Multihull

Modern pioneers of multihull design include James Wharram (UK), Derek Kelsall (UK), Tom Lack (UK), Lock Crowther (Aust), Hedly Nicol (Aust), Malcolm Tennant (NZ), Jim Brown (USA), Arthur Piver (USA), Chris White (US), Ian Farrier (NZ), LOMOcean (NZ), Darren Newton (UK), Jens Quorning (DK) and Dick Newick (USA).

Model of a wa, a single-outrigger vessel, from Woleai in the National Museum of Ethnology (Japan)

A single-outrigger canoe is a canoe with a slender outrigger ("ama") attached by two or more struts ("akas"). This craft will normally be propelled by paddles. Single-outrigger canoes that use sails are usually inaccurately referred to by the name "proa". While single-outrigger canoes and proas both derive stability from the outrigger, the proa has the greater need of the outrigger to counter the heeling effect of the sail. The outrigger on a proa can either be on the lee or windward side, or in a tacking proa, interchangeable. However, more recently, proas tend to keep the outrigger either to leeward or to wind which means that instead of tacking, a "shunt" is required, whereby the bow becomes the stern, and the stern becomes the bow.

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. Catamaran configurations are used for racing, sailing, tourist and fishing boats.

The hulls of a catamaran are typically connected by a bridgedeck, although some simpler cruising catamarans[10] simply have a trampoline stretched between the crossbeams (or "akas").[11] Small beachable catamarans, such as the Hobie Cat, also have only a trampoline between the hulls.

Catamarans derive stability from the distance between the hulls—transverse clearance—the greater this distance, the greater the stability.[12] Typically, catamaran hulls are slim, although they may flare above the waterline to give reserve buoyancy.[13] The vertical clearance between the design waterplane and the bottom of the bridge deck determines the likelihood of contact with waves. Increased vertical clearance diminishes such contact and increases seaworthiness, within limits.[14]

A 60' trimaran with high aspect fractional Bermuda rig

A trimaran (or double-outrigger) is a vessel with two outrigger floats attached on either side of a main hull by a crossbeam, wing, or other form of superstructure. They are derived from traditional double-outrigger vessels of maritime Southeast Asia.[2][3][4] Despite not being traditionally Polynesian,[5][6] western trimarans use traditional Polynesian terms for the hull (vaka), the floats (ama), and connectors (aka).[15] The word trimaran is a portmanteau of tri and (cata)maran,[16] a term that is thought to have been coined by Victor Tchetchet, a pioneering modern multihull designer, born in Ukraine (at that time part of the Russian Empire).[17]

Some trimaran configurations use the outlying hulls to enhance stability and allow for shallow draft, examples include the experimental ship RV Triton[18] and the Independence class of littoral combat ships (US).[19]

Some multihulls with four (quadrimaran) or five (pentamaran) hulls have been proposed; few have been built.[20] A Swiss entrepreneur is attempting to raise €25 million[21] to build a sail-driven quadrimaran that would use solar power to scoop plastic from the ocean; the project is scheduled for launch in 2020.[22] A French manufacturer, Tera-4, produces motor quadrimarans which use aerodynamic lift between the four hulls to promote planing and reduce power consumption.[23]

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.[24][25] BMT Group, a shipbuilding and engineering company in the UK, has proposed a fast cargo ship and a yacht using this kind of hull.[26][27]

A narrow waterline distinguishes a SWATH ship from a conventional catamaran

Multihull designs may have hull beams that are slimmer at the water surface ("waterplane") than underwater. This arrangement allows good wave-piercing, while keeping a buoyant hydrodynamic hull beneath the waterplane. In a catamaran configuration this is called a small waterplane area twin hull, or SWATH.[28] While SWATHs are stable in rough seas, they have the drawbacks, compared with other catamarans, of having a deeper draft, being more sensitive to loading, and requiring more power because of their higher underwater surface areas.[29] Triple-hull configurations of small waterplane area craft had been studied, but not built, as of 2008.[30]