Bulb keel

A bulb keel is the ballast arrangement of a sailing yacht in which a heavy bulb, usually lead, is hung at the bottom of a slender keel fin. Concentrating the ballast as low as possible maximizes the righting moment for a given ballast weight by lowering the center of gravity and lengthening the lever to the center of buoyancy, so the yacht carries more sail before heeling too far. The fin above the bulb provides the lift that resists leeway, while the bulb is shaped for least drag. This is a device unique to ballasted sailing craft; it is unrelated to the bilge keel of a ship, an unballasted roll-damping fin on the turn of bilge.