A Multihull Primer
© 2004 by Steven K. Roberts
Nomadic Research Labs
There is a great philosophical rift in the nautical world, a guaranteed
argument-starter analogous to the perennial Mac-vs-Windows debate that
has ruined many an otherwise benign social encounter. (Um, surely you
don’t have to ask, but for the record, I’m a diehard Mac user—more so
than ever now that the OS is wrapped around Unix to give us a lickable
GUI atop a proper kernel. But I digress.)
I’ve noticed that multihulls are to monohulls as recumbent bicycles are
to diamond frames: they're faster, more intriguing, and annoying
to old-timers. They also cover a wide quality range from
exquisite to junk (a few ancient famous examples of the latter
conveniently serving to support the arguments of
traditionalists). 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. And they are both so undefined that designs have yet to
converge upon established standards, but instead show up in ever more
radical configurations as marine architects are drawn to the challenge
of pushing the envelope.
In short, both are where the action is.
Of course, it’s not like we modern high-tech sailors can lay any claim
to the multihull concept; the idea of making fast, 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—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 to 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, rigged her as a schooner,
and 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,
his 25-foot catamaran, he was banned from all future races. This
sort of attitude can put a damper on creativity…
The nondescript, half-Catamaran
[sic], 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 boatbuilders 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 rotomolded polyethylene WindRiders and their speedy hydrofoil
descendants, exquisite folding trailerable 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 a Multihull?
Given the fact that I’m not getting any younger and this project has
taken ten years, I should address one question before we go any
further: why not just grab any of the thousands of inexpensive
single-hulled vessels out there, slap on a computer, and get on with
it? The technology is well-established, they are easy to
find,
and they are most certainly simpler: if the objective is to stuff
some electronics on a boat and go play, it would seem sensible to take
a low-cost, off-the-shelf approach by starting with a capable beach
cruiser or trailerable daysailer. 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. 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.
Catamarans have two hulls
connected by some kind of structure, which in
large boats can include a spacious bridgedeck; trimarans have a center
hull flanked by two smaller ones, supported by crossbeams (see Figure 1). 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
their ongoing argument.
Figure
1: Monohulls, catamarans, and trimarans
(sketch by Don Elliott)
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 accommodations. This is not a fundamentally complex
piece of hardware, although you can spend any conceivable amount of
money on one (a fact that has led to no end of nautical
clichés: everyone knows that a boat is a hole in the water
into which you pour money, and you can simulate sailing by standing in
a cold shower and tearing up $100 bills—or $1,000 bills if you want to
simulate racing). Still, inflated boat bucks notwithstanding,
monohulls are pretty simple machines.
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 (carry around a
sheet of plywood on a windy day if you don’t believe me). Most
sailing rigs are stayed,
meaning that they are held in place by the
balanced tension between rigging wires that run fore and aft as well as
athwartships; 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 trailered. Small wonder that these things
have major geek appeal, but it comes at a stiff price: all other
things being equal, a multihull is typically twice the cost of a
similarly-scaled monohull.
Speed and acceleration
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. 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 (and yes, the sleek little Microship
trimaran is much heavier than it should be, significantly reducing its
alacrity). 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 a
counter-argument.
I should make one other comment on speed. Multihull sailing
involves a somewhat different set of skills from monohull
sailing—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 us less inertia to carry the boat through the wind if we screw up
and find ourselves in irons,
sail flogging as the bows point directly
into the wind and we’re going nowhere but backwards, slowly.
Usually this is just a matter of skill-development, but some multis 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, or (in my case) surreptitiously deploying the pedal drive
and giving ‘er a few cranks to power through the “eye of the wind” if I
haven’t timed the tack just right.
Sailing attitude and comfort
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 cats and tris,
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 sleep). On the Microship, this
phenomenon mostly impacts the solar panels, but those can fold out of
the way.
Draft
This one’s easy. Because of their keels, monohulls extend deep
below the surface of the water—sometimes many feet. This makes
them more likely to hit bottom, 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 siltation, this can require considerable
attention.
Of course, finding oneself “on the bricks” is a bad thing in any boat,
and just because the draft,
or depth, of a multihull is considerably
less doesn’t mean that we can ignore the problem. In fact, some
multihulls have appendages (daggerboards and rudders) that are much
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.
Figure
2: “Shoal Draft” implies the ability to explore shallow waters without
hitting bottom.
(Natasha Clarke cartoon)
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 my case, the 11-foot beam provides
space for relatively huge solar arrays, and catamarans of cruising
scale have palatial accommodations compared to monohulls of comparable
length. Still, this comes at a price: wide boats are harder
to park, more difficult to tack, and less likely to fit in tight little
spots whilst happily engaged in the aforementioned pleasure of
gunkholing. 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.
Still, I consider this a win for the multihull, as there’s nothing like
lounging about on the nets between hulls while underway. In the
Microship case, we have the best of both worlds: these boats are
so small (11’ beam) that docking is not a problem, as a pair of them
can fit end-to-end in a standard 12’-wide marina slip, in addition to
being readily beachable. They can also drop their own anchors if
necessary, or simply deploy landing gear and roll up onto land.
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.
Now. 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,
linked below) 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, I’d rather fight
for my life on an inverted trimaran that still carries most of my
stores and tools than suddenly find myself becoming shark bait when my
boat sinks out of sight below me, taking everything with it.
There’s another safety issue here that affects monohulls and multihulls
more or less equally: dealing with heavy weather. I’m not even
going to begin to get into this argument; 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, multis 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.
A sailor with no schedule always
has fair winds.
—Source unknown
One of the saddest sights I’ve 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. The multihulls don’t fare any
better—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.
So it’s better to avoid the situation entirely. In the Microship
system, we have the luxury of small scale, and this opens a new option
best expressed by the classic line:
Run away!
—Monty Python and the Holy Grail
Assuming we aren’t stupid enough to be bobbing along some vast exposed
rocky coast, blithely ignoring NOAA weather broadcasts as heavy weather
looms, we should be able to deploy the landing gear and haul out with
our own straining muscles—either finding a launch ramp and hoofing it
down the road as far from angry water as possible, or dragging the
boats up a beach and tying them off to trees while hoping for a
merciful storm surge. Hey, at least it’s a fighting chance.
So you can see that the whole multihull-monohull argument is rife with
trade-offs and matters of personal preference. In the Microship
application, the scales are tilted strongly in favor of a multihull:
shoal draft and light weight making the system amenable to operation by
a single human without needing external vehicles or special shore
facilities. The broad beam gives us room for solar panels, speed
is a nice bonus, and sailing dry and flat makes it more likely that
on-board computers will actually be used underway rather than just
demonstrated at trade shows. We are willing to accept the added
complexity and cost to have the other benefits.
Figure
3: Monohulls require special care when beaching.
(Photo taken near Langley on Whidbey Island, WA, by Wm Leler)
Catamaran vs. Trimaran
I should mention one other set of trade-offs. Now that it’s clear that
the Microship substrate needs to be a small multihull, the question
remains whether it should be a cat or a tri. As it turns out,
this is not a difficult decision at this scale…
While building two hulls seems initially more appealing than building
three, small tris have much more useful space than small “beach
cats.” The latter consist of hulls that are too narrow for
comfort, requiring that the accommodations be built atop the bridgedeck
that connects them. (I should note that the converse is also
true: for a given length, large cats tend to have more room than
large tris. But the Microship is way below the point at which the
curves cross.)
Cats also have two centerboards, two rudders, and too much redundancy.
Of course, tris have three hulls,
but the outer ones can be passive and
simple. Cats 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 (very sexy,
but much more complex). The main hull of a trimaran is a torque tube,
with centralized torsion and better load distribution. Cat crossbeams
are severely loaded by rigging stresses, and there is no practical way
to implement a simple freestanding unstayed
rig (without wires holding
it up)—a key requirement for flexibility while traveling since it may
be necessary to drop the mast frequently to sneak under bridges or drag
the boats down the road.
Small trimarans are also less prone to underwing slamming, since the
gaps between floats are narrower. And they are much easier to
fold for land mode.
Looks like the choice is clear... and here's the
tale of how we did it.
