MULTIHULLS

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A multihull is a sailing ship 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 keel and/or ballast for this purpose.

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.

DID YOU KNOW?

The catamaran and trimaran multihull forms and also comes in a 'power' flavor with no sailing apparatus. In fact, pure power catamarans are now becoming a common sight in international charter fleets in the Caribbean and Mediterranean. Further, a new breed of catamarans has 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. The birth of these two designs is now being reported widely on sites such as www.catamarans.com and www.2hulls.com.

So far power trimarans are electric.  See Solar Navigator and PlanetSolar.

ADVANTAGES

Multihulls are substantially faster than monohulls, because the absence of ballast reduces their weight and the amount of drag through the water considerably, without reducing the amount of sail that they can carry, and because the waterline to width ratio is so large.

POPULAR USES

Multihulls are quite popular for racing, especially in Europe and Australia, and are somewhat popular for cruising in the Caribbean. They're not seen very often in the United States, although they're gradually getting more popular. 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.

ISSUES

Multihulls' width can be a problem, especially when docking. They are also more expensive to produce than a monohull of the same length.

Unfortunately, it is common wisdom (among monohull sailors, at least) that in the open ocean, multihull craft are unsafe. If a storm or wave capsizes a small monohull, it may recover, if it does not broach and sink. The rigging will probably be severely damaged, but the crew will be able to jury-rig and reach a port. Multihulls can capsize but they rarely sink. Some rescued crews (in races) have reported that they were unable to dismount the deck-mounted liferaft or emergency radio from the mass of broken, submerged rigging under the capsized craft. Another risk in a multihull is the pitch pole, when the bow of the boat buries itself into a wave and the stern flips over putting the boat into a forward somersault; this tends to be more common in smaller racing multihulls. This rarely effects monohull's because they carry more ballast which prevents the tendency to flip, and helps them power through waves.

Proponents argue, with some justice, that no careful captain ever finds himself in capsizing conditions. Most crews that have capsized in the open ocean found it an extremely traumatic event no matter what type of boat they sailed.

Proponents also argue that capsize is only one of many types of catastrophe that can befall yachts at sea. In other types (for example hull compromise by hitting submerged debris) multihulls are substantially safer than monohulls because they don't carry ballast and can therefore float even when severely damaged.

POPULAR BOATS

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.

Larger boats include Corsair Marine (mid-sized trimarans), and Privilege (large, luxurious catamarans). But also the much larger French trimarans of the ORMA racing cirquit and round the world record attemps 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.

NEW AND OLD

The great divide in the nautical world, is between die hard traditional sailors and the technophobes using more than one hull.

Multihulls are to monohulls as recumbent bicycles are to diamond frames, or Mac users to PC owners.  Multihulls are more intriguing and annoying to old-timers.  They also cover a wide quality range from exquisite to junk, but this holds true with many monohulls.  Both multihulls and recumbents attract wizards as well as nutcases, leading to odd alliances against Old Methods.  Both have been banned from sanctioned races after blowing everyone else off the course. As the breed are still relatively undefined, designs have yet to frozen as established standards, but instead show up in ever more radical configurations as marine architects are drawn to the challenge of pushing the envelope. Hence, this is where the design action is.

Of course, multihulls are nothing new. The idea of making faster, stable, shallow-draft boats dates back many centuries.  Single-outrigger canoes, called proas, were first observed by Europeans in 1521 during Magellan’s voyage through Micronesia.  These zippy little native craft routinely achieved blazing speeds of 20 knots, and embarked on open-ocean voyages covering thousands of miles.  Not only were they highly seaworthy boats, but their navigators possessed skills that we are still trying to understand, passed from one generation to the next in song form.  Such incantations integrating celestial observations, clouds, and complex wave patterns to make distant landfall with a level of precision rivaling today’s computerized methods.  As you can imagine, all this was astonishing to sailors on slow lumbering ships that could barely beat to windward and were in danger of breaking up if they ever felt the bottom in any but the most benign conditions.

Nifty though they may have appeared at the time, multihulls didn’t have much effect on European boat building traditions.  Although, a fellow named Sir William Petty built a crude 30’ catamaran (two hulls) in 1662, easily beating all other entrants in a race hosted by the Royal Society and thus annoying them no end.  A couple hundred years later, another creative chap built a most curious vessel with a trio of 25-foot long, 30-inch diameter hulls and rigged her as a schooner. He proceeded to make an eastward crossing of the Atlantic in an unbelievably quick 51 days (it has recently been done in less than 7, but sailing was different back then).  Heads were starting to turn, but when Nathanael Herreshoff soundly thrashed the competition in a New York regatta with Amaryllis, a 25-foot catamaran, he was banned from all future races.  They also banned rear engines racing cars to begin with.

For as long as history is recorded, old thinking has put a damper on creativity:

"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."

Source: 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, and through the next couple of decades there were a few more notable experiments—always accompanied, of course, by the vocal ridicule of the yachting community.  But it wasn’t until the late 1950s that the groundwork was laid for the explosive growth of the field:  James Wharram crossed the Atlantic in a 23-foot Polynesian-style catamaran that he built for $420, then made a circuit from the Caribbean to New York and thence to England in a 40-footer named Rongo.  Shortly thereafter, the legendary Arthur Piver made a crossing in a simple homebrew trimaran, launching a nautical phenomenon that coincided perfectly with the ‘60s.

It was a time of exploring radical lifestyle alternatives, and an alluring meme spread quickly among the counterculture:  one could build a boat with little money and no experience, then homestead the ocean and travel freely.  All that was needed was a set of plans, a stack of plywood, a few buckets of goo, and a bit of scrounged marine hardware—people who had never even sailed a dinghy set up shop in garages and derelict industrial spaces, partying into the night while building their boxy and often fanciful escape pods.  Enclaves of stoned boat-builders popped up in the San Francisco Bay Area and elsewhere, yielding a few beautiful successes, some spectacular failures, and a lot of unfinished projects.

Still, the technology’s time had at last come, and since then the field has gone mainstream.  The fast Hobie and Nacra “beach cats,” low-cost roto-molded polyethylene Wind-Riders and their speedy hydrofoil descendants, exquisite folding trailer-able trimarans from Corsair, big luxury cruising catamarans, and those gonzo “maxi-cats” that race nonstop around the globe and seem to break records every season…there’s no going back to an all-monohull world.  But there are serious design issues to consider, and it’s not hard to build something that sails like a pig or is even prone to disaster.

WHY CHOOSE A MULTIHULL?

What’s the big deal with multihulls, anyway?

The fundamental difference between the two broad classes of boats is the method used to provide righting moment, which balances the heeling moment applied by the wind’s force on the sails - assuming a sailing boat, but equally valid for a solar boat with large flat panel areas.  If you just took a canoe and attached a mast with a big piece of cloth, it would sail downwind just fine… but trying to sail “off the wind” would likely lead to immediate capsize.  Monohulls solve this problem with a heavy keel (usually made of lead to maximize the ratio of weight to surface area and resulting hydrodynamic drag); multihulls solve it with additional buoyancy located away from the centerline of the boat - the resulting lift, or buoyancy of the outriggers (or opposite hull) equaling the force of the wind.  

Catamarans have two hulls connected by some kind of structure, which in large boats can include a spacious bridge-deck; trimarans have a center hull flanked by two smaller ones, supported by crossbeams as per the diagram below.  Obviously, either approach increases complexity and incurs the fabrication costs of more hulls.  Beginning with this, let’s run through the issues that give monohull and multihull advocates an endless supply of material for ongoing argument.

Monohulls, catamarans, and trimarans

COMPLEXITY and COST

A single-hulled sailboat is basically a tub shaped to fit the intended application, a stick poking up in the air with a sail attached, a weight hanging off the bottom to keep it from blowing over, and suitable accommodation shoehorned into place.  This is not a fundamentally complex piece of hardware, although you can spend huge  amounts of money on one. I.e. a hole in the water into which you pour money.  Simulated sailing = standing in a cold shower and tearing up £50 notes - multiply by 10  if you want to simulate racing.

Multihulls, on the other hand, consist of two or three hulls linked by something strong enough to withstand the substantial forces that result from trying to keep a sail upright in a stiff breeze.  I'm sure we've all experienced the considerable forces when carrying a sheet of plywood on a windy day. If not try it. Most sailing rigs are stayed, meaning that they are held in place by the balanced tension between rigging wires that run fore and aft and beam to beam; to keep these from slapping around and coming apart in feisty conditions. The underlying structures have to be extremely stiff.  Some trimarans add additional complexity by being foldable, allowing these relatively light boats (by monohull standards) to be hauled out and towed or placed on a sturdy roof rack.  Small wonder that these things have major geek appeal, but it comes at a stiff price.  All things being equal, a multihull is typically twice the cost of the equivalent monohull.

PERFORMANCE

Without the lead keel, a multihull weighs but a fraction of what a comparably scaled monohull does - translating into less wetted surface to cause drag through the water as well as faster acceleration when motive force is applied.  A heavy boat has to push more water out of the way as it moves (an amount equal to its displacement, to be precise), and this means bigger waves and more friction.  When a boat moves through the water, it creates a bow wave and a stern wave, and at some point it gets trapped between the two and can go faster only with great difficulty; this is known as the boat’s hull speed, or the speed length ratio.  As it turns out, the value can be determined with a simple formula:  1.34 times the square root of the waterline length for a typical boat, although that becomes inaccurate for skinny hulls.  

Unless it has enough power to plane above the water, a heavy monohull is pretty much stuck in its own wave trough (watch an unencumbered tugboat cruising at maximum speed sometime), but it’s important to note that 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 exactly this as a sort of rule-bending hack, and the difference can be substantial.

"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

But multihulls manage to sidestep the whole problem.  Because they are so light, they don’t make very big bow and stern waves, and their long skinny hulls can easily climb out of the little hole in the water.  As such, a 30-foot catamaran can go a lot faster than a 30-foot monohull, even with the same size sail.

But there’s a downside to this lightness: multihulls are more sensitive to overloading than monohulls, since a given mass of cargo represents a much larger percentage of the boat’s overall weight.  Many once-speedy cats have turned into dogs because their owners have stuffed them full of cruising gear.  Even though cruisers may have no interest in racing and would thus discount the importance of this, there are still safety issues.  Fast boats stand a better chance of getting out of dangerous situations, whether crossing a busy shipping lane or outrunning bad weather.  Of course, crashing into something at high speed is worse than doing so at low speed, so as usual there is room for argument.  All design is of course a compromise.

One other comment on speed.  Multihull sailing involves a somewhat different set of skills from monohull handling - traditional methods will backfire.  It’s sometimes smarter, for example, to beat to windward by sailing farther off the wind and taking advantage of the higher boat speed to offset the additional distance traveled, while allowing more successful tacks.  This process, tacking, is fundamental to sailing, and is how you make your way upwind.  Tacking a multihull can be a bit tricky, as long skinny hulls are harder to turn and the lighter weight gives less flywheel inertia to carry the boat through the maneuver if not executed with some skill. You could end up sail flogging as the bows point directly into the wind, when you'll be going nowhere but backwards, slowly.  This is just a matter of skill-development, but some multihulls are notoriously hard to tack (like Hobie cats) and sailors must resort to tricks like grabbing the boom and pushing it into the wind to force the bow around.  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.  Unfortunately, with PlanetSolar as a serious rival, we cannot afford to reveal how.

STABILITY

This is a subjective area, but on one topic there can be little disagreement:  when the wind is blowing from abeam (one side or the other), multihulls sail flat and monohulls sail heeled over.  For most people, the former is much more comfortable; life is easier when things aren’t sliding off tabletops and the cockpit is not tilted at a 45° angle.  Of course, that can be exciting, and the pure visceral rush is part of the appeal of sailing.  We have to be careful not to make value judgments here, lest we get drawn into religious, rather than rational arguments.

But the attitude of the vessel relative to the earth’s surface is not the only comfort issue… the physical motion of the boat is perhaps the most significant factor affecting the pleasure (or pain) of sailing.  Certain kinds of kinesthetic input coupled with uncorrelated visual data can have a rather profound affect on the vestibular system, and some boats have the amazing ability to extract lunch from even the hardiest souls.  Under feisty conditions, the motions of monohull and multihull couldn’t be more different:  the former has a pendulous, rounded sort of character, combining movement in all axes with X, Y, and Z translations—but it does it with a sort of measured grace, low-pass filtered by the mass of the keel.  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 at times be dramatic, with violent deceleration as the hulls go airborne and then collide with waves.  “Underwing slamming” is an issue with both catamarans and trimarans, where waves rise between the hulls and slap the supporting structures (which, in larger boats, are likely to contain accommodations, and thus humans who may be trying to rest).  

With Solar Navigator and PlanetSolar, this phenomenon mostly concerns the solar panels.

DRAFT

Because of their keels, monohull sail boats extend deep below the surface of the water - sometimes many feet.  This makes them more likely to hit the sea bed, an event whose impact can range from embarrassing to catastrophic, and generally limits them to dredged channels and waters that are known to be safely deep.  In tidal zones or those prone to silting, this can require considerable attention.

Of course, finding oneself “aground” is a bad thing in any boat, and just because the draft, or depth, of a multihull is considerably less doesn’t mean that a captain can ignore the problem.  Some multihulls have dagger-boards and rudders that are actually more fragile than the big lead keel of a monohull, so there’s no reason to get cocky here. Grounding is grounding, and something will probably break.

A ' shoal draft' (not draught) allows exploration of 

shallow waters with less chance of grounding

But the good news is that shoal draft (an inexact term referring to an amount of draft that allows entering shoal, or shallow, waters) is amazingly liberating.  Many of the most interesting places to explore are near shore or otherwise in thin water, and multihulls are generally better suited to such poking around, a pursuit known as gunkholing. This greatly reduces the need for a dinghy for every trip to shore, and wee boatlets like Microships are easily beachable… as long as we don’t get careless and land on sharp rocks or forget to pay attention to tidal cycles and find ourselves stranded until the next flood.

BEAM

But what multihulls lack in depth they make up for in width, although that can be convenient in the case of a solar powered boat where a generous beam provides space for relatively huge solar arrays.  This is why catamarans of cruising scale have palatial accommodations compared to monohulls of comparable length.  But, this comes at a price because wide boats are harder to park, more difficult to tack, and less likely to fit in tight little spots.  The “parking” issue is particularly significant, and the larger cruising-scale multihulls are usually restricted to relatively scarce end ties at marinas - those open spots at the very end of each dock.  Marina operators recognize the rarity and convenience of these spaces and often charge more for them, yet they are exposed to more wakes from passing traffic.

SAFETY and CAPSIZE

This is perhaps the biggest issue—the one that always dominates arguments between proponents of the different broad categories of boats.  Ask any monohuller what she thinks of multihulls, and you will probably hear, “well, they can capsize.”  This is true.

A catamaran or trimaran has two stable states:  right side up or upside down.  Being in the latter state is not much fun.  On a tiny boat like a Hobie Cat or Microship, there is actually some hope of re-righting without outside help (if you’re thinking clearly and not flailing around in hypothermia-inducing waters while fumbling with boat parts), but anything much larger is pretty much stuck in the turtled position until a power boat happens along to help “tow it over” or drag it ignominiously to the safety of land, the rig scraping bottom or lost entirely, topsides trashed, naked hulls gleaming in the sunlight and glinting in the eye of every monohull sailor in sight who is then obliged to nod sagely and observe, “yep, now that’s the problem with those things.”  

There have even been some gripping sea stories of life aboard a capsized multihull, such as the Rose-Noëlle that drifted around the South Pacific upside-down for 119 days while her crew clung to survival - at last breaking up on a rocky Australian shore while the scraggly humans swam for safety.

The less-publicized other side of this grim reality is that monohulls also have two stable states:  floating on the surface of the sea or lying on the bottom.  There are dozens of books (most notably Callahan's excellent Adrift) that relate epic survival in a life raft after a collision at sea holed a trusty sloop that sank within minutes, dragged down by thousands of pounds of lead.  Given the choice, you might prefer to fight for your life 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 below you, taking everything with it.

Some multihulls are designed to be self righting, such as the Solar Navigator.  However, this design is still an industrial secret.  It certainly helps if the crew of a vessel has some underwater experience in any of the situations described here, and I'd recommend every sailor undergoes some basic diver training.

There’s another safety issue here that affects monohulls and multihulls more or less equally, dealing with heavy weather.   Sailors vastly more experienced than I have compiled entire books on the subject and successfully surviving a storm is a function of not only the boat, but seamanship, tools, and luck.  Any boat can be trashed by some combination of wind and wave, and although you may be able to tip the odds with knowledge and preparation, sometimes it’s better to not be there in the first place.  To this end, multihulls might have the advantage since they can escape more quickly, but evil conditions can materialize with frightening speed and it would be arrogant to suppose that there’s always a way out.

One of the saddest sights ever seen is a pile of boats stacked like cordwood in the aftermath of an east-coast hurricane, hulls gored, rigs snarled, dreams shattered.  Hauling out involves large machines and facilities, and in most places isn’t even an option - skippers must instead scramble to deploy lines, anti-chafing gear, and ground tackle, while fervently hoping that their marina neighbors have done as well. Then one yacht breaks loose and is driven before the storm into the others, breaking them loose, until the whole snarled mass of lead and fiberglass ends up being ground to bits on the granite riprap of a breakwater.  Multihulls don’t fare any better in such conditions and some even become airborne, landing in a broken heap on shore.  Force 10 winds and 20-foot shore breaks atop a wicked storm surge are just too big to fight.  Solar Navigator can flood compartments to lower itself in the sea, where the hull acts like a giant sea-anchor.

Assuming we aren’t stupid enough to be bobbing along some vast exposed rocky coast, blithely ignoring weather broadcasts as heavy weather looms, most captains should be able to avoid nasty conditions.  The advent of weather faxes and other modern communications is certainly a help here, provided you have the power on-board to operate such equipment.

So you can see that the whole multihull Vs monohull argument is rife with trade-offs and matters of personal preference.   

Monohulls need more care when beaching

CATAMARAN vs. TRIMARAN

Weighing the advantages, it’s clear that a performance ship needs to be a multihull.  The question remains whether it should be double or triple hulled.

While building two hulls seems initially more appealing than building three, small trimarans have much more useful space than, for example, small “beach cats.”  The latter consist of hulls that are too narrow for comfort, requiring that the accommodations be built atop the bridge-deck that connects them.  The converse is also true:  for a given length, large cats tend to have more room than large trimarans considering only conventional designs.

Sailing catamarans usually also have two centerboards, two rudders, and too much redundancy. Of course, trimarans have three hulls, but the outer ones can be passive and simple floats.  Catamarans must be stronger (and thus heavier) to make up for an intrinsically weaker structure and the support of a central rig.  Unless the boat is set up as a biplane with two rigs as per Team Phillips a set-up which proved rather more complex than the designers envisaged - although, before breakages this boat certainly performed well. 

The main hull of a trimaran is a torque tube, with centralized torsion and better load distribution. Catamaran crossbeams are severely loaded by rigging stresses, and there is no practical way to implement a simple freestanding un-stayed rig, without wires holding it up.

Trimarans are also less prone to under-wing slamming, since the gaps between floats are narrower.  And they are much easier to fold for land mode, if considering a portable break down vessel.

REFERENCE and LINKS

• 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

This website is Copyright © 1999 & 2006  NJK.   The bird logo and name Solar Navigator are trademarks. All rights reserved.  All other trademarks are hereby acknowledged.       Max Energy Limited is an educational charity.

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