When water flows past a keel (a foil), lift is created because the movement of water over the keel creates a low-pressure side and a high-pressure side. This is fine, but there is a problem (you knew it was coming). Because a keel is in a three-dimensional environment (water) it looses lift to tip vortex.

Tip vortex is the event of water curling around and off the end of the keel. When this happens, the water flows from the high-pressure side to the low-pressure side. This water flow from the high-pressure side to the low-pressure side generates trailing vortices and a loss of lift. Another influence to a keel's lift is the keel's aspect ratio.

Aspect ratio refers to the length of the keel compared to its width and the average chord length. It is the span of a keel's length with the chord from the fore and aft width of a straight tapered fin. This is the average of the chord at the boat's hull and at the keel's tip. Since water is flows around an asymmetrical object (a foil), the shape of the foil creates lift.

So what can be done to stop or at least minimize the loss of lift at the ends of a keel? The best way is to put end plates on a keel, doing so will solve some of the problem. What is an end plate? Well, for most model yachts, keels already have end plates, the first is the boat hull itself and the keel bulb. An end plate creates a wall that prevents the water from flowing from the high-pressure to the low-pressure side. We won't worry about the keel where it meets the boat's hull, instead let's talk about the bulb.

The majority of keels for model yachts use round torpedo shaped lead bulbs. Having a bulb at the end of the keel provides some end-plate effect. You can't create a perfect end plate because of the need of weight and added drag due to extra wetted service. But by knowing the effect a bulb has (not only just the needed weight), you can add this knowledge into the design and construction of the total keel.

Just by having a bulb at the end of a keel provides some end-plate effect to reduce tip vortex, this helps prevent the loss of lift to the keel. This lift is vital to the performance to the type of model yachts (36/600s, Marbleheads, and others) we sail. There is no way to completely eliminate tip vortex with a keel bulb; we have the best design with a round torpedo shaped lead bulb.

There have been some attempts to put winglets on keel bulbs. These winglets are used to combat tip vortexes that come off the end of the keel. Winglets are being used on America's Cup yachts with success. As for model yachts, I have seen only one trial but do not know if performance improved. Now this does not mean a forward thinking skipper should not try winglets. The opposite is true. Outside of the amount of testing for improved results, winglets also provide more wetted surface for the model yacht designer to worry about. To add to that, there may not be enough measurable performance for winglets on a model yacht bulb due to the small size. Who knows, with some effort, winglets on model yacht's keel bulb may be the next advancement in technology for model yachting!