching twin keels and reading all I could. Opinions are all over the place... everything from "There ain't no good twin keel" (with apologies to Hal Bynum and Dave Kirby) to Hunter claiming their twin keel designs being almost as efficient as their fin keels *shrug*. Only things folks seem to be able to agree upon: twin keels don't make any given design faster; a lot comes down to the sailor, not the boat. The secondary features to the twin keel sound nice, but whether they make up for any loss in performance... well, suppose I'll find out when I sail her. It's also been mentioned that I might do some good by fairing the keels to a proper NACA airfoil; might also be that would simply be putting lipstick on a pig.

Best case scenario, I'm happy with it. Midline: I sell it off and recoup (some) of the refit costs. Worst: I market it as a playground for some redneck kids on forty acres (being unable to sell it otherwise) after stripping her, sell off the nice galvanized tandem axle trailer that came with it, and I eat the repair costs as an educational expense.

I've used regular Rustoleum on my skin on frame kayaks... haven't been exceedingly happy with it, but it's outperformed everything else I've tried. Can't see putting it on fiberglass, though, when Rustoleum makes their TopCoat product and it's not exceedingly expensive.

Gimme a few years, and I'll report back!
I would like to comment on Bilge keels (Twin Keels). I know that there are people who advocate for bilge keels, and have made extraordinary claims about their performance, but these claims are often bumping up against the basic principles of physics. I apologize that this is draft of an article that I had written for another purpose but it talks about some of the issues surrounding bilge keels.

"Bilge keels (or twin keels for our English friends) are a pair of keels (usually fins these days) that emerge on either side of the boat and angle out. They offer some advantages. If you let the boat dry out (stand on the ground when the tide goes out) the boat can stand on the two keels and wait the next tide. They tend to have shallower draft than a fin keel or even a long keel on the centerline.

But any advantage they offer comes with some serious liabilities. From a sailing point of view, bilge keels have significantly greater drag in most sailing conditions. This results from a number of factors. To begin with, while either of the two bilge keels will have less wetted surface than a center line fin, combined they have significantly greater wetted surface area and much larger frontal area than a fin keel with similar lift. They also produce a lot more induced drag since each keel will generate its own very large tip vortex, and being shallow, they tend to be low aspect ratio with a long bottom on each keel and so tends to produce larger tip vortexes as well.

Then there is the asymmetry issue. This one is a little difficult to explain. As a boat moves through the water, it does not move parallel to the center line of the boat. In order for the keel to work, the boat makes leeway and so is passing through the water at an angle. That angle creates drag. A number of strategies are used on a bilge keel to minimize drag and that typically includes some mix of using an asymmetrical foil shape, angling the front of the foil towards the centerline slightly, and canting the bottom of the keel outward so that it is more perpendicular side forces against the water. While this helps the leeward keel work more effectively, it places the windward keel in an orientation that it is working to create more drag There are dubious theories about increased efficiency since one keel is vertical like a good leeway resisting foil and one is canted like a good stability inducing foil. With computer modeling there has been greater success in approaching that theory on larger bilge keel boats.

While a lot has happened to optimize both fin and bilge keels, the increased drag of having the extra foil in the water is hard to overcome. While bilge boards (operable) are being used on modern race boats these are raised and lowered on each tack. A good example of this are the last generation of Volvo Ocean Race and Open Class boats which use bilge dagger-boards in concert with an operable keel that can swing a big weight to windward. The tremdous stability gains of an articulated keel allows enough sail area to overcome the greater drag of the extra bilge foil.

One of the biggest disadvantages of a bilge keel is that when you run a bilge keeler aground accidentally, you are seriously aground and there is no easy way out. On a center line keel you can heel the boat to reduce draft. That is not an option with a bilge keel boat because one keel digs in harder while the other is being lifted only slightly. Similarly with a fin keel you can often ‘fish tail’ yourself free. With two keel bottoms planted you better hope you've grounded on a rising tide.


In the side by side sailing that I have done in identical models except that one had bilge keels and the other a fin keel, the fin keel had greater speed, pointed higher, tended to roll less, and made less leeway. I used to teach sailing at a small charter fleet down in South Florida that used a small fleet of Westerly's. They had a mixture of both fin and bilge keel versions of the same model. It was very apparent that the bilge keel model was much slower, more tender and did not go to weather as well.

Structural design and engineering of bilge keel boats is tricky as well. The transverse loads need to resisted at two separate locations that are close together resulting in high sheer and bending moments between the keels

More controversial are issues related to interaction between the two foils. As a boat moves through the water there is a zone of turbulent and disturbed water that extends outward of the foils. I have seen it posed that on small boats with bilge boards there is an interference between the fins that further reduces lift and further adds to drag.

Lastly there is the issue of stability vs weight. In and of itself, weight does absolutely nothing good for a boat. It does not improve motion comfort, stability or strength. Secondary stability comes from the shift in position of buoyancy relative to the center of gravity. The lower the center of gravity the greater the relative shift. In calculating the righting moment, each weight in the boat is multiplied by its weight times its distance to a the center of buoyancy. Since a deep keel allows the ballast weight to be lower than a shallower keel less weight can be used to achieve the same righting moment. In the case of bilge keels these are generally shallower draft and so the ballast weight is carried higher than on a deeper fin, and while the longer keel bottom may permit the ballast to spread out longitudinally, this does not offset the advantage that a deeper fin tends to have. "


Respectfully,
Jeff


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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay

Last edited by Jeff_H; 04-23-2019 at 08:49 AM.
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