MULTIHULLS

Vessels with more than one hull are termed multi-hulled

A multihull is any vessel with more than one hull. Additional hulls provide stability to hold a vessel upright, most notably against the sideways force of the wind from sails. By contrast monohulls use a keel and/or ballast to counteract wind induced heel.

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. 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, 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.

Multihulls include: proas with two differently sized hulls, catamarans, and trimarans, which have a larger hull in the center and two smaller ones on either side. 

Multihull sailing boats are usually very much wider than a monohull of similar draft, hence need no ballast. The reduced hull drag makes them faster than monohulls that may be raced in the America's Cup and Volvo ocean races.

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.

POWER

Catamaran and trimaran multihull forms are also used by power-boaters. Pure power catamarans are now a common sight in the Caribbean and Mediterranean as charter fleets. There are also super catamarans, a definition reserved for boats over 60 feet in length. Such designs are now reported widely on websites as www.catamarans.com and www.2hulls.com. Solar Navigator was originally a catamaran, as is PlanetSolar.

NEGATIVES

The width of a multihull can be a problem when docking. They are also more expensive to produce than an equivalent monohull.

It is held that in the open ocean, multihull craft are less safe. If a storm capsizes a small monohull it can in some cases recover, if it does sink. Multihulls can capsize but they rarely sink. Thye are though difficult if not impossible to self right and/or launch deck-mounted liferaft or emergency radio. Pitch-poling is another risk in a multihull, where the bow of the boat dives into a wave so deeply that the stern permorms a forward somersault.

SMALLER POPULAR BOATS

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. Larger boats include Corsair Marine (mid-sized trimarans), and Privilege (large, luxurious catamarans) and the much larger French trimarans of the ORMA racing circuit.

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).

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.

This autonomous SWASH hull design can self-right robotically

NEW Vs OLD

The idea of making faster, stable, shallow-draft boats dates back many centuries to the outrigger canoes, called proas. These were seen by Europeans in 1521 during Magellan’s voyage through Micronesia.  Some if these native craft achieve speeds of 20 knots. They were used on open-ocean voyages thousands of miles in duration.

European boat building traditions were not affected by these revelations. Sir William Petty built a crude 30’ catamaran in 1662, with which he beat all other entrants in a race hosted by the Royal Socieyt. One can just imagine how that was received. Two hundred years later a 25 footer with 30-inch diameter hulls (rigged as a schooner) made an eastward crossing of the Atlantic in 51 days (reduced to 7 days by Ellen MacArthur).

It's strange how progress is held up by traditionalists. When Nathanael Herreshoff thrashed the competition in a New York regatta with a 25ft catamaran called Amaryllis, he was banned from future races. This happened in the automotive world when rear engined cars that beat front engined cars were banned to begin with.

"The nondescript, half-Catamaran, half-Balsa and wholly life-raft constructed by Mr. Herreshoff, of Providence, whether ruled out by the judges or counted in, can justly claim to be the fastest thing of her inches under canvas that floats, and it is doubtful if there are any steamers of her size that could out-speed her in a straight reach with the wind abeam. Whether she is ruled out of this race or not need make but little difference to her owner, as he can justly lay claim to a medal and diploma of the Exposition as presenting the fastest sailing craft in the world: That she is this every one of the many thousand that witnessed her performance yesterday will admit."

Quote: Anon. (R. F. Coffin?). "A Yachting Wonder. Sudden Development of the Fastest Craft in the World. The Reveille, Susie B., Amaryllis and Victoria Win the Second Centennial Regatta." The World, June 24, 1876, p. 2.

In 1937, a Frenchman built a 35’ double canoe in Hawaii and made an epic 264-day voyage back to his homeland via the Indian Ocean. In the late 50s the groundwork was re-laid for the popularity of multihulls by James Wharram, when he crossed the Atlantic in a 23-foot Polynesian-style catamaran that he built for just $420. After this James made a circuit from the Caribbean to New York and to England in a 40-footer named Rongo.  Soon after Arthur Piver made an Atlantic crossing in a simple home built trimaran in the ‘60s.

Since N. G. Herreshoff, in 1877, many have 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. 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.

SAILOR'S CHOICE

In sailing the difference between monohull and multihull is the mechanism used to provide righting moment, to balance heeling moment generated the wind on the sails.

This is equally valid for a solar boat with large flat panel areas.  Where monohulls solve the stability problem with a heavy keel of lead and more hydrodynamic drag; multihulls use mechanical advantage in the positioning of buoyancy away from the centerline of the boat - the resulting leverage (buoyancy thus generated) must equal the force of the wind with something in reserve.  

One advantage of a catamaran we'd not come across: a garage

PERFORMANCE

Without a lead keel, multihulls weighs considerably less than a comparable monohull. Less mass = less wetted surface area to cause water drag as faster acceleration. When a boat moves through the water, it creates a bow wave and a stern wave, and at some point it can get trapped between the two. The length of a boat therefore determines practical speed. This natural phenomenon is known as the boat’s hull speed, or the speed length ratio.  The value can be determined with a simple formula: 1.34 times the square root of the waterline length for a typical boat shape, that becomes less applicable to ultra thin hulls.

Unless a boat has enough power to plane above the water, a monohull is caught in its own wave trough. That said, the length of the waterline may change considerably when a sailboat is heeled over in the wind—many racing hulls are deliberately designed to do this as a sort of rule-bending ruse.

"In a sense, hull speed is related to going supersonic—exceeding the wave propagation limit of the medium you are traveling through.  This is really a statement about how fast you can go before you must push water aside faster than it can get out of its own way."

— Bryan Willman, posting to the “Human-Powered Boats” mailing list, February, 2002

Multihull sailing involves a different set of skills from monohulls where traditional tactics may backfire. Tacking a sail powered multihull can be tricky, as long thin hulls are harder to turn and the lighter hulls have less inertia to carry a boat through if not executed with skill. A solar powered electric vessel avoids the need for such skills.  In the case of the Solar Navigator, the vessel can automatically counter for such situations. PlanetSolar managed to contend with all situations when setting the first solar powered multihull record in May of 2012.

STABILITY

When the wind is blowing from one side or the other multihulls sail flat and monohulls sail heeled over.  Sailing flat is more comfortable for most sailors. Crockery and maps don't slide off work surfaces.  This may reduce the excitement for the purist but here we are talking about stability.

Multihulls, lacking a big lead weight and rounded hull, tend to dance around on the surface, kicked to and fro by waves.  Since they’re also usually moving faster, this can be dramatic, with violent deceleration as the hulls go airborne and then collide with waves. With Solar Navigator and PlanetSolar, this phenomenon mostly concerns the solar panels and avoiding damage and contact with seawater..

Exploration of shallow waters is likely to lead to grounding - monohulls need more care when beaching.

SAFETY

Given the choice, you might prefer to brave the rigors of a capsize  on an inverted trimaran that still carries most of your stores and tools than suddenly find yourself becoming shark bait when your boat sinks out of sight.

Some multihulls are designed to be self righting, such as the Solar Navigator.  However, this design is still under industrial development.  It helps if the crew of a vessel has some underwater experience. Every sailor should undertake some basic diver training.

Any boat can come a cropper with heavy weather.  Successfully surviving a storm is a function of not only the boat, but seamanship, tools, and determination - a quality that is drained in cold inhospitable water.

The Solar Navigator can flood compartments to lower itself in the sea, where the hull acts like a giant sea-anchor.

Monohulls need more care when beaching

CATAMARAN vs. TRIMARAN

With the above in mind it should be clear that if you are looking for performance, you should be considering a multihull.  Then there is the choice between catamaran and trimaran.

Sailing catamarans usually also have two sets of control surfaces: centerboards and rudders. Trimarans may have three hulls, but the outers can be simple floats devoid of complications.  Catamarans with a central rig must be stronger to make up for a weaker structure, unless a boat is set up with two rigs as per Team Phillips, which proved rather more complex than the designers envisaged - even though it performed well before delamination. 

The main hull of a trimaran is a torque tube, with centralized torsion and better load distribution. Trimarans are also less prone to under-wing slamming, since the gaps between floats are narrower.

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.

REFERENCE and LINKS

http://en.wikipedia.org/wiki/Multihull

MYCQ - Multihull Yacht Club Queensland - Australia's leading Multihull club

http://www.catsailor.com/forums/ubbthreads.php - the most active forum page for all small sailing catamarans (beach cats).

http://www.cruisincats.com/ Cruisincats.com - 101 reasons why catamarans are better,faster and sexier than...

http://www.catamarans.com/ World's largest exclusive catamaran site

http://www.supercatamarans.com/ Home of the mega catamaran

http://www.multihull-maven.com/ Articles and news on multihulls, profiles of boats, designers, yards, etc.

http://www.catamaran.co.uk/main.htm

http://www.sailing.org/ International Sailing Federation

A list of different hull designs

http://www.boatdesign.net/forums/multihulls/historical-multihulls-42019-3.html

http://www.f-boat.com/safety/

Multihull

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