Multihull

Multihull and monohull ship layouts

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

Description

Multihull ships (vessels, craft, boats) comprise a great variety of types, sizes and applications. A very specific sub-group of multihulls is the Small Waterplane Area (SWA) multihulls.^[1] A twin-hull version of this subgroup is commonly known as SWATH (Small-waterplane-area twin hull). At the present time, the main interests in shipbuilding and shipping industries of multihull vessels are focused on engine-powered ships.

Today thousands of catamarans are used as racing, sailing, tourist, and fishing boats. Hundreds (about 70%) of fast passenger and car-passenger ferries are catamarans. About 300 semi-submersible drilling and auxiliary platforms are deployed at sea. Some ships with outriggers are built, including the experimental ship Tritone (UK), and the first and second sister-ships of the series of Littoral Combat Ships (USA). About 70 SWA ships of various purposes and displacement are built.

Their specifics in all aspects of naval architecture are described in detail in a monograph from Multi Hull Ships,^[2] which is a reference source akin to the SNAME's 3-volume Principles of Naval Architecture. According to this reference, the entire breed of multi hull ships can be classified by the number of hulls, by mutual arrangement of the hulls, and by shapes and sizes of the hulls. Joint classification is shown in Fig.1 and Fig.2, showing some arrangements of hulls for conventional and SWA multihulls, both reproduced from Ships with Outriggers.^[3]

Fig.1: Hull Arrangements for Multihull Ships
1) catamaran: two identical conventional hulls
2) catamaran with non-symmetric hulls
3) trimaran: three identical symmetric hulls
4) trimaran with flat outer boards of side hulls
5) catamaran with shifted hulls
6) proa or a vessel with bigger main hull and smaller side hull (an outrigger)
7) ship with outriggers, which is a triple-hull ship with its side hulls being significantly smaller than the central hull.

Fig. 2: Hull Arrangements for Small-Waterline-Area Multihull Ships
1) twin-hull SWA ship; each hull consists of the under-water volume (gondola or under-water volume or pontoon) and one long strut; the type is named after the first built ship: duplus
2) twin-hull SWA ship with two short struts on each hull; the type is called trisec
3) triple-hull SWA ship with identical hulls; called a tricore
4) SWA ship with (two) outriggers
5) a vessel with a conventional central hull and SWA outriggers
6) a monohull SWA vessel with foils

Each type of multihull differs from the other and from the monohulls. For example, a catamaran has the biggest transverse stability of all ships of the same displacement. A duplus has the biggest transverse stability of all SWA ship types. A trisec can have the smallest waterplane area of all multihulls and monohulls of the same displacement. A catamaran with longitudinally shifted hulls and any triple-hulled ships can ensure the most favorable interaction of wave systems generated by the vessel at speed. A ship with outrigger(s) (proa or trimaran in Western sense) can have the smallest relative mass of hull structures.

Engine-powered multihull terms

Two-hull design

Sometimes, the term catamaran is applied to any ship or boat consisting of two hulls. A deeper understanding requires detailed terminology. The other twin-hull ships are proa, duplus, and trisec.

The history of commercial catamarans began in 17th century England. Some separate attempts at steam-powered catamaran application were carried out by the middle of the 20th century. However, such ships needed better materials and more developed hydrodynamic technologies for successful application. The second half of the 20th century was the time of wide and largely successful application of various catamarans on the base of developed science.

Three-hull design

Sometimes the term trimaran is used for any triple-hull ships or boats. A deeper understanding of ship type characteristics requires more detailed definition the types. Hereafter the term trimaran is used only for a ship or boat with three identical hulls of traditional shape. The other triple-hull ships or boats are outrigger ship and tricore.

The main trait specific to a trimaran is that it has the widest range of possible interactions of wave systems generated by hulls at speed. The interactions can be favorable or not favorable ones, depending on relative arrangement of hulls and relative speed. Today no authentic historical trimarans exist. Only model test results and corresponding simulations provide estimates on the power of the full-scale ships. The calculations show big enough possible advantages at a defined band of the relative speeds.

A new type of super-fast vessel, the wave-piercing trimaran (WPT) is known as air-born unloaded (up to 25% of displacement) vessel, which can ensure twice the speeds achievable today with a permissible relative power.

Outrigger configurations

Proa is a ship or boat from one hull and one outrigger (small side hull). The engine in the main hull is more convenient, but not obligatory.

A ship with one main hull of conventional shape and two small side hulls (outriggers) is called an outrigger ship. The same type is identified as trimaran in English language publications.

Small waterplane area design

Hulls with beam designs that are narrower at the waterplane level than below can be classified as hulls with decreased or small waterplane area. More often the term small waterplane area hull means a hull with an underwater volume (gondola) and one or more thin strut(s), which connect the gondola to the above-water platform. Any ship or boat consisting of hulls with small waterplane area can be identified as small waterplane area ship or SWA ship.

Any twin-hull SWA ship is called a SWATH. A SWATH ship or boat with one long strut on each hull is specifically called a duplus (named after the first drilling ship of the type). Today the duplus is the most common type of SWA ships built.

The term trisec is specifically used for a SWATH ship with two struts on each gondola. A trisec can have a minimal area of waterplane and, therefore, have minimal motions in waves, resulting in higher effectiveness of motion control.

A triple-hull SWA ship is called a tricore, regardless of the number of struts on each gondola. The term is used for a ship or boat with three identical hulls with small waterplane area. There are no built tricores, but towing tests of models show the possibility of sufficient advantage from a power point of view in defining band of the relative speeds.

An outrigger ship can have a main hull and/or outriggers with small waterplane area.

Mutual arrangement of hulls

The hulls are connected by the above-water structure called the platform or above-water bridge. The structure can be watertight partially or as a whole, or can consist of separate frames.

The distance between the hulls is called the transverse clearance and can be measured by various methods: between center planes of hulls or between the inner boards.

The distance between the design waterplane and the bottom of the above-water platform (wet deck) is called the vertical clearance.

The longitudinal distance between a triple-hull ship is called the longitudinal clearance and can be measured between the middle of the hulls or between their fore perpendiculars.

Characteristics of multihulls

The variations in geometry define some sufficient differences in ship characteristics. Some differences ensure some technical and exploitation advantages and disadvantages for ships of various purposes.

All multihulls have the following common traits:

• Widest possibility of geometry variations.
• Relatively bigger area of decks (upper and inner ones) than in the comparable monohulls.
• It means the possibility of relatively bigger inner volume for the watertight above-water platform, which connects the hulls.
• Independence of transverse stability from single hull width; the stability strongly depends from the transverse clearance.
• It means a possibility of simple enough ensuring of transverse stability of any multihull type.
• It means the possibility of a wide variation of hull lengths for higher performance.
• Higher non-sinkability because of big enough volume of the above-water platform.
• Higher safety and survivability of energy sources, because distributed power engine components are distant each from the other.
• Relatively bigger overall beam.
• More or less higher seaworthiness (depending on the multihull type) in comparison with the monohull of the same displacement.

All ships with small-waterplane area (SWA ships) have smaller motions of any kind, with less external wave forces and moments, except in the case of sailing in following waves. All systems of motion mitigation are most effective in SWA ships.

Advantages and disadvantages of multihulls

As follows from Multi Hull Ships,^[2] the specificity of multi-hulls can be an advantage for defined purposes, as below.

• The greater range of geometric variability allows to designers wide possibilities of characteristic optimization in accordance with practical needs.
• The possibility of lower wave-making resistance (as a sufficient part of towing resistance ) is a sufficient advantage for high-speed and fast ships.
• The towing resistance can be significantly decreased by a rational selection of hull aspect ratio (twin-hull ships) and arrangement of hulls to take the most benefits of the interference between the wave systems generated by each hull (triple-hull ships).
• Significantly greater area of decks and volume of closed (including watertight) and open spaces for payload; as a result, a multi-hull vessel can be a preferred choice for shipping high-volume and light-specific-weight payload (passengers, automobiles, etc.).
• Transverse stability is independent of the aspect ratio of any hull, but depends on the transverse distance between the hulls at degree two; as a result, the transverse stability can be comparable with longitudinal stability, or even exceed it in some cases; it means wider possibility of dimension optimization without any restrictions by the transverse stability.
• A possibility of increasing the non-sinkability of the vessel by providing watertightness to the volume of the above-water platform and its rational dividing by water-tight bulkheads; it means higher safety of all multi-hulls in a comparison with the monohulls.
• Higher safety and survivability of power plant machinery due to greater variability of the safest accommodation of its main components; it means higher survivability of all multi-hulls, especially of combat ones.
• Higher seaworthiness means higher part of time of sailing, when all needed demands of passenger and crew comfort are fulfilled.

The main disadvantages of multi-hull ships, as compared to monohulls, are the following:

• The wider overall beam of a multihull vessel is often a problem for conventional docking.
• The relatively greater total wetted area increases the frictional component of water resistance of the vessel, and thus the total resistance at low speeds.
• The weight of hull structures per ton of displacement is greater than that of monohulls.
• Assembly of a multi-hull vessel in a shipyard takes a wider area; as a result, the construction cost and maintenance (in dry dock) can be greater.
• All multihulls are less applicable ships in ice than monohulls.

The main problem of designing is the restriction of all disadvantage results and full application of advantages.

Sailboats and small vessels

Multihull sail boats and small craft 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.^[4]

Multihull sail boats and small craft's 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.^[5]

• Aka^[5] - 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^[5] - The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka^[5] 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^[5] - 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.^[6]

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 of sail boats and small craft

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. 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, 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, versus 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 is then called a raft, with all the drag problems that rafts have.

Advantage of multihull sail boats and small craft

• 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 multihull sail boats and small craft

• 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 will help drive the vessel even after the wind has dropped. This will aid a monohull to drive through ocean waves. Where 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, New Zealand 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 and the recent popularity of the Weta trimaran (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. 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. Perhaps the best known builder of these more luxurious vessels is Pachoud Yachts from New Zealand.

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), LOMOcean (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, in 1877, many have been convinced about the superiority of multihulls.^{[citation needed]} In 1978, 101 years after catamarans like Amaryllis were banned from any and all forms of yacht racing^[7] 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

Notes

References and Bibliography

• Jim Howard, Charles J. Doane. Handbook of offshore cruising: The Dream and Reality of Modern Ocean Cruising. Sheridan House, Inc. p. 280. ISBN 1-57409-093-3. 
• C. A. Marchaj. Aero-Hydrodynamics of Sailing. Tiller Publishing. ISBN 1-888671-18-1. 
• C. A. Marchaj. Sail Performance. McGraw Hill. p. 400. ISBN 0-07-141310-3. 
• C. A. Marchaj. Seaworthiness:The Forgotten Factor. Tiller Publishing. p. 372. ISBN 1-888671-09-2. 
• Gavin le Sueur. Multihull Seamanship. John Wiley & Sons. p. 144. ISBN 1-898660-31-X. 
• Harvey, Dereck, Multihulls for Cruising and Racing, Adlard Coles, London 1990, ISBN 0-07136-6414-2

External links

Wikimedia Commons has media related to Multihulls.

• 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.
• Multihulls designer & builder
• 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)