Capsize – how it happens
Planning for an unplanned inversion
By John Vigor
Article taken from Good Old Boat magazine: Volume 3, Number 6, November/December 2000.
Capsize: how it happens, and what you can do to survive it
When Isabelle Autissier’s 60-foot racer capsized in the Southern
Ocean, it sent a chill of fear through the sailing community. Sailors
don’t like to think of capsize. But here was a big, well-found boat,
a Finot-designed Open 60 Class flier, wallowing upside down in huge
frigid swells, with her long thin keel jutting toward heaven. It was
a bizarre and frightening sight.
Autissier was lucky. She was taking part in the Around Alone race, so
her million-dollar boat was equipped with emergency satellite
transmitters, position recorders, and lots of other equipment that no
normal cruiser is likely to be able to afford or fit on board. She
was eventually rescued in a wonderful feat of seamanship by Giovanni
Soldini, a fellow competitor.
So what went wrong? And could it happen to you? It depends where you
sail, but if you sail out of sight of land, whether at sea or on a
lake, the answer is yes, it could. And you should always be prepared
for it to happen. The good news is that most yachts of classic
proportions will survive a capsize. Unlike Autissier’s extreme
design, they will right themselves, although some might take longer
You can form a crude idea of what went wrong with Autissier’s boat by
imagining a long plank floating in the water. It doesn’t care which
side is up. It’s happy floating either way up. That’s Autissier’s
boat. Now imagine a plank with a heavy weight attached along one
side, so the plank floats on edge. If you turn it upside down, the
ballast quickly pulls it back again. That’s your normal yacht design.
Autissier’s racer was shaped too much like a wide plank – too beamy
and too light to recover from an inverted position, despite the long
heavy keel. It’s one of the paradoxes of naval architecture that an
excessively beamy boat, while hard to capsize in the first place, is
unseaworthy if she is inverted.
Furthermore, a light, shallow, beamy boat capsizes more easily than a
narrow, deep, heavy boat because she offers the seas more leverage to
do their work, and because she is quicker to respond to the upward
surge of a large swell.
Designers create racing boats like Autissier’s because that shape
gives them the ability to plane at high speeds. In other words, they
deliberately sacrifice seaworthiness on the altar of speed, and the
boats rely on the skill of their crews to keep them upright.
Unfortunately, singlehanders have to sleep now and then, so they
can’t be on watch all the time.
While it’s true that a good big boat is less likely to capsize than a
good small boat, there is no guarantee that even the largest yachts
are immune from capsize. It’s not the wind that’s the problem. It’s
Tests carried out at Southampton University in England have shown
that almost any boat can be turned turtle by a breaking wave with a
height equal to 55 percent of the boat’s overall length.
Even if you don’t like to think about it, you know in your heart that
it’s a reasonable finding. It means your 35-footer could be capsized
through 180 degrees by a 20-foot wave. Even a 12-foot breaking wave
would roll her 130 degrees from upright – from which position she may
turn turtle anyhow.
And if you imagine you’re never going to encounter a 20-foot wave,
think again. Waves of that size can be generated in open water by a
40-knot wind blowing for 40 hours. And a 12-foot wave is the result
of a 24-knot wind blowing for 24 hours. Plenty of those around.
Large waves are formed in other ways, too. A current flowing against
the wind will create seas that are much larger and steeper than
normal. And the old stories about every seventh wave being bigger
than the rest have a basis of truth, although it’s not necessarily
the seventh wave. It could be the fifth or the ninth. The point is
that wave trains occasionally fall in step with each other at random
intervals, literally riding on one another’s backs, to form an
exceptionally high wave. We call that a freak wave, but it’s actually
more normal than we care to admit.
Scientists calculate that one wave in every 23 is more than twice as
high as the average. One in 1,175 is three times bigger. And one in
300,000 is four times the average height. They may be far apart, but
they’re out there, and many big ships have been lost to them.
John Lacey, a British naval architect, put forward an interesting
proposition after the 1979 Fastnet Race, in which 63 yachts
experienced at least one knock-down that went farther than 90 degrees
and remained upside down for significant periods.
He explained that the old International Offshore Rule for racers had
radically changed the shape of yacht hulls by greatly increasing the
proportion of beam to length, which gave them more power to carry
sail without the need for additional ballast. It also gave them more
room below, of course.
But the flatiron shape of the hull made it very stable when it was
inverted. To bring the boat upright again would require about half
the energy needed to capsize the yacht in the first place, Lacey
“Since the initial capsize may have been caused by a
once-in-a-lifetime freak wave, one could be waiting a long time for a
wave big enough to overcome this inverted stability,” he commented.
Autissier’s experience bore out that prophetic statement. Her boat
was still upside down when she abandoned it.
Lacey did some more sums and figured that a narrower cruising hull
with a lower center of gravity than a typical IOR boat would require
only one-tenth of the capsize energy to recover from a 180-degree
“It therefore seems, in my opinion, that we should tackle the problem
from the other end, and design yachts for minimum stability when
upside down,” he concluded.
His recommendation is not likely to be taken too seriously, but he
certainly does have a point. You could make an inverted yacht
unstable with narrow beam, a very deep keel with a lot of weight at
the very end of it for righting leverage, and a deep-vee cabintop, or
at least one that was narrow on top and broad at deck level. For the
same reason, flush-decked yachts should be avoided, because they’re
likely to be much more stable upside down.
But as in everything to do with sailboats, there are compromises to
be made. Deep narrow hulls might recover quickly from inversion, but
as sailors discovered a century ago when they were all the rage,
they’re lacking in buoyancy. They’re also wet, and they have very
Two basic design features probably govern the probability of capsize
more than any others. The first is inertia and the second is the
shape of the keel.
Inertia is not generally well understood, but it’s the first line of
defense against a wave impact. In simple terms, inertia is resistance
to change. The inertia of a moving boat works to keep her moving on
course, even though other forces are trying to halt or divert her.
The inertia of a boat at rest resists any sudden attempt to start her
Obviously, because inertia varies with mass, a heavy boat has more
inertia than a light boat, so a wave hitting her from the side is
going to get a slower response. Light-displacement boats are more
likely than heavy boats to be picked up and hurled over by a plunging
Narrow beam is a help, too, because the force of a breaking wave is
concentrated nearer the centerline of the yacht, where it has less
The way weight is distributed on a boat also affects its inertia. A
wide boat with a light mast and a shallow keel will respond very
quickly to every wave with a lively, jerky motion. A boat with a
heavier mast and a deeper keel has its weight spread out over a
greater span, and it’s more difficult to change its speed or
direction, so the force of a breaking wave may be dissipated before
it has a chance to overturn the boat. Inertia, incidentally, is what
keeps a tightrope walker aloft. It’s contained in that long stick. If
you push down on one side of it suddenly to regain your balance, it
almost bounces back at you. It will subsequently move slowly away,
but you can recover it with a long gentle pull as you lean the other
A long, old-fashioned keel resists sudden rolling simply because it’s
difficult to move anything that big sideways through the water. A fin
keel, with its meager surface area, is much more easily moved when
it’s stalled; thus, the boat to which it’s attached is more easily
overturned. But a fin keel that’s moving through the water acquires
much more stability, which is why fin keelers should be kept moving
in heavy weather.
Capsize screening formula
The maximum beam divided by the cube root of the displacement in cubic feet, or
Maximum beam (feet) = less than 2 3÷Displ/64
The displacement in cubic feet can be found by dividing the displacement in pounds by 64. The boat is suitable for offshore passages if the result of the calculation is 2.0 or less, but the lower the better.
Although there are design factors that improve seaworthiness (usually
at the expense of speed and accommodation), and although there are
tactics you can use in a storm to minimize the chances of
overturning, no boat is totally capsize-proof. That is not to say
that every boat is going to capsize, of course, even the ones most
likely to. After all, hundreds of yachts cross oceans every year
without mishap. But prudent sailors keep the possibility in mind and
do what they can to forestall any problems and to lessen any damage
resulting from an inversion.
If you have never given any thought to inversion, the results of a
capsize can be devastating, not only on deck but down below as well.
Not many people realize what large forces are involved in a capsize,
especially the head-over-heels capsize called a pitchpole. It’s not
just a gentle rolling motion. The contents of lockers and drawers can
be flung long distances in the saloon, and you could easily find
yourself standing in a state of disorientation on the overhead in a
seething mess of battery acid, salt water, clothing, ketchup,
mayonnaise, diesel fuel, paint thinner, knives, forks, and shards of
broken glass. There will be no fresh air entering the cabin to
dissipate the fumes. And it will be dark because your ports will be
So, first things first: presuming you haven’t been injured by flying
objects, can you lay hands on flashlights? Were they stored safely in
a special place that you can reach without having to shift a wodge of
soaked bunk mattresses? Is there one for every member of the crew?
Are the batteries fresh? You may not stay upside down for long. But
if you’re unlucky, like Isabelle Autissier, you will find you need a
flashlight more than anything else on earth.
There are some other things you should think about before you ever
set sail. And there are some precautions you can take.
- Avoid heavy weather. “The most dangerous thing on a boat is
an inflexible schedule.” Thanks to Tony Ouwehand for this observation.
- Avoid taking large waves abeam, particularly breaking waves.
- When caught in heavy weather:
- Heave to.
- Run (down wave) using a drogue to keep speed down to 3 to 5 knots.
- Use a sea anchor from the bow or a series drogue from the
stern. (Practice rigging and deploying these in moderate conditions.)
- Is your rig as strong as possible? Will it withstand the tremendous
forces of a capsize?
- Do you have a plan to free a toppled mast from alongside, where it
can batter holes in your hull? Have you ever thought how difficult it would be to cut the rigging, even with a decent pair of bolt cutters,
on a slippery deck that’s suddenly rolling viciously?
- Do you have material on board for a jury rig? Have you thought about
how you would use it?
- Will your radio transmitter’s antenna come down with the rig? Do you
have a spare?
- Will your EPIRB start working automatically because it’s been under
water – whether you want it to or not?
- Are your cockpit lockers waterproof? Can you imagine how quickly
you’d sink if one of them was open at the time of capsize?
- Do your companionway hatchboards lock in position? Have you ever
thought how much water would get below if one or more fell out as you
- What have you done about waterproofing the cowl vents for the engine?
Those are huge holes in what would become the bottom of the boat.
(The same goes for Dorade boxes, incidentally. Each one is a
potential three- or four-inch hole in the bottom. Fit them with deck
plates for sea work, on deck and down below.)
- If you’re in the cockpit when the boat capsizes, will you be attached
by a harness? Will you be able to free yourself if you’re trapped
under water and the boat stays inverted for some time?
The anchor locker
- If the anchors and chain are not fastened down securely they could
bash their way through the locker lid and cause all kinds of havoc.
- Is your self-draining deck anchor locker waterproof? Many aren’t completely sealed at the top, where wires for pulpit-mounted running lights come though, and would let in water.
The engine room
- Is your engine mounted securely enough to withstand a capsize? I know
of one boat in which the engine was hurled from its mounts during a
pitchpole, causing great destruction.
- What if the engine’s running during a capsize? Could you switch it
off quickly, with everything upside down? Would the oil run out?
Would the fuel drip out of the tanks? Are your breathers inside or
- Are the batteries fastened down firmly enough? Can you imagine what
damage they could do if they got loose? And will they drip acid if
they’re upside down? (Newer batteries – gel cells and AGMs will not
spill acid when inverted. -Ed.)
- Can you turn the stove off? If there’s a smell of gas, can you deal
with it? Have you made sure the galley cupboards can’t fly open
during a capsize and turn the saloon into a sea of broken glass and
- Can you lay hands on a fire extinguisher quickly? It could save your life.
- Have you figured out a way to keep all those loose tops in place in
the saloon – the boards that cover access to storage under bunks, the
bilge boards, and so on? Some boats have inside ballast, and many
have heavy objects, such as storm anchors, stowed in the bilges. Make
sure they stay there, because if they get loose they can come
crashing through the overhead (your new “floor”) and sink the boat
- Make sure your bunk mattresses will stay in place, too, otherwise
they will greatly hamper your attempts to get around.
- Have you figured out a way to pump bilge water out of an inverted
boat? Think about it. It’s not easy.
- Most books could escape from their racks during a capsize and become
potentially harmful flying objects. Have you solved that problem?
- The ship’s papers and your own personal documents should be in a
watertight container in a secure locker, one that is not too high up
in the boat because that’s where the water will be when you capsize.
There are many other systems and pieces of gear on a boat that could
be affected by a capsize. When you use them, think inverted. Imagine
what would happen if they got loose. Invent ways to keep things in
their places during an unplanned inversion. Don’t ever imagine it’s
wasted work. It’s one of the unspoken rules of the sea that if you’re
prepared, the worst is not likely to happen. If you’re not, you’re
bound to attract trouble.
More on the subject
Tami Ashcraft wrote a compelling story of the realities of inversion
and its aftermath in her book, Red Sky in Mourning: The True Story of
a Woman’s Courage and Survival at Sea, reviewed in our May 2000 issue. John Vigor goes into more depth about preparation for
capsize in his book, The Seaworthy Offshore Sailboat.
John Vigor is a professional journalist. The author of The Practical
Mariner’s Book of Knowledge, The Boatowner’s Handbook, Twenty Small
Boats to Take You Anywhere, and The Seaworthy Offshore Sailboat, he
has worked for major newspapers around the world and is a frequent
contributor to leading sailing magazines. He has sailed for more than
40 years in boats 11 to 40 feet in length and logged some 15,000
miles of ocean voyaging. In 1987 he and his wife, June, and their
17-year-old son sailed their 31-foot sloop from South Africa to the
U.S. John’s books are available from the Good Old Boat Bookshelf.