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A multihull is a ship or boat with more than one hull, whereas a vessel with a single hull is a monohull.
The first multihull vessels were Austronesian canoes. The builders hollowed out logs to make canoes and stabilized them by attaching outriggers to prevent them from capsizing. This led in due course to the proa, catamaran, and trimaran.
A catamaran is a vessel with twin hulls. Commercial catamarans began in 17th century England. Separate attempts at steam-powered catamarans were carried out by the middle of the 20th century. However, success required better materials and more developed hydrodynamic technologies. During the second half of the 20th century catamaran designs flourished. Nowadays, catamarans are used as racing, sailing, tourist and fishing boats. Cruising catamarans are becoming important in the holiday charter market. Some 70% of fast passenger RoRo ferries are catamarans.
A catamaran's hulls are slim, although they may flare above the waterline to give reserve buoyancy. Catamarans are prone to "slamming", an unpleasant phenomenon where the waves slam against the underside of the bridge deck. The distance between the design waterplane and the bottom of the bridgedeck is called the "vertical clearance"; and the greater this distance, the less slamming will be encountered. Although a large vertical clearance normally increases a catamaran's seaworthiness, the designer must take care not to raise the overall CoG too much.
The trimaran has the widest range of interactions of wave systems generated by hulls at speed. The interactions can be favorable or unfavorable, depending on relative hull arrangement and speed. No authentic trimarans exist. Model test results and corresponding simulations provide estimates on the power of the full-scale ships. The calculations show possible advantages in a defined band of relative speeds.
Design concepts for vessels with two pair of outriggers have been referred to as pentamarans. The design concept comprises a narrow, long hull that cuts through waves. The outriggers then provide the stability that such a narrow hull needs. While the aft sponsons act as trimaran sponsons do, the front sponsons do not touch the water normally; only if the ship rolls to one side do they provide added buoyancy to correct the roll. BMT Group, a shipbuilding and engineering company in the UK, has proposed a fast cargo ship and a yacht using this kind of hull.
From the earliest times, monohulls (whether or not fitted with sails) were stabilized by carrying ballast (such as rocks) in the bilges; and all modern monohull yachts and ships still rely on ballast for stability. Naval architects arrange the vessel's centre of gravity to be well below the hull's metacenter. The low centre of gravity acts as a counterweight as the craft heels around its centre of buoyancy; that is, as a monohull heels, its ballast operates to restore it to its upright position.
Having no ballast, multihulls that become holed or inverted have a high rate of survivability; water-tight bulkheads should prevent sinking if the hulls fail. Catamarans may have increased reliability because most have an engine in each hulls. Whereas capsized monohulls typically right themselves, capsized multihulls remain inverted. Large multihulls may have escape hatches in the hulls or bridgedeck.
Racing catamarans and trimarans are popular in France, New Zealand and Australia. Cruising cats are commonest in the Caribbean and Mediterranean (where they form the bulk of the charter business) and Australia. Multihulls are less common in the US, perhaps because their increased beam require wider dock/slips. Smaller multihulls may be collapsible and trailerable, and thus suitable for daybooks and racers. Until the 1960s most multihull sailboats (except for beach cats) were built either by their owners or by boat builders; since then companies have been selling mass-produced boats, of which there are more than 150 models.