The Wing Keel

In 1983, the Australian team manning the boat Australia II shocked the sailing world by taking the year's competition for America's cup. Up until that year, the world's most prestigious yacht regatta had returned year after year to the United States. Most experts point to the innovation of the winged keel the Australian yacht had been equipped with, as responsible for this win. In fact, this theory has proved so strong that today many sailing boat owners are interested in using the wing type keel for their own vessels. Example of a wing keel.

The Catalina 22 MkII was fitted with a highly efficient wing keel as one of the options, perfected from many years of design refinement by one of the world's largest yacht builders.

The part of the keel that is referred to as the wing is actually a horizontal foil adapted to fit a traditional shoal keel. There are several advantages to this adaptation.

Wing keels provide a decrease in draft without decreasing efficiency. Traditional shoal keels make it harder to steer a boat properly, but with the wing effect this type of keel is much easier to maneuver. The aspect ratio is doubled. Aspect ratio refers to the ability of the keel to resist pressure from the water, dividing in a flowing manner along the sides of the boat. The higher the number, the greater the glide. Additional downward lift is provided, and this makes the boat much more stable (it sits more comfortably in the water).

Owners of smaller wing keel boats also find them ideal for trailering. The Catalina 22 is one of the world's premier trailer sailors, and the wing keel's lack of moving parts and through hull fittings (always a concern for developing leaks), are one the major advantages over the often problematic swing keel design.

The greatest advantage of the winged keel is when one is sailing upwind. The greater stability and the side force produced by the horizontal foil allow a racing boat an advantage in conditions that are not ideal to the sail force.

Of course, the advantages of the winged keel are only obvious under the right conditions, and they may not be for every vessel. Lighter, non-racing craft, for example, may find that the advantages this type of keel represents do not transfer under ideal racing situations.

Nevertheless, for long voyage sailing in deep water a winged keel is virtually guaranteed to provide an advantage in terms of both speed and stability. That's what won the Australians the World Cup almost three decades ago, and it remains the strong selling point of wing keels to this day.



Winged Keels Pros & Cons
     Jeff_H Super Moderator, SailNet forums
     www.sailnet.com

I think that wing keels offer a lot of promise in reducing draft while offering close 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 not always realized.

To explain:
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 a 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 wings 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 can have 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 really jam 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.

Jeff_H Super Moderator, SailNet forums
www.sailnet.com


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