Join Date: Feb 2000
Location: Annapolis, Md
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Winged Keels Pros & Cons
(The ''War and Peace'' version)
I think that wing keels are generally being way over sold. At their most extreme, they offer a lot of promise in reducing draft while offering closer to fin keel performance. In theory wing keels can do three things to improve performance over an equal draft equal area keel; 1) Lower the vertical center of gravity, 2) reduce edge leakage and tip vortex, and 3) when heeled over, increase the foil (wing) area working against leeway. But that promise is rarely realized.
1) Lower the vertical center of gravity.
First of all the wings, typically being made of lead and occurring right near the bottom of the keel, allows the wing keel to have a similar vertical center of gravity to a deeper fin.
2) Reduce edge leakage and tip vortex;
This second theoretical purpose for a wing keel is far more important than it might sound. If you had equal area rectangular shaped keels, one horizontal (low aspect ratio) and the other vertical (high aspect ratio, all other things being equal, the vertical one would slide sideward less (make less leeway) and have less drag. There are several reasons that high aspect ratio keels will make less leeway and have less drag. To understand this you need to look at the front, top and bottom of the keel. If we start at the top, the area adjacent to the hull is highly turbulent and so the portion of the keel that is near the hull generates less lift than the portion of the keel operating in non-turbulent water. The high aspect ratio keel has a greater portion of its area operating in non-turbulent water.
The leading edge of the keel is the primary source of lift (resistance to sideward motion). The higher aspect ratio keel has more leading edge and so simply has more lift in relationship to the drag created by its surface area.
But it''s the bottom of the keel that is the big frontier for wing keels. When a boat keels over and makes leeway, water from the high-pressure side of the keel slips under the keel into the low-pressure side of the keel. This slippage reduces the effectiveness of the bottom of the keel in creating lift. Beyond that this water than slips under the keel is highly turbulent and tumbles away from the boat creating a large vortex. There is a large amount of drag involved in towing this tumbling mass of water through the water. The longer the bottom of the keel in relationship to its height, the larger the area of the keel operating in reduced efficiency and the greater the tip vortex and consequently the greater the drag. To reduce this problem a simple plate can be added to the bottom of the keel that would stick out to either side and this plate would direct the water aft over the end of the keel and help to reduce the losses in efficiency of the keel tip. This is called an ''end plate effect''. The problem with a simple end plate is that they have their own tip vortex and that can increase drag. With carefully modeled wing shaped end plate, the tip vortex of both the low aspect ratio keel and the end plate can be greatly reduced to a level closer to a simple fin.
3) When heeled over, increase the foil (wing) area working against leeway.
This theoretical advantage is easy to imagine but very hard to make work as promised. If you visualize a keel with two long wings protruding out either side and then you think of this boat as being heeled over, the lower wing will end up closer to vertical and if designed correctly can help generate lift that would resist leeway. In reality it takes pretty long wings to achieve any significant lift and they need to be configured so that they indeed develop lift in the right direction. Beyond that these long winds represent a lot of drag and so really need to be optimized to provide sufficient lift to overcome the losses of their drag. There''s the rub. Cruising boats go through so many changes in trim that it is really not possible to get that configuration right for all or even most situations.
As a result, most wing keels are little more than a specialized type of bulb keel. At best the average production wing keel concentrates weight lower in the keel, like a bulb and perhaps when properly designed they also are moderately efficient in reducing tip vortex.
Wing keels come with a price. No matter how carefully designed, a wing keel will have greater drag than a simple fin keel with equal lift. They are a little harder to design structurally and they have real problems in a grounding situation, where their greater surface area on their bottom makes them harder to free. In rare grounding situations, when heeled over, the lower tip can rally jamb into the bottom making getting free even harder. From talking to a lot of wing keel owners that just does not seem to happen that often.
So, in conclusion, wing keels offer an improved windward performance over a similar depth and aspect ratio keel. They will have greater drag than a deeper fin of equal lift and so will be slower on all points of sail. Not all wings are created equal. Some are wildly better than others. They potentially offer a lot of advantages, but they are heavily dependent on the quality of the design and I really think that many, if not most, wing designs are not really working to their potential nor offering anything like the promised performance. In testing tank studies of various keel types, carefully designed keel/centerboarders generally offered a greater potential for higher performance than your average wing but that comes at the price of more complexity and more moving parts to break. In yacht design, as in most things, there''s no such thing as a free lunch.