Multihull

A multihull is a vessel that uses more than one hull to stay afloat. Most boats are monohulls, which have only a single hull.

Multihulls are designed to solve specific problems of stability and speed. By spreading the boat's weight across several parts, these vessels can achieve things a single hull cannot. They are used for many purposes today. You might see them in professional ocean racing, as tourist ferries, or even as small fishing boats.

To understand how they work, you must look at displacement. Most ships stay afloat by displacing their weight in water. A monohull must be deep, long, or wide to displace enough water to support its weight. This often results in a large bow wave, which creates resistance and limits speed. A multihull divides that displacement between multiple, narrow hulls. These slim hulls create very small bow waves and wakes. This happens because of a favorable Froude number, which is a way to measure a ship's drag and wave-making resistance. Because they do not have to fight large waves of their own making, multihulls can often move much faster than monohulls.

There are several distinct types of multihulls based on their configuration. The most common is the catamaran, which has two hulls. These hulls are typically connected by a bridgedeck, though some use a trampoline stretched between crossbeams.

Another type is the trimaran, or double-outrigger. A trimaran has a main hull with two smaller floats, called amas, attached to the sides by crossbeams, or akas.

Some specialized designs even use four hulls, called quadrimarans, or five hulls, known as pentamarans. A pentamaran uses a narrow central hull for cutting through waves and two sets of outriggers for stability.

Multihull designs have a very ancient history. The Austronesian peoples developed single-outrigger canoes, double-canoes, and double-outrigger boats during their expansion between 3000 and 1500 BC.

These vessels allowed them to colonize regions like Micronesia, Polynesia, and Madagascar. Even ancient Greek sources mention large catamarans from the third century BC. One such ship, the Syracusia, was built under the supervision of the mathematician Archimedes. In modern times, designers like James Wharram, Ian Farrier, and Victor Tchetchet helped advance these technologies. Tchetchet is even credited with coining the term "trimaran."

Stability is one of the most important features of these boats. In a catamaran, stability comes from the transverse clearance, which is the distance between the two hulls. The greater this distance, the more stable the boat becomes. Some designs, known as SWATH, use a small waterplane area twin hull.

This means the hulls are slimmer at the water's surface but wider underwater. While SWATHs are very stable in rough seas, they require more power because they have more surface area underwater. This can make them more sensitive to how much weight is loaded onto the ship.

Multihulls have changed the world of competitive sailing. In 1978, the trimaran Olympus Photo won the first Route du Rhum race. This victory marked a turning point in ocean racing. Since then, no open ocean race has been won by a monohull. The speed difference is massive. For example, the winning time for the race dropped by 70% between the early days and 2006. In 2016, engineers even began building large, wind-driven racing catamarans that use foils to lift the hulls out of the water at high speeds.

Today, multihulls are found in many different industries. In the Caribbean and Mediterranean, cruising catamarans are a major part of the charter business.

They are popular with families because they do not heel, or tip, as much as monohulls. This makes sailing much more comfortable and less tiring for the crew. They also do not need heavy ballast to stay upright, which makes them much lighter. Whether they are used as fast cargo ships or stable fishing platforms, multihulls continue to offer unique advantages in maritime travel.