Multihull

A multihull is a ship, vessel, craft or boat with more than one hull.

Multihulls include: Proas, which have two differently shaped or sized hulls with lateral symmetry; catamarans, which have two hulls with longitudinal symmetry; and trimarans, 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 sails. This stability is provided in multihulls by the weight of the boat itself, in contrast to monohull sailcraft which typically use an underwater counterweight, a ballasted keel 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's "Amaryllis", also 1988 America's Cup). 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 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.

• Aka - 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 - The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka 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 - 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.

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

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.

Fundamental departure in concept

File:Multihull.svg

Multihull and monohull ship layouts

The differences between monohulls 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. This can only be achieved by adding weight, or ballast.

What you then have, in essence, is a small passenger compartment, at waterline, dangling a long pendulum with a bob at the end. In terms of our round log; we've added a lever arm to one side, with a great weight attached. The log floats lower in the water but it will not capsize.

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, 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, catamaran, and trimaran, plus all the other various outriggers seen throughout the Pacific. Even the Chinese Junk 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.)

• Monohulls 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 Amas further apart, using longer Akas.

A monohull may be stabilized by making it substantially wider than it is tall. However, it's 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). The waterline 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 capsizes a small monohull, it may recover when the weight of the ballast in the keel rights the boat, if it does not broach 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 dampens a great deal of oscillations and other surface effects. 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.

• Multi-hulled vessel may be harder to tack than monohulls.

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 Olympic multihull called Tornado.

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. 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 largest manufacturer of large multihulls is Fountaine Pajot in France. But also the much larger French trimarans of the ORMA racing circuit and round the world record attempts are included in this.

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.

The performance record

Since N.G. Herreshoff, in 1877, many have always been convinced about the superiority of multihulls. In 1978, hundred and one years after catamarans like Amarylis were banned from any and all forms of yacht racing^{[broken footnote]}, 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. 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

• Catamaran
• Trimaran
• monohull
• Monocat
• Sailing
• Round Texel
• International C-Class Catamaran Championship
• International Catamaran Challenge Trophy

References and Bibliography

• Jim Howard, Charles J. Doane. "Handbook of offshore cruising: The Dream and Reality of Modern Ocean Cruising". Sheridan House, Inc.. pp. 280. http://books.google.ch/books?id=NB4uFQuUlnEC&hl=en. 
• C. A. Marchaj. "Aero-Hydrodynamics of Sailing". Tiller Publishing. http://books.google.ch/books?id=KYUDAAAACAAJ&dq=Aero-hydrodynamics+of+Sailing&lr=lang_nl%7Clang_en&hl=en. 
• C. A. Marchaj. "Sail Performance". McGraw Hill. pp. 400. http://books.google.ch/books?id=Z7JIAAAACAAJ&dq=Sail+Performance&hl=en. 
• C. A. Marchaj. "Seaworthiness:The Forgotten Factor". Tiller Publishing. pp. 372. http://books.google.ch/books?id=uLz2IAAACAAJ&dq=seaworthiness&hl=en. 

External links

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• 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://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