Sailing boats can have one, two, or three hulls. Boats with one hull are known as monohulls, while those with two or more are known as multihulls. Multihulls can be further subdivided into catamarans (two hulls), and trimarans (three hulls). A sailing boat is turned by a rudder, which itself is controlled by a tiller or a wheel, while at the same time adjusting the sheeting angle of the sails. Smaller sailing boats often have a stabilising, raisable, underwater fin called a centreboard; larger sailing boats have a fixed keel. As a general rule, the former are called dinghies, the latter keelboats. However, up until the adoption of the Racing Rules of Sailing, any vessel racing under sail was considered a yacht, be it a multi-masted ship-rigged vessel, a sailboard or remote-controlled boat, or anything in between.

Sailing Hull

Photo: multihulls.net.au

Multihulls use flotation and/or weight positioned away from the centre line of the sailboat to counter the force of the wind. This is in contrast to heavy ballast that can make up to ? of the weight of a monohull sailboat. In the case of a standard catamaran there are two similarly sized and shaped slender hulls connected by beams, which are sometimes overlaid by a deck superstructure. Another catamaran variation is the proa.

In the case of trimarans, which have an unballasted centre hull similar to a monohull, two relatively smaller amas are situated parallel to the centre hull to resist the sideways force of the wind. The advantage of multihulled sailboats is that they do not suffer the performance penalty of having to carry heavy ballast, and their relatively lesser draft reduces the amount of drag, caused by friction and inertia, when moving through the water.