Ever been to a regatta, and when you have looked at all of the model yachts, have you ever noticed the different keel bulb shapes? I have seen long bulbs, short bulbs, fat bulbs, and skinny bulbs. Why are there so many different shapes? Which bulb shape is best?

If you remember, when an object is moving in water, drag is created, and slows the boat down. For model yachts to keep upright while sailing, they require a weight at the bottom of the keel. All weights are made of lead, and are in the shape of a torpedo. The question is what is the best torpedo shape that gives the best performance with the least drag.

Torpedo keel bulbs don't provide any lift to the keel, they are just there for the ride to keep the boat upright. The goal of designing a bulb is to minimize drag. According to the U.S. National Advisory Committee for Aeronautics (NACA), if the bulb's volume of weight is constant, the size and length does make a difference.

A long bulb with a small diameter produces a large wetted surface and frictional drag, but is low in form drag. A bulb that is short and fat produces a low wetted surface and frictional drag, but is large in form drag. Both of these two extremes will not give the performance that the model yachter wants. There must be a happy medium between the two.

It is a trade-off between chord length and thickness. To give a good idea of this trade-off, the maximum foil thickness of the bulb should be at 45% of the chord length (see figure 1). To further illustrate this look at the plot chart below. You can see the drag plot line is lowest right at the #2 bulb to 2.5. Any thing before and after of the range of 2 to 2.5 creates more drag (see figure 2).

Hopefully this information can be of some help in the designing and building of better keel bulbs. Any way to reduce drag on the keel can only result in improved performance of the model yacht.