Multihull

From Wikipedia, the free encyclopedia

A multihull is a ship or boat with more than one hull. The additional hulls provide stability, typically to hold the vessel upright against the sideways force of the wind on the sails. This is in contrast to monohulls which typically use a ballasted keel for this purpose, especially on larger sailboats.

Multihulls include: proas, which have two differently sized hulls; catamarans, which have two similar hulls; and trimarans, which have a larger hull in the center and two smaller ones on either side. Multihull sailboats are typically much wider than the equivalent monohull, which allows them to carry no ballast, so they are typically faster than monohulls under equivalent conditions (see Nathanael Herreshoff's "Amarylis", also 1988 America's Cup). It also means that multihulls are less prone to sink than monohulls when their hulls are compromised. There are also multihull powerboats, both for racing and transportation.

... 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.[1]

Contents

[edit] Multihull component terms

There are three terms that describe the components of modern multihulls. 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.[2]

  • Aka[2] - 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[2] - The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka[2] 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[2] - 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. [3]

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.

[edit] Fundamental departure in concept

The differences between monohulls and multihulls are due to a fundamental difference in their underlaying 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.

[edit] 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, Phonecians, 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 center of buoyancy. 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.

[edit] Geometery 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 monohull. With the addition 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 increased drag and weight. Europeans also went through the raft stage.

Back to our two logs, separating them 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 expand 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.

[edit] 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.

[edit] 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 utilised 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 usually 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.

[edit] 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. Modern multihulls are not self-righting and can capsize but they don't sink; instead they float upside-down, providing a raft for the survivors to hang on to, that is much easier to find than a liferaft or single individual in a survival suit[4]. 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 ocillations 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.

[edit] Myths and their rejoinders

All the bad press and publicity against multihulls have been political, with not a shred of real science behind it from the very beginning:

While AMARYLLIS won easily boat to boat, she was protested by several of the competitors and subsequently ruled out, the prize being given to the PLUCK AND LUCK. At that time, the papers called AMARYLLIS a life raft and several things, but created all at once an interest in catamarans, so that during the next ten years there were about twenty of them on the Hudson River and the head of Long Island Sound. However, their popularity was short lived, principally because they were barred from all the regular classes, although the Newburgh Bay Yacht Club ran special classes for catamarans for a few years.[5]

It should be noted that N.G.HERRESHOF built and patented the very first modern Catamaran Amaryllis and the second one Tarantella (US Pat No. 189,459, 10Apr1877). They outclassed their competition so badly that the other yachtsmen were faced with the instantaneous obsolescence of their very expensive toys, plus having to learn a completely new technology paradigm. In their mind, suppressing the technology, in any way they could, was the preferred option. It worked, for over 100 years.

[edit] Windward performance

Some claim, to this very day, that multihulls sail poorly against the wind.

It is true that the enormous disparity of speed between the catamaran and an ordinarily built boat is most noticeable when sailing with the wind a little abaft of beam. Sailing to windward is a paradox at best, and a small amount thus gained is a greater triumph than much greater distances gained in the headlong, free wind sailing. Windward sailing is not a weak point of the catamaran. I can, with a good whole sail breeze, beat to windward faster, by a mile an hour at least, than any sailing vessel afloat, or I can beat the WM. R. BROWN, the WM.T.LEE, the SUSIE S., DARE DEVIL, or any other boat of that class that can be named, one-quarter, or five miles to their four, under the conditions before mentioned. I'm not making an idle, empty boast. I know well of what I am writing. I have sailed every class of vessel, from the small cat-boat up to the first-class yacht, and their performances are individually familiar to me. And further, if the owners of the boats whose names I have mentioned, want to be practically convinced of this, that is, of the speed of the windward sailing of the catamaran, the best way for them is to try it on. I shall be only too happy to do so anywhere and at any time.[6]

[edit] Pitchpoling

And now, whilst we are flying along, with the waves lifting and breaking high under the after tie-beam, let us overhaul another of the alleged failings of the catamaran, to wit: their tendency to turn over endwise or pitchpole. Now, the center of effort of the sails of the TARANTELLA is 14 '6" above the waterline. With the wind abaft of beam, the tendency to bury the bows of the hull is quite obvious. This desire to bury forward is corrected, in a measure, first by having more than an ordinarily large jib, which, on account of its inclined position, lifts strongly that part of the boat. Then the midship link, at which point is imparted most of the press of the sails upon the leeward boat, is so placed in relation to the displacement of the hulls that the downward push (to which the force of the wind on the sails is resolved) presses more toward the stern, so the leeward boat always keeps in good fore-and-aft trim. The trouble then lies only in the lifting of the stern of the windward hull. Of course, if you lift the stern of the boat, and thus make the bow bury itself, the effect is just the same, and just as unpleasant as when the bow sinks for want of buoyancy with the trim of the stern where it should be.[6]

Of course, the modern answer to this is to have huge crash compartments in the bows of both Vaka and Ama, distributing the weight towards the stern.

[edit] 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.

[edit] 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 cirquit 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.

[edit] 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[5], 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.

[edit] See also

[edit] Notes

  1. ^ Handbook of offshore cruising: The Dream and Reality of Modern Ocean Cruising (English).
  2. ^ a b c d e A primer on proas. Retrieved on 2007-10-30.
  3. ^ The Tridarka Raider. Retrieved on 2007-10-30.
  4. ^ A fact borne out multiple times in Open Ocean multihull racing, with numerous rescues of crews, from capsized craft, in the same races that sank monohulls.
  5. ^ a b L. Francis Herreshoff. The Spirit of the Times, November 24, 1877 (reprint) (English). Marine Publishing Co., Camden, Maine.
  6. ^ a b N.G.HERRESHOF. The Catamaran Chronicle, Bristol, R.I. 1. Nov. 10, 1877 (reprint) (English). Marine Publishing Co., Camden, Maine.

[edit] References and Bibliography

[edit] External links

Wikimedia Commons has media related to:
v • d • e
Classes and types of catamaran, trimaran and multihull (worldwide list)
A-Catamaran • ARC 17 • ARC 21 • ARC 22 • Astus 14.1 • Astus 16.1 • Astus 20.1 • Beachcat • Sprint 15 • Dart 16 • Dart 18 • Dragonfly 800 • Formula 16 • Formula 18 • Hobie • Mystere • Proa • RC 27 • RC 30 • Taipan 4.9 • Tornado • Open Ocean Performance Sixties • Unicorn
v • d • e
Types of sailing vessels and rigs
Barca-longa · Barque · Barquentine · Bermuda rig · Bilander · Brig · Brigantine · Caravel · Carrack · Catamaran · Catboat · Clipper · Dutch Clipper · Cog · Corvette · Cutter · Dhow · Dinghy · Felucca · Fifie · Fluyt · Fore & Aft Rig · Frigate · Full Rigged Ship · Fusta · Gaff Rig · Galeas · Galiot · Galleon · Gunter Rig · Hermaphrodite Brig · Herring Buss · Hoy · Jackass-barque · Junk · Ketch · Longship · Lugger · Mast Aft Rig · Mersey Flat · Multihull · Nao · Norfolk Punt · Norfolk Wherry · Pink · Pinnace · Pocket Cruiser · Polacca · Pram · Proa · Punt · Sailing hydrofoil · Schooner · Ship of the Line · Sixareen · Sloop · Sloop-of-war · Smack · Snow · Square Rig · Tall Ship · Thames Sailing Barge · Trailer sailer  · Trimaran · Vinta · Wherry · Windjammer · Windsurfer · Xebec · Yacht · Yawl · Yoal