Significance of keel surface area

[miked12]

I understand that it is the action of the keel against the water which causes foward motion and prevents side swiping. I have also noticed that many of the new racing designs have keels with a very thin and long pedistal with a lead bulb on the end. What is the significance of the lateral surface area of the keel?

[tspeer]

The side force developed by the keel must equal the side force applied to the boat by the sail rig. If you have enough lateral area to generate the force without stalling, plus a margin to account for motion through waves and maneuvering, then a larger keel simply adds wetted area and drag.

The deeper the keel is, the less drag will be created by the sideforce on the keel, irrespective of the amount of area. This means that a deep keel with adequate, but not excessive, area will be the most efficient.

The most effective place to put the ballast is at the end of the keel because this gives it the most leverage - it can generate the righting moment with a smaller heel angle. So the highest performance keel is very long and narrow with a bulb at the end.

[gonzo]

A very important thing to consider in keel design is the density of the water. The resistance of the water on the keel is directly proportional to it. Therefore a deep keel has more lateral resistance for the same area. A deep keel needs less area for the same resistance. Another advantage of a deep keel, is that at pronounce angles of heel it ventilates less. That is there is, proportionaly, less exposed keel. This advantage is only for upwind performance. A traditional deep bilge design has less wetted surface for its displacement. It will be a faster hull in a close reach or more down wind.