foil keel

[nordvindcrew]

I have been batting an idea around in my mind for a while. On a fin keel, would there be any advantage to have a system of cams in the hollow fin that could shape either side of the fin to create a lifting foil. As the boat tacks, the cams would be operated to change the foil shape to create lift on that side of the boat to resist or even cancel leeway. I am just wondering if it has already been done, and if so what the results were, or if it hasn't been done, what do the forum members think of the idea

[marshmat]

Seems like an interesting idea.

I've seen fins with a small trim tab on the trailing edge, but never one with this kind of flexibility. There are, of course, symmetrical foils that pivot to cancel leeway- CBTF for example.

There might be a bit of a compromise in that any attempt well below the surface to reduce leeway will also tend to increase heeling moment.... not sure how that would optimize out in the end.

[Eric Sponberg]

Been done, at least studied in theory, and I wouldn't be surprised if someone has built one. Look in C.A. Marchaj's book "Aero-Hydrodynamics of Sailing", Photo 3.25 on page 617 of the 1st edition. It shows a keel with moveable leading and trailing edge sections which effectively make a cambered foil when in operation. The photo shows a towing tank model (sorry, my wife has appropriated my scanner so I can't readily scan it at this time), so that is why I am not sure that one has actually been built.

On Project Amazon (my aluminum Open Class 60 of 10 years ago) I designed the keel trim tab as the last 30% of the chord length in what was a relatively fat section such that when the trim tab was fully deflected, it did NOT create a discontinuity on the leeward side. The keel blade was a hollow aluminum structure, the inside of which was a fuel tank for the generator. The ballast was the bulb casting at the bottom. The trim tab was activated by a vertical hydraulic cylinder and a cam plate hidden in the recess between the two foil sections. This worked so remarkably well that the Race Committee for the 1998-99 Around Alone race certified that the keel alone qualified as the emergency rudder. The owner went out into Charleston Bay with the Committee Boat to prove that he could turn circles by keel trim tab alone.

The attachment is an excerpt from an article I wrote for Marine Technology, the quarterly journal of the Society of Naval Architects and Marine Engineers (SNAME) in 2000. I have pdf copies of the article if anyone would like--simply send me an email with your address and I'll be happy to send it to you by return, no cost. The article describes all of the design features of Project Amazon, hull, rig, structure, etc.

Eric

[nordvindcrew]

I thought that the idea probably had been explored, and wondered if it had any value. I was prompted to think about this when I was told that Hobie catamarans sailed to windward due to asymetrical hull shape that created a foil to help the boat sail to windward. thanks

[messabout]

Yes it has been tried but only in a homely way. Years ago I had a similar brain fart that resulted in a daggerboard with just such an internal mechanism. the board did seem to reduce leeway. Alas, the boat was slower than before. I finally figured out that the flex skinned board with the internal cam arrangement was too thick. It was of necessity about a 12% cambered board. It was used on a 16 foot dinghy. The board, being too thick for the case, had to be inserted from the bottom. No problem as it was purely an experimental proposition.

Another madcap experiment had me building a dagger board that had all the camber on one side. No internals this time and it was much thinner at about 6%. It worked rather well as long as frequent tacking was avoided. In order to tack we had to pull the board and re-insert it end for end. The board had a constant section along the entire length so flipping end for end put the camber on the advantaged side. The idea had some limited merit and slightly outperformed similar boats if always on the same tack. The advantage was lost entirely when sailing to windward on a closed course. Too much time lost during tacks.

[PortTacker]

Also every America's Cup boat since the 80s has trim tabs on the keel.

[Eric Sponberg]

Well-designed twin keel boats also have asymetric keels, and these are toed inward about 2 degrees at the leading edge to further improve windward ability. These work very nicely and have the benefit of reduced draft. They are common in England (and probably France) which experience great tidal changes--17-20' or so on the English side of the Channel, and 40'+ on the French side.

Also, Open Class boats are sometimes fitted with asymetric daggerboards up forward to add lateral area that is otherwise lost when canting a keel. Racing multihulls, too, typically have asymetric daggerboards.

Eric

[JesperW]

The Americas cup trim tabs are often designed according to Gabriel Heymans Patent.

See http://www.heymanyachtdesign.com/in3a.html for an extensive (but rather outdated) discussion of the design.

(Gabriel has designed parts of the hull for my Journeyman 60 design)