A multihull is a ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

, vessel, craft or boat with more than one hull.

Description

Multihulls include: Proa

Proa

A proa, also seen as prau, perahu, and prahu, is a type of multihull sailing vessel.While the word perahu and proa are generic terms meaning boat their native language, proa in Western languages has come to describe a vessel consisting of two unequal length parallel hulls...

s, which have two differently shaped or sized hulls with lateral symmetry; catamaran

Catamaran

A catamaran is a type of multihulled boat or ship consisting of two hulls, or vakas, joined by some structure, the most basic being a frame, formed of akas...

s, which have two hulls with longitudinal symmetry; and trimaran

Trimaran

A trimaran is a multihulled boat consisting of a main hull and two smaller outrigger hulls , attached to the main hull with lateral struts...

s, which have a main hull in the center and symmetric stabilizing hulls on either side.

These types of boats have several advantages compared to single-hull boats. The increased distance between the center of gravity and the center of buoyancy provide higher stability compared to boats with a single hull. This allows multihulls to have narrower hulls and thus substantially less wave-forming resistance, which in turn results in greater speed without applying more effort.

In the case of boats under sail, stability serves to hold the vessel upright against the sideways force of the wind on the sail

Sail

A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...

s. This stability is provided in multihulls by the weight of the boat itself, in contrast to monohull

Monohull

rightA monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another.-Fundamental concept:...

 sailcraft which typically use an underwater counterweight, a ballasted

Sailing ballast

Ballast is used in sailboats to provide moment to resist the lateral forces on the sail. Insufficiently ballasted boats will tend to tip, or heel, excessively in high winds. Too much heel may result in the boat capsizing. If a sailing vessel should need to voyage without cargo then ballast of...

 keel

Keel

In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...

 for this purpose, especially on larger sailboats.
Multihull sailboats are typically much wider than the equivalent monohull, which allows them to carry no ballast, and the reduced weight also makes them faster than monohulls under equivalent conditions (see Nathanael Herreshoff

Nathanael Herreshoff

Nathanael Greene Herreshoff I , was an American naval architect-mechanical engineer. "Captain Nat," as he was known, revolutionized yacht design, and produced a succession of undefeated America's Cup defenders between 1893–1920....

's "Amaryllis", also 1988 America's Cup

America's Cup

The America’s Cup is a trophy awarded to the winner of the America's Cup match races between two yachts. One yacht, known as the defender, represents the yacht club that currently holds the America's Cup and the second yacht, known as the challenger, represents the yacht club that is challenging...

).
It also means that multihulls need not sink or be abandoned if flooded, as opposed to ballasted monohulls who do indeed sink when flooded.
The comfort of more onboard accommodation space and more level boats under sail offer substantially improved conditions for crew and passengers, which also contributes to the greatly increasing popularity of multihull sailboats during the past few decades.

There are also multihull powerboats, usually catamarans (never proas), both for racing and transportation. Speed, maneuverability, and space onboard are the main factors for choosing multihull design in powerboats.

... the weight of a multihull, of this length, is probably not much more than half the weight of a monohull of the same length and it can be sailed with less crew effort.

Multihull component terms

There are three terms that describe the components of modern hull. The term vaka, like the related terms aka

Aka (sailing)

The aka of a multihull sailboat is a member of the framework that connects the hull to the ama . The term aka originated with the proa, but is also applied to modern trimarans....

 and ama

Ama (sailing)

The term ama is a word in the Polynesian and Micronesian languages to describe the outrigger part of a canoe to provide stability. Today, among the various Polynesian countries, the word ama is often used together with the word vaka or waka or va'a , cognate words in various Polynesian languages...

, come from the Malay

Malay language

Malay is a major language of the Austronesian family. It is the official language of Malaysia , Indonesia , Brunei and Singapore...

 and Micronesian language group

Malayo-Polynesian languages

The Malayo-Polynesian languages are a subgroup of the Austronesian languages, with approximately 385.5 million speakers. These are widely dispersed throughout the island nations of Southeast Asia and the Pacific Ocean, with a smaller number in continental Asia...

 terms for parts of the outrigger canoe

Outrigger canoe

The outrigger canoe is a type of canoe featuring one or more lateral support floats known as outriggers, which are fastened to one or both sides of the main hull...

, and vaka can be roughly translated as canoe

Canoe

A canoe or Canadian canoe is a small narrow boat, typically human-powered, though it may also be powered by sails or small electric or gas motors. Canoes are usually pointed at both bow and stern and are normally open on top, but can be decked over A canoe (North American English) or Canadian...

 or main hull.

• Aka
  Aka (sailing)
  The aka of a multihull sailboat is a member of the framework that connects the hull to the ama . The term aka originated with the proa, but is also applied to modern trimarans....
  
   - The aka of a multihull sailboat is a member of the framework that connects the hull to the ama(s) (outrigger). The term aka originated with the proa, but is also applied to modern trimarans.
• Ama
  Ama (sailing)
  The term ama is a word in the Polynesian and Micronesian languages to describe the outrigger part of a canoe to provide stability. Today, among the various Polynesian countries, the word ama is often used together with the word vaka or waka or va'a , cognate words in various Polynesian languages...
  
   - The term ama comes from the proa. The vaka
  Vaka (sailing)
  The vaka is the main hull of a multihull vessel.-Origin and use of the term:The term vaka is used in the Polynesian, Malay and Micronesian languages for a 'canoe', 'ship' or 'boat.' Other parts of a traditional vaka can include the aka and ama .A proa consists of a vaka, the main canoe-like...
  
   is the main hull
  Hull (watercraft)
  A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...
  
  , the ama is the outrigger, and the aka
  Aka (sailing)
  The aka of a multihull sailboat is a member of the framework that connects the hull to the ama . The term aka originated with the proa, but is also applied to modern trimarans....
  
   or iako (Hawaiian) is the support connecting the two (not three) hulls. The term ama and aka have been widely applied to modern trimarans.
• Vaka
  Vaka (sailing)
  The vaka is the main hull of a multihull vessel.-Origin and use of the term:The term vaka is used in the Polynesian, Malay and Micronesian languages for a 'canoe', 'ship' or 'boat.' Other parts of a traditional vaka can include the aka and ama .A proa consists of a vaka, the main canoe-like...
  
   - A proa consists of a vaka, the main canoe-like hull; an ama, the outrigger; and akas, the poles connecting the ama to the vaka.

Catamaran

Catamaran

A catamaran is a type of multihulled boat or ship consisting of two hulls, or vakas, joined by some structure, the most basic being a frame, formed of akas...

s and trimaran

Trimaran

A trimaran is a multihulled boat consisting of a main hull and two smaller outrigger hulls , attached to the main hull with lateral struts...

s share the same terminology, with a vaka, ama, and aka.

Semantically, the catamaran

Catamaran

A catamaran is a type of multihulled boat or ship consisting of two hulls, or vakas, joined by some structure, the most basic being a frame, formed of akas...

 is a pair of Vaka held together by Aka, whereas the trimaran

Trimaran

A trimaran is a multihulled boat consisting of a main hull and two smaller outrigger hulls , attached to the main hull with lateral struts...

 is a central Vaka, with Ama on each side, attached by Aka.

Fundamental departure in concept

The differences between monohull

Monohull

rightA monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another.-Fundamental concept:...

s and multihulls are due to a fundamental difference in their underlying design principles, which can even be traced back to the days of the dugout canoe.

Going to the basic round log; logs are generally unstable and they tend to roll over (capsize) without too much provocation. Hollowing out this log, into a dugout canoe, doesn't help this much, unless the hollowed out section penetrates below the log's center of buoyancy. Any load carried by the dugout then actually stabilizes the craft.

Weight stabilized (monohull)

It is in finding the solution to this basic problem that highlights the difference in fundamental principles. Western European cultures expanded on this concept by filling the bottoms of their watercraft with rocks and other ballast. This can be traced back to the Romans, Phoenicians, Vikings, and even further, as a common fundamental concept. Even modern powered Ocean liners carry tons of ballast, in order to maintain their stability. Naval architects go to great lengths to assure that the center of gravity of their designs remains substantially below the metacenter

Metacentric height

The metacentric height is a measurement of the static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre . A larger metacentric height implies greater stability against overturning...

. This can only be achieved by adding weight, or ballast. The low centre of gravity acts as a counterweight as the craft rotates around its centre of buoyancy, creating a restorative force as the craft deviates from its stable position.

Geometrically stabilized (multihull)

The Polynesians approached the problem in a completely different way. They discovered that two round logs tied together don't roll, or capsize, as easily as a single log. With the addition of more logs, one creates a flat raft, which is extremely stable. To add buoyancy one simply hollows out the logs, at no sacrifice in stability. However, this requires a lot of work and it has other problems such as increased drag and weight. Europeans also went through the raft stage.

Separating the two logs by a pair of sticks, called Akas

Aka (sailing)

The aka of a multihull sailboat is a member of the framework that connects the hull to the ama . The term aka originated with the proa, but is also applied to modern trimarans....

, one actually increases the stability manifold with no increase in weight and with a lot less overall work (no rocks to haul and no more trees to cut). Cover the intervening distance with a frame and platform creates a craft with almost the same level of stability of a raft and same performance. In addition to this, one can carry a tremendous amount of cargo without being bothered by having to expend a large part of cargo allowance in ballast.

From this basis, came the proa

Proa

A proa, also seen as prau, perahu, and prahu, is a type of multihull sailing vessel.While the word perahu and proa are generic terms meaning boat their native language, proa in Western languages has come to describe a vessel consisting of two unequal length parallel hulls...

, catamaran

Catamaran

A catamaran is a type of multihulled boat or ship consisting of two hulls, or vakas, joined by some structure, the most basic being a frame, formed of akas...

, and trimaran

Trimaran

A trimaran is a multihulled boat consisting of a main hull and two smaller outrigger hulls , attached to the main hull with lateral struts...

, plus all the other various outriggers seen throughout the Pacific. Even the Chinese Junk

Junk (ship)

A junk is an ancient Chinese sailing vessel design still in use today. Junks were developed during the Han Dynasty and were used as sea-going vessels as early as the 2nd century AD. They evolved in the later dynasties, and were used throughout Asia for extensive ocean voyages...

 was built on these concepts, being a derivative of the ancient cargo raft.

Design summary

On one hand is a round log, with a long attachment, and a great weight at the end, hanging straight down from the log, into the water. Unfortunately, it cannot be parked too close to the shore because of the long arm at the bottom. On the other hand are two round logs, separated by a framework that carries a platform; two simple craft and two entirely, almost mutually alien, concepts of design and construction.

Balance is achieved in both designs. Essentially, both craft are stable. The single round log achieves this by being careful that the center of gravity remains well below the center of buoyancy at all times. This results in a narrow craft that has a deep draft and a fair amount of weight below the waterline. The other is a beamy craft, whose center of gravity may even be slightly above its center of buoyancy but, achieves its stability purely by the geometry of its wide stance on the water, to keep its rollcenter below its center of gravity. The difference is between balancing on one foot, using a heavy balance bar, verses standing in a wide two-footed stance.

(The following two statements are simplified and intended to highlight the fundamental difference between the two approaches.)

• Monohull
  Monohull
  rightA monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another.-Fundamental concept:...
  
  s are stabilized by keeping their center of gravity well below their center of buoyancy. This is done by adding ballast weight.
• Multihulls are stabilized by keeping their rollcenter below their center of gravity. This is done by spreading the Ama
  Ama (sailing)
  The term ama is a word in the Polynesian and Micronesian languages to describe the outrigger part of a canoe to provide stability. Today, among the various Polynesian countries, the word ama is often used together with the word vaka or waka or va'a , cognate words in various Polynesian languages...
  
  s further apart, using longer Akas
  Aka (sailing)
  The aka of a multihull sailboat is a member of the framework that connects the hull to the ama . The term aka originated with the proa, but is also applied to modern trimarans....
  
  .

A monohull

Monohull

rightA monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another.-Fundamental concept:...

 may be stabilized by making it substantially wider than it is tall. However, it is then called a raft, with all the drag problems that rafts have.

Advantage of multihulls

• Multihulls are inherently substantially faster than monohulls, because the absence of ballast reduces their weight and the amount of drag through the water considerably (see hull speed
  Hull speed
  Hull speed, sometimes referred to as displacement speed, is the speed of a boat at which the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag...
  
  ). The waterline
  Waterline
  The term "waterline" generally refers to the line where the hull of a ship meets the water surface. It is also the name of a special marking, also known as the national Load Line or Plimsoll Line, to be positioned amidships, that indicates the draft of the ship and the legal limit to which a ship...
  
   to width ratio is larger, allowing the thinner hulls to be driven through the water at higher speeds, as each works somewhat independently of the other(s).

• Multihulls have a shallower draft which enables shallower waterways to be utilised, along with shallower moorings. It also reduces the risk of underwater collision.

• The stable platform of a multihull makes seasickness less likely, reduces the fatigue of 'walking on walls' and increases safety due to stability in a seaway. The stable platform is wide enabling deck area to be effectively utilized for solar panels and fresh water collection. By not heeling, instruments that require a level platform work more effectively (e.g. radar).

• Duplication of systems enables backups should failures occur. In a catamaran (the most popular multihull), most have twin engines and thus almost always a way of getting home. Many essential items are able to be duplicated e.g. water tanks, fuel tanks and toilets.

• Most multihulls are positively buoyant. Should hull penetration occur the vessel is unlikely to sink thus enabling time for rescue or repair. In fact, most modern production multihulls are officially rated as unsinkable, by various regulatory agencies around the world.

Disadvantage of multihulls

• The width of a multihull vessel is often an issue, especially when docking. They are also more expensive to produce than a monohull of the same length.

• If a storm or wave capsize
  Capsize
  Capsizing is an act of tipping over a boat or ship to disable it. The act of reversing a capsized vessel is called righting.If a capsized vessel has sufficient flotation to prevent sinking, it may recover on its own if the stability is such that it is not stable inverted...
  
  s a small monohull, it may recover when the weight of the ballast in the keel rights the boat, if it does not broach
  Broach (sailing)
  A sailboat broaches when its heading suddenly changes towards the wind due to wind/sail interactions for which the rudder cannot compensate. This causes the boat to roll dangerously and if not controlled may lead to a capsize...
  
   and sink before it recovers. It is difficult to right a multihull and the larger ones could even require the use of a crane.

• The inherent inertia of a monohull
  Monohull
  rightA monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another.-Fundamental concept:...
  
   will help drive the vessel even after the wind has dropped. This will aid a monohull to drive through ocean waves. Were as a lighter Multihull relies more on the wind for drive. For example, monohulls can power through waves that a multihull would be forced to ride over. This means that multihulls are more prone towards hobby horsing especially when lightly loaded and of short overall length.

• Smaller differences in payload make a larger difference in performance.

Popularity

Multihulls are quite popular for racing, especially in Europe and Australia, and are somewhat popular for cruising in the Caribbean and South Pacific. They appear less frequently in the United States, although they are gradually becoming more popular, in Florida and the Caribbean. Until the 1980s most multihull sailboats (except for beach cats) were built either by their owners or by boat builders on a semi-custom basis. Since then several companies have been successful selling mass-produced (by boat industry standards) boats.

Popular multihulls

There are many types of multihulls in different categories. Among the small sailing catamarans, also called beach catamarans, the most recognized racing classes are the Hobie Cat 16, Formula 18 cats, A-cats and the ex Olympic multihull class called Tornado

Tornado (sailboat)

The Tornado is an two person multihull class recognised as an International Class by the International Sailing Federation. It was used for the Olympic Catamaran discpline for over 30 years.-Background:...

 and the recent popularity of the Weta trimaran

Weta trimaran

The Weta is a 14 foot sailing Trimaran designed in New Zealand by Roger and Chris Kitchen. The Weta is a one-design boat sailed internationally. It is recognised as one design class by Yachting New Zealand. The boat is constructed from fibre glass and carbon fibre, and is popular as a racing boat...

 (NZ).

Pure power catamarans are becoming a common sight in international charter fleets in the Caribbean and Mediterranean. A new breed of catamarans has also now started to take shape in the form of the mega or super catamaran

Catamaran

A catamaran is a type of multihulled boat or ship consisting of two hulls, or vakas, joined by some structure, the most basic being a frame, formed of akas...

. This definition is reserved for those catamarans over 60 feet in length. It usually takes one year to build these huge vessels and often a large amount of customization takes place at the request of the owner who commissions the vessel.

Larger boats include Corsair Marine (mid-sized trimarans), and Privilege (large, luxurious catamarans). The Seawind, Perry, Lightwave production catamarans from Australia, The largest manufacturer of large multihulls is Fontaine-Pajot in France (the much larger French trimarans of the ORMA racing circuit and round the world record attempts are included in this total).

In the powerboat part of the multihull spectrum we find a range of boats from small single pilot Formula 1 power boat series to the large multi-engined or even gas turbined power boats that are used in the off-shore powerboat racing series and that are piloted by 2 to 4 pilots.

Multihull Designers that have made their mark and are considered the pioneers of multihull design and the ones that have made multihulls so popular today are: James Wharram (UK), Derek Kelsall (UK), Loch Crowther (Aust), Hedly Nicol (Aust), Malcolm Tennant (NZ), Jim Brown (USA), Arthur Piver (USA), Chris White (US), Ian Farrier (NZ). There are a number of more recent multihull designers who have created a range of designs popular because of the vessels' proven abilities.

The performance record

Since N. G. Herreshoff

Nathanael Herreshoff

Nathanael Greene Herreshoff I , was an American naval architect-mechanical engineer. "Captain Nat," as he was known, revolutionized yacht design, and produced a succession of undefeated America's Cup defenders between 1893–1920....

, in 1877, many have always been convinced about the superiority of multihulls. In 1978, 101 years after catamarans like Amaryllis were banned from any and all forms of yacht racing history was to prove them right. Starting with the victory of the little trimaran Olympus photo, skippered by Mike Birch in the first Route du Rhum

Route du Rhum

The Route du Rhum is a transatlantic single-handed yacht race, which takes places every 4 years in November. The course is between Saint Malo, Brittany, France and Pointe-à-Pitre, Guadeloupe, France...

. Light and slender Olympus photo seemed to devour the sea, running away from all other types of boats. No great open ocean race would be won by a monohull, ever again. In addition, winning times have dropped by 70%, since 1978, from Olympus photo's 23d 6h 58'35" to Gitana 11's 7d 17h 19'6", in 2006.

See also

External links

• http://www.mocra-sailing.co.uk The Multihull Offshore Cruising & Racing Association
• http://www.catamaran.co.uk/main.htm The UK Catamaran Racing Association
• http://www.mycq.org.au/ The Multihull Yacht Club of Queensland (Australia)
• http://multihullboatbuilder.org Multihull Boatbuilding Information / Community
• http://www.multihull-maven.com Articles and news on multihulls, profiles of boats, designers, yards, etc.
• http://www.sailing.org International Sailing Federation
• http://www.multihulls-world.com/us/ The multihulls reference magazine
• http://www.multihull.com.au The multihulls reference magazine (Australia)