Multihull

An 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. If the craft has a sail, it is known as a proa. While 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 difficulty with the proa is that the outrigger must be on the lee side to be effective, which means that a change of tack will need the sail to be rearranged.

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.

The hulls of a catamaran are typically connected by a bridgedeck, although some simpler cruising catamarans[4] simply have a trampoline stretched between the crossbeams (or "akas").[5] Small beachable catamarans, such as the Hobie Cat, also have only a trampoline between the hulls.

Catamarans have no ballast and their stability is derived from the width between the hulls. The distance between hulls is called the "transverse clearance", and the greater this distance, the more stable the catamaran will be.

A catamaran's hulls are slim, although they may flare above the waterline to give reserve buoyancy.[6] 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.[7] Although a large vertical clearance normally increases a catamaran's seaworthiness, the designer must take care not to raise the overall CoG too much.

A 60' trimaran with high aspect fractional Bermuda rig

A trimaran is a vessel with three hulls. Unlike a catamaran where the hulls are mirror-images of each other, a trimaran is rather like a monohull with two slim outriggers.[8] A trimaran has less accommodation space than a catamaran, but may be capable of even faster speeds.

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.

A new type of super-fast vessel, the wave-piercing trimaran (WPT) is known as an air-born unloaded (up to 25% of displacement) vessel, that can achieve twice the speed with a relative power.

Some trimaran configurations use the outlying hulls to enhance stability and allow for shallow draft, examples include the experimental ship RV Triton[9] and the Independence class of littoral combat ships (US).[10]

Some multihulls with four (quadrimaran) or five (pentamaran) hulls have been proposed; few have been built.[11] A Swiss entrepreneur is attempting to raise €25 million[12] 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.[13] A French manufacturer, Tera-4, produces motor quadrimarans which use aerodynamic lift between the four hulls to promote planing and reduce power consumption.[14]

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.[15][16] BMT Group, a shipbuilding and engineering company in the UK, has proposed a fast cargo ship and a yacht using this kind of hull.[17][18]

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.[19] 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.[20] Triple-hull configurations of small waterplane area craft had been studied, but not built, as of 2008.[21]

Multihulls have a broader variety of hull and payload geometries. Compared to a monohull, they have a relatively large beam, deck area (upper and inner), above-water capacity, shallower draft (allowing operation in shallower water) but a limited payload.

Early Austronesians discovered that round logs tied together into a raft were more stable than a single log. Hollowing out the logs further increased buoyancy and payload. Separating two logs by a pair of cross-members (or "akas") further increased stability. Spanning the intervening distance with a platform provides space for accommodation.

Compared to monohulls, multihulls are much less prone to heeling (tilt); a sailing catamaran will rarely heel more than 5° whereas a monohull will frequently heel to 45°. This is particularly noticeable when running the wind; a monohull will roll incessantly, while a catamaran will remain upright. A catamaran's stable motion reduces seasickness and tiredness of the crew, making it safer and more suitable for family cruising. The stability also allows more efficient solar energy collection and radar operation. However, shorter multihulls may be more prone towards an uncomfortable motion called hobby horsing, especially when lightly loaded.

Being heavier (because of its ballast), a monohull's momentum will temporarily maintain progress if the wind drops, while a (lighter) multihull has less momentum and may be prone to going "in irons" when going about. (Multihulls need to keep the jib "aback" to complete the turn. However, multihull skippers will frequently choose to "gybe" instead, as gybing is much less of an event in a multihull than in a monohull. Multihulls "ghost" well under sail as they respond readily in light airs.

A 45' catamaran under sail with minimal bow wave and wake, thanks to slim hull sections, light displacement and a long LWL.

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.

By contrast, a multihull's stability is derived from its width, and modern multihulls are much wider than earlier designs, with the beam sometimes more than half the LOA. Should the weather hull lift from the water, the weight of the vessel will seek to restore the multihull to its normal position. However, unlike a monohull, a multihull has a "point of no return" which will lead to the vessel becoming inverted. A catamaran dinghy may be righted by its crew, but a large cruising cat will normally remain inverted unless it can be craned upright.

A catamaran that is being pressed too hard may "pitchhole (capsize)", a disastrous event whereby the lee bow (downwind bow) digs into the water and trips, followed by the stern rising and the entire vessel somersaulting.

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.

Having a low displacement,[22] and long, narrow hulls, a multihull typically produces very small bow waves and wakes, a consequence of a favorable Froude number.[23][24][25] Vessels with beamy hulls (typically monohulls) normally create a large bow wave and wake. Such a vessel is limited by its "hull speed", being unable to "climb over" its bow wave unless it changes from displacement mode to planing mode. Vessels with slim hulls (typically multihulls) will normally create no appreciable bow wave to limit their progress.

In 1978, 101 years after catamarans like Amaryllis were banned from yacht racing[26] they returned to the sport. This started with the victory of the trimaran Olympus Photo, skippered by Mike Birch in the first Route du Rhum. Thereafter, no open ocean race was won by a monohull. Winning times dropped by 70%, since 1978. Olympus Photo's 23-day 6 hr 58' 35" success dropped to Gitana 11's 7d 17h 19'6", in 2006. Around 2016 the first large wind driven foil-borne racing catamarans were built. These cats rise onto foils and T-foiled rudders only at higher speeds.