A multihull is a sailing ship with more than one hull. The additional hulls provide stability, typically to hold the vessel upright against the sideways force of the wind on the sails. This is in contrast to monohulls which typically use a keel and/or ballast for this purpose.
Multihull sailboats are typically much wider than the equivalent monohull, which allows them to carry no ballast, so they are typically faster than monohulls under equivalent conditions It also means that multihulls are less prone to sink than monohulls when their hulls are compromised. There are also multihull powerboats, both for racing and transportation.
Multihulls are substantially faster than monohulls, because the absence of ballast reduces their weight and the amount of drag through the water considerably, without reducing the amount of sail that they can carry, and because the waterline to width ratio is so large.
Multihulls are quite popular for racing, especially in Europe and Australia, and are somewhat popular for cruising in the Caribbean. They're not seen very often in the United States, although the multihull yacht sales gradually increase. Until the 1980s most multihull sailboats (except for beach cats) were built either by their owners or by boatbuilders on a semi-custom basis. Since then several companies have been successful selling mass-produced (by boat industry standards) boats.
A trimaran is a multihull boat consisting of a main hull and two smaller outrigger hulls (amas), attached to the main hull with lateral struts (akas).
The first trimarans were built by indigenous Polynesians almost 4,000 years ago, and much of the current terminology is inherited from them. Multihull sailboats (catamarans and trimarans) gained favor during the 1960s and 1970s. Modern recreational trimarans are rooted in the same homebuilt tradition as other multihulls, though there are a number of production models now on the market, such as the folding, trailerable trimarans from Corsair Marine, Quorning Boats, Performance Cruising Inc., and Ian Farrier.
The trimaran design is also becoming more widespread as a passenger ferry. Designs which can carry over 300 cars are now in production. The trimaran concept has also been considered for modern warships.
Trimarans have a number of advantages over comparable monohulls (conventional, single-hulled sailboats). Given two boats of the same length, the trimaran has a shallower draft, a wider beam, less hull area, and is able to fly more sail area. In addition, because of the wide beam, trimarans do not need the weighted keel required in monohulls. As a result, the trimaran offers much better straight-line performance than a monohull, is able to sail in shallower water, and maintains its stability in stronger winds. However, its wider beam makes it a little more cumbersome to maneuver, so tacking and jibing can be trickier, and the narrower hulls provide less living space than an equivalently-sized monohull.