1992 Ameracat Cat 30

A catamaran is a multihulled vessel consisting of two parallel hulls of equal size.

A catamaran is geometry-stabilized, that is, it derives its stability from its wide beam, rather than having a ballasted keel like a monohull. Being ballast-free and lighter than a monohull, a catamaran can have a very shallow draught. The two hulls will be much finer than a monohull's, the reduced drag allowing faster speeds in some conditions, although the high wetted surface area is detrimental in lower wind speeds. A sailing multihull will heel much less than a sailing monohull of the same length, so on a windward course its sails spill less wind and are more efficient. The limited heeling means the ride may be more comfortable for passengers and crew, although catamarans can exhibit an unsettling "hobby-horse" motion and have a violent unsettling diagonal pitching motion that is unpredictable and makes moving around hard in rougher conditions, the noise is also much more than a monohull experiences.

A catamaran's two hulls are joined by some structure, the most basic being a frame, formed of akas. More sophisticated catamarans combine accommodation into the bridging superstructure. Catamarans may be driven by sail and/or engine. Originally catamarans were small yachts, but now some ships and ferries have adopted this hull layout because it allows increased speed, stability and comfort.

The catamaran concept is a relative newcomer for Western boat designers, although they have been used since time immemorial among the Dravidian people, in South India. Catamarans were developed independently in Oceania, where Polynesian catamarans and outrigger canoes allowed seafaring Polynesians to voyage to the remotest Pacific islands.