Bulb keel
A bulb keel is a keel, usually made with a high aspect ratio foil, that contains a ballast filled, usually teardrop shaped bulb at the bottom. The purpose of the bulb keel is to place the ballast as low as possible, therefore gaining the maximum possible amount of leverage and thus the most righting moment. A racing class using a bulb keel is the International 110 class, which uses a 300 lb. (136 kg) cast iron bulb keel on a boat whose minimum racing weight is 910 lbs. (414 kg).
Since bulb keels work best on long, thin keels or centerboards, they are generally not used on sailboats intended for shallow waters, but are most often found on offshore racing craft. Centerboards built like bulb keels are often referred to as "lifting keels", and they can be retracted into the boat to reduce the draft, and to allow the boat to be loaded onto a trailer. Lifting keels are also more likely to be found on craft built for speed, as a winch or a set of pulleys is required to provide purchase to lift the heavy keel.
Some boats with lifting keels go so far as to allow the keel to be canted side to side, allowing the bulb to be placed to windward to reduce heel in high winds.