Design for DSS-Foil Assist for Keelboats

- that's what my boat's designer tells me I should do, but it hasn't got enough sail for that at present; I need to find a bigger lake with fewer trees and bridges to make it worth while. As a kayaker I am used to progress on the level . . .

The fixed foil plan looks good and should yield a much faster boat than mine. It's easier to add a moveable DSS with a flattie but the fixed foil eliminates all the worry about re-deploying it during a change of tack, which can be exciting enough without additional worries . . .

Would it be wise to make the foils removable for trailering and Winter storage? It would then be possible to compare performance with and without DSS.

 

Dss

Welbourn used fixed foils on at least one rc model-they worked well enough to keep him interested. They were angled up so the lee foil was level at "X" angle of heel.....

Here are some pictures of a model with fixed foils-scroll down a bit:

 

Yes, you're right, Doug, slight lift to leeward when heeled ... but then that was what i was warned about with my old (since changed) out sloping (and far more acutely angled) foils on Groucho ... and no one could feel anything wrong beating to windward. My thoughts are that the angle out grips the water, stopping leeway and therefore compensates or cuts leeward movement. So you could take that theoretical loss on the less sloping quasi DSS and say BS.
But scrolling down 2 pages on your pdf; there's a shot of pretty much the same proposed skimmer deal on one of the Wellbourne dinghies. However why not slope them up slightly like the team have later done? That is definitely a better answer, no theoretically leeward lift and windward foil flying more clear of the surface. Interesting.

 

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This is cool! We're getting stuff done around here. You've got me thinking about fixed boards or maybe removable boards that could be inserted into a trunk before going out. Like you said-interesting....

 

Folding DSS?

Possible concept:

click on image-

 

With down-sloping foils the boat must heel to raise the windward foil above the waterline. When this happens the downward angle of the lee foil will increase. If the angle reaches 30 degrees as an example, half of the lift force will be vectored horizontally, toward the lee direction. This will increase the leeway, requiring more windward lift from the daggerboard resulting in increased drag.

If the foils are angled up, the heeling angle to immerse the leeward foil is greater. As the daggerboard moves windward from the centerline it generates vertical lift on the wrong side counteracting the RM of the leeward DSS foil, again increasing drag.

The optimum condition is with the DSS is horizontal and zero heel. In this condition there is no RM or windward lift cancellation between the DSS and daggerboard.

 

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You're right but: every DSS boat designed so far(as far as I know) is designed to sail with a small angle of heel with the foil deployed. On a hull like mine it appears to be impossible to get the foil low enough below the surface w/o at least 8 degrees angle of heel. Otherwise, the chine is too low for a good planing hull. The chine is 3" below the surface when the boat is level. I tried 5" and that was way too low for a 12' planing hull.
On DSS boats the slight component of lift working against RM is easily made up for by the foil. In my opinion, that is far less of a sailing problem than if the foil contributes to leeway. Like everything else in sailboat design there are some compromises in using DSS....
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PS: also most DSS designs, including mine, use a curved foil to allow the foil to be relatively parallel to the surface of the water at the designed angle of heel.

Illustration of "generic" DSS keelboat:

 

The hinged foil provides a way to try out a DSS as a modification with less impact on the boat; just a few screw holes, but the foils can still fold out of the way for transport.

Would I be correct to assume that the optimum for-and-aft location for DSS foils is someplace close to but forward of the CoG?

 

Dss

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Thats right, AK. What I've done(after talking to the inventor) is put the foil just forward of the static CB. By "static CB" I mean the center of buoyancy
when the transom/bottom intersection and the bow/bottom intersection are just touching ( or equidistant from the water with the boat fully loaded). Welbourns foils are, generally, right behind the keel but that may not work in every application. Keep the CL around .4-.6(easy if it is adjustable-if not use .6).
Use a thin foil-10% max t/c ratio.
As a suggestion: if you use a folding version the strut may be a pain since it will catch anything on the surface as well as, possibly, acting as a conduit for foil ventilation. Also, consider mounting the hinge to a piece of wood that can allow the angle of incidence to be adjusted. Instead of a hinge, you could also consider a small sealed trunk on each side and devise a way to just plug the foil in.

 

- one way to eiminate the strut for a folding foil would be to use the foil on the windward side with an inverted profile, then it can use a simple cord.

 

Dss

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AK,good thinking except: with the foil pulling down on the windward side
it will add RM but it will also add weight to the boat since the force generated is added to boat weight. Probably not a good idea.....
That's one of the key-and very important-elements of DSS: it provides vertical lift that reduces hull displacement and wetted surface.
A 1.5 sq.ft. foil at 10 knots will generate 257lb of vertical lift up or down. If down that is a lot of extra weight....

 

Actually I was thinking of trying it to windward as a test of foil effectiveness. If the RM is significant at the speeds I can achieve on my boat then it will be worthwhile installing it in a proper trunk to leeward. If it's a failure, I only have a few screw holes to fill and paint over, and only my friends need know . . .

 

Dss

Here s a picture of a DSS foil by Jim Bowland, left and one from Ocean Navigator, right:

click on image-

 

Dss

For future reference: DSS can also be configured like a centerboard with a slot on each side allowing the foil to pivot aft. At this point, I think the "normal" version is better but who knows what may come up. This gives a bit more latitude in design.

 

DSS Design: Infiniti Yachts

New DSS design: http://www.infinitiyachts.com/home.html