A catamaran (from Tamil kattu "to tie" and maram "wood, tree") is a type of boat or ship consisting of two hulls joined by a frame. Catamarans can be sail- or engine-powered. The catamaran was the invention of the paravas, a fishing community in the southern coast of Tamil Nadu, India. Catamarans were used by the ancient Tamil Chola dynasty as early as the 5th century AD for moving their fleets to conquer such Southeast Asian regions as Burma, Indonesia and Malaysia.

Catamarans are a relatively recent design of boat for both leisure and sport sailing, although they have been used for millennia in Oceania, where Polynesian catamarans and outrigger canoes allowed seafaring Polynesians to settle the world's most far-flung islands. Catamarans have been met by a degree of scepticism from some sailors accustomed to more "traditional" designs.

Although the principles of sailing are the same for both catamarans and monohulls, there are some "peculiarities"to sailing catamarans. For example:

Catamarans can be harder to tack (turn through the wind). Because they are lighter in proportion to their sail size, they have less momentum to carry them through the turn when they are head to wind. Correct use of the jib sail is often essential in successfully completing a tack without ending up stuck in irons (pointing dead into the wind and sailing backwards, see: No Go Zone). They have a higher average speed. Catamarans are less likely to capsize in the classic 'beam-wise' manner but often have a tendency to 'pole-axe' (or 'pitchpole') instead - where the leeward (downwind) bow sinks into the water and the boat 'trips' over forward, leading to a capsize. Teaching for new sailors is usually carried out in monohulls as they are thought easier to learn to sail, a mixture of all the differences mentioned probably contributes to this.

Catamarans, and multihulls in general, are normally faster than single-hull boats for four reasons:

Each hull of a catamaran is (typically) thinner in cross section than those of monohulls; catamarans are lighter due to the fact there is no keel counterweight. catamarans have a wider beam (the distance from one side of the boat to the other), which makes them more stable and therefore able to carry more sail area per unit of length than an equivalent monohull. the greater stability means that the sail is more likely to stay upright in a gust, drawing more power than a monohull's sail which is more likely to heel (lean) over. A catamaran is most likely to achieve its maximum speed when its forward motion is not unduly disturbed by wave action. This is achieved in waters where the wavelength of the waves is somewhat greater than the waterline length of the hulls, or it is achieved by the design piercing the waves. In either case pitching (rocking horse-like motion) is reduced. This has led to it being said that catamarans are especially favourable in coastal waters, where the often sheltered waters permit the boat to reach and maintain its maximum speed.

Catamarans make good cruising and long distance boats: The Race (around the world, in 2001) was won by the giant catamaran Club Med skippered by Grant Dalton. It went round the earth in 62 days at an average speed of eighteen knots.

Catamaran designs
Popular small racing catamaran sailboats include:
The Hobie cats (especially the 16-foot long Hobie 16, as well as many other designs including 14, 17, 18, and 20 ft models.)
TOPCAT
Nacras
Prindles
International A-Class (open design; length 5.49 m (18 ft), beam 2.3 m (7 ft 6.5 in), weight 75 kg, sail 13.94 m2 (150 ft2))
Formula classes (F16, F18, F20)
Dart 15 and Dart 18
Tornado, Olympic class racing catamaran.
Small sailing catamarans that are mass-produced, trailerable, and can be beached on sandy shores are commonly called "beach cats".