A trimaran is a multihulled boat consisting of a main hull (vaka) and two smaller outrigger hulls (amas), attached to the main hull with lateral struts (akas). The design and names for the trimaran components are derived from the original proa constructed by native Pacific Islanders.
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 but there are also a number of production models on the market. A number of trimarans in the 19 – 36 foot lengths have been designed over the last 30 years to be accommodated on a road trailer. These include the original Farrier - Corsair folding trimarans - and original John Westell swing-wing folding trimaran (using the same folding system later adopted also on Quorning Dragonfly) and like trimarans. Many sailboat designers have also designed demountable trimarans that are able to be trailered (like the SeaCart 30 by Oceanlake Marine ).
The trimaran design is also becoming more widespread as a passenger ferry. In 2005 the 127 metre (417 ft) trimaran Benchijigua Express was delivered by Austal to Spanish ferry operator Fred.Olsen, S.A. for service in the Canary Islands. Capable of carrying 1280 passengers and 340 cars, or equivalents, at speeds up to 40 knots, this boat was the longest aluminum ship in the world at the time of delivery. The trimaran concept has also been considered for modern warships. The RV Triton was commissioned by British defense contractor QinetiQ in 2000. In October 2005, the United States Navy commissioned for evaluation the construction of a General Dynamics Littoral Combat Ship (LCS) trimaran designed and built by Austal.
There are three terms that describe the components of modern multihulls. The term vaka, like the related terms aka and ama, come from the Malay and Micronesian language group terms for parts of the outrigger canoe, and vaka can be roughly translated as canoe or main hull.
Semantically, the catamaran is a pair of Vaka held together by Aka, whereas the trimaran is a central Vaka, with Ama on each side, attached by Aka.
The above section reflects American usage. In the UK the main hull of a trimaran is called simply the main hull or centre hull. The side hulls are floats. The structures between the main hull and the floats are called the wings and the structural portions thereof are beams. In cruising trimarans the wings are solid and cabin accommodation extends over them, while in racing trimarans accommodation is limited to the main hull and the wings are open sheets of netting.
Two types of trimaran exist: the regular trimaran and the open trimaran, which features a trampoline between the hulls instead of plating.
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 wetted area, and is able to fly more sail area. In addition, because of the righting moment provided by the wide beam, trimarans do not need the weighted keel that is required in monohulls, often resulting in unsinkable designs. As a result of the wide beam, 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 requires more space to maneuver, so tacking and gybing can be trickier in confined areas and the narrower hulls provide less living space than an equivalently-sized monohull. Trimarans also require more docking space in marinas, unless the ama can be folded to reduce the beam.
As the righting moment (the force that resists the opposite torque of the wind on the sails) is produced by a float on either side called an ama and not a heavy protruding keel, trimarans are lighter and faster than a monohull of equivalent length. A lightweight retractable keel or foil, referred to as a centerboard or Daggerboard is often employed to resist lateral movement, making many models easily beachable. Most trimarans are difficult to flip sideways given a reasonable degree of caution, however, trimarans can reach speeds so great in high winds that they can plow into the back of a wave and flip end-over-end (Pitchpole). This hazard is especially dangerous for a multihull that is using a spinnaker in high winds and large seas. To avoid this unfortunate scenario trimaran sailors are advised to reduce sail and to always have all sails easily released. The use of trampolines with a large weave, to allow water to easily pass through, and the deployment of parachute anchors drogues and sea anchors whenever appropriate should reduce the risk to an acceptable degree.
The father of the modern sailing trimaran is Victor Tchetchet, a Russian émigré and a strong proponent of multihull sailing. Mr. Tchetchet, who was a fighter pilot during the First World War in the Czar’s Air Force, lived in Great Neck, New York from the 1940s until his death. He built two trimarans while living in the US, Eggnog 1 and 2. Both boats were made of marine plywood and were about 24 feet long. Mr. Tchetchet is credited with coining the name trimaran. Aside from boat design Mr. Tchetchet earned his living as a landscape and portrait painter. About the same time, Arthur Piver was also building trimarans in the USA and created many early plywood designs to which amateurs built their boats. Many successfully crossed oceans despite being relatively heavy and inferior compared to those of more modern design. The homebuilt cruiser movement survived his death in 1968, with designers Jim Brown, John Marples, Jay Kantola, Chris White, Norman Cross and Richard Newick bringing the trimaran cruiser to new levels of performance and safety.
Several manufacturers build trimarans in which the floats can be folded near to the main hull. This allow them to be trailerable and/or to fit in a normal monohull space in a marina. At least 5 technologies are in use:
Although it is possible for a trimaran to capsize, this is less frequent than with monohull boats because of the greater resistance to rolling that the amas offer. Most trimaran designs are considered nearly unsinkable because even when filled with water, the flotation of one ama is enough to keep the entire vessel afloat. Because of their stability and safety, special trimarans such as the Challenger, have become popular with sailors who have restricted mobility.
The greater speed compared to monohulls can also become important for safety when weather conditions are bad or threaten to deteriorate because the boat can leave the area of danger faster.
Potential buyers of trimarans should look for one that is designed with amas with multiple sealed partitions, controls that all run to the cockpit, a collision bulkhead, partial or full cockpit coverings or windshields, and drain holes in the cockpit that can adequately drain the cockpit quickly, among other things.
Trimarans capsizes are more likely to be of the pitch-pole type than a roll to one side due to their higher sideways stability and speeds. Capsized trimarans are harder to turn upright than monohull boats. A capsized trimaran should not be righted by sideways rotation as this usually causes heavy damage of the mast and rigging. Harnesses pulling on the stern toward the bow, or from the bow toward the stern of capsized trimarans have been shown to be able to successfully turn them end-over-end. Several design features reduce the chance of pitch-pole capsize. These include having wing nets with an open weave designed to reduce windage and decks and nets that shed water easily. The best way to avoid capsize is to reduce the efficiency of the sails in heavy weather conditions.
In their early days, multihulls including trimarans ran a greater risk of material damage than monohulls. For ocean-going trimarans, even some trimaran sailors still considered this to be true.
Trimarans at anchor or mooring may follow the wind due to their light weight and shallow draft while monohulls usually follow the tides. This can cause collisions if the trimaran is close to another vessel and the swing circles overlap. A bridle to the anchor line may assist in reducing this swing.
Competing with a giant trimaran the BMW Oracle Racing team of the Golden Gate Yacht Squadron won the America's Cup on February 14, 2010, off Valencia, Spain, beating the giant catamaran Alinghi 2-0 in the best-of-three series and becoming the first American syndicate to win the cup since 1992. The competing boats, Alinghi 5 and USA 17 were both 90-foot multihulls computing in a duel characterised by monohull boats. The rigid wing sail of the USA 17 trimaran provided a decisive advantage and the trimaran won the America's Cup by a considerable margin.
Francis Joyon holds the new world record for solo circumnavigation of the world, set on January 20, 2008. The 51-year-old Frenchman circled the planet alone in 57 days, 13 hours, 34 minutes, 6 seconds in a trimaran. He beat British sailor Ellen MacArthur's record set in February 2005 for which she spent just over 71 days at sea.
Littoral combat ships built by General Dynamics at Bath Iron Works will be of a trimaran design. The USS Independence (LCS-2) is the first of these ships. Littoral combat ships built by Lockheed will be of a monohull design.