A
multihull is a
shipA ship is a large vessel that floats on water. Ships are generally distinguished from boats based on size and passenger capacity. Ships may be found on lakes, seas, and rivers and they allow for a variety of activities, such as the transport of people or goods, fishing, entertainment, public...
, vessel, craft or boat with more than one hull.
Multihulls include:
ProaA proa or prau is a type of multihull sailing vessel. While the word proa just means boat in its native language, the term 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;
catamaranA 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
trimaranA 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
sailA sail is any type of surface intended to generate thrust by being placed in a wind—in essence a vertically-oriented 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
monohullrightA 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
ballastedBallast 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. When sailing vessels carried cargo, it was at times necessary to sail to a...
keelIn 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, the construction is dated from this event, with only the ship's launching considered more...
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 HerreshoffNathanael Greene Herreshoff , 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 CupThe America’s Cup is the most prestigious regatta and match race in the sport of sailing, and the oldest active trophy in international sport, predating the Modern Olympics by 45 years. The sport attracts top sailors and yacht designers because of its long history and prestige...
).
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 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 multihulls. The term
vaka, like the related terms
akaThe 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-History:The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka or iako is the support connecting the two hulls...
, come from the
MalayMalay is a group of languages closely related to each other to the point of mutual intelligibility but that linguists consider to be separate languages. They are grouped into a group called "Local Malay", part of a larger group called "Malayan" within the Malayo-Polynesian branch of the...
and
Micronesian language groupThe Malayo-Polynesian languages are a subgroup of the Austronesian languages, with approximately 351 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 canoeThe 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
canoeA canoe is a small narrow boat, typically human-powered, though it may also be powered by sails or small electric or gas motors. Canoes usually are pointed at both bow and stern and are normally open on top, but can be decked over .In its human-powered form, the canoe is propelled by the use of...
or main hull.
- Aka
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
-History:The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka or iako is the support connecting the two hulls...
- The term ama comes from the proa. The vaka-Origin and use of the term: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. A proa consists of a vaka, the main canoe-like hull; an ama, the outrigger; and...
is the main hullA hull is the watertight body of a ship or boat. Above the hull comes the superstructure and deckhouse. 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 akaThe 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
-Origin and use of the term: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. A proa consists of a vaka, the main canoe-like hull; an ama, the outrigger; and...
- 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
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 trimaranA 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
catamaranA 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
trimaranA 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
monohullrightA 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
metacenterThe metacentric height is the distance between the center of gravity of a ship and its metacenter. The GM is used to calculate the stability of a ship and this must be done before it proceeds to sea. The GM must equal or exceed the minimum required GM for that ship for the duration of the...
. 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.
Geometry 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.
Back to our two logs, separating them by a pair of sticks, called
AkasThe 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
proaA proa or prau is a type of multihull sailing vessel. While the word proa just means boat in its native language, the term proa in Western languages has come to describe a vessel consisting of two unequal length parallel hulls...
,
catamaranA 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
trimaranA 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
JunkA junk is a Chinese sailboat design dating from ancient times and still in use today. Junks were originally developed during the Han Dynasty and were used as ocean going vessels as early as the 2nd century AD. They were further evolved in the later dynasties, and were built and used throughout...
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
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
-History:The term ama comes from the proa. The vaka is the main hull, the ama is the outrigger, and the aka or iako is the support connecting the two hulls...
s further apart, using longer AkasThe 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
monohullrightA 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'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
Hull speed, sometimes referred to as displacement speed, is a rule of thumb used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design...
). The waterlineThe International Load Line or Plimsoll Line , positioned amidships, indicates the legal limit to which a ship may be loaded for specific water types and temperatures. To an observer on the ship the water appears to rise or fall against the hull. Temperature affects the level because warm water...
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 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.
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
Capsizing refers to when a boat or ship is tipped over until disabled. 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 broachA sailboat broaches when its heading suddenly changes towards the wind due to wind/sail interactions for which hull hydrodynamic reactions cannot compensate. This happens when the aerodynamic force on the rig greatly exceeds the hydrodynamic force on the hull, usually due to a sudden increase in...
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. 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
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:...
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.
- 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
TornadoThe Tornado is an olympic class sailing catamaran, with a crew of two. It was in the Olympic Games from 1976 through 2008, when multihulls were deselected for the 2012 games. It was designed in 1967 by Rodney March of Brightlingsea, England, with help from Terry Pierce, and Reg White, specifically...
.
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
catamaranA 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 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. HerreshoffNathanael Greene Herreshoff , 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, 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 RhumThe 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...
. 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
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...
- 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...
- 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:...
- Monocat
A monocat is a type of multihull with a single hull fore and two hulls aft. One could say that it begins like a monohull and ends like a catamaran. Hence the name....
- Sailing
Sailing is the art of controlling a boat with large foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to change the direction and speed of a boat...
- Round Texel
Round Texel is the biggest catamaran race in the world with an annual average of 600 participants.The Ronde om Texel is a regatta for catamarans around the island of Texel in The Netherlands. Round Texel was first organized in 1978. In that year Sigi Lach won in a Hobie 14 in a field of 84...
- International C-Class Catamaran Championship
The International C-Class Catamaran Championship, or ICCCC or I4C has taken over from the International Catamaran Challenge Trophy for C Class catamaran match racing of highly innovative catamarans, predominantly with a wing rig instead of a conventional sail plan.The prior racing scheme...
- International Catamaran Challenge Trophy
The International Catamaran Challenge Trophy is the formal name for the more familiarly known Little Americas Cup and started in 1961 as a match racing series between two C Class catamarans, modelled on the Americas Cup series for yachts...
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://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