This term is generally associated with boats that have more then one hull and can have three different hull forms. They can be either displacement hulls or planing hulls. The Proa has a narrow single main hull called a vaka joined to a smaller hull or outrigger by cross-members called akas. A catamaran is a twin-hulled vessel. The twin identical hulls of the catamaran are called amas. Trimarans have three hulls. The larger center hull is the vaka and the two outer hulls are the amas. The hulls placed apart in such a way gives additional stability to the multihull as opposed to a monohull. This additional stability given by the general greater overall width of the multihull gives a significant advantage in that, unlike a displacement monohull, they do not have to be ballasted down at the keel to hold them upright and also the lack of a deep keel and appendages helps reduce resistance. This makes them lighter and faster.