Keeping Her Afloat
Butler, drawings by Marya Butler
from Small Boat Journal #26 Sept. 1982
Flotation was taken for granted,
even after some Neanderthal
scooped out the inside of his log
float. But it wasn't long before another
"innovator" split his log into strips and
hammered them back together, hollow
on the inside, then hung heavy rocks
on the bottom to keep his funny log
from rolling over and dumping him out
whenever he tried to stand up. Then a
really strange thing started happening:
logs started sinking. To the bottom!
Fill them with water and the things
would spiral down into the deep blue
before you could jump off. This was a
serious problem. Then, making things
even worse, other simpletons began
building boats of stuff that doesn't
even float, like steel, fiberglass, and
cement — things sane people made
Boats that wouldn't float when flooded needed some kind of positive flotation, an add-on that would keep the
hulk afloat until help came, or until
she could be patched up and pumped
out. With positive flotation, boats could
once again be almost as good as what
boaters started with in the first place.
Positive flotation means a boat will
not sink, even with radical hull damage. Depending on the amount and
location of the flotation, the hull might
bob with the decks awash, or higher.
The point is that it won't keep going
down; what you have is a raft — but a
To get an idea of the flotation requirements for an individual boat, divide it's
loaded displacement in pounds by 64,
the weight of one cubic foot of seawater. Use 62-1/2 pounds per cubic foot
if you sail only in fresh water. The
answer you get is the number of cubic
feet of flotation needed to support the
boat on her designed waterline. Depending on a number of other factors, such
as hull and furniture material, tankage, cargo, and the like, a fraction of
this amount — judiciously placed —
would keep the boat from sinking. But
it's difficult, if not outright dangerous,
to calculate the minimum amount of
flotation that will keep the boat afloat.
Compensate on the safe side. Flotation
equalling about 30 to 70 percent of
displacement seems to be average for
The problem is how to produce the
amount of flotation needed in a small
boat where space is at a premium.
We've tried various approaches — some
common, some not so common — in
boats we've owned or built for clients.
We present them here along with some
thoughts on other possible systems.
Filling the required space with foam is one approach. A catalyzed mixture
squirted through a wand to fill lockers
and odd spaces seems the ideal solution. Certainly it can be made to work
and is good for isolated areas, but there
are some less appealing aspects to
The stuff is expensive. For a 29-foot
boat we once owned, a foam contractor
quoted $800 (at 1979 prices) for roughly
80 cubic feet of foam. The price was
just the beginning of the bad news. He
couldn't guarantee that the stuff would
not eventually break down and absorb
water. He had no information on the
fumes given off by the foam, though he
thought it had a urea-formaldehyde
base. He said there was a danger of heat
build-up while foaming the larger bunk
areas and that we'd better stand by
with the fire extinguishers just in case!
And if that weren't enough, we just
couldn't bear to seal all that wood, all
that storage space, away forever.
We did eventually experiment with
foaming some small spaces with aerosol can formulations. True to our expectations, it wasn't the answer for us —
especially in smaller boats where space
is already at a premium.
We searched for alternatives, finally
realizing that the flotation does not
have to be in large, fixed chunks. Why
not 50,000 ping-pong balls, for instance,
poured into some unused space and
secured with a lid or net? The flotation
material would be removable at any
time for inspection, would never absorb
water, and conforms to any shape. We
began looking for some closed-cell,
lightweight, inexpensive material that
was available in small pieces. One day
our smiling U.P.S. lady delivered an
order of deck hardware in a box stuffed
with little "peanuts" of the same foam
they make disposable coffee cups from.
Foam packing material, in its various forms, seems to have good possibilities. It can be dumped into any
compartment with a lid or a door. In
most cases, it is so light that it's hard to
weigh. It does take a while to pack
down though, and you have to keep
adding more to top off the compartment, but it eventually settles. True, it
has a little less flotation capability
than solid foam because of the space
between the pieces (which would hold
water if flooded), but a well-packed
compartment would come close to its
maximum lifting potential nonetheless.
With a big vacuum, you could empty
or fill a compartment in seconds. And
packing material is cheap, available
almost for the asking from shipping
departments. The alleys of industrial
areas seem full of it, spilling out of
every dumpster. It can be bagged in
plastic zip-locks with air drawn out by vacuum, or it can be contained in fine-mesh nets. We ran across dozens of
different kinds. We experimented by
soaking them in salt water, squeezing
'em, freezing 'em, and so on. Some
worked better than others; some seemed
to hold a little water; some didn't.
Next began our sealed tank experiments. We were building ultra-light-
weight fuel and water tanks for a multihull from plywood and epoxy, sheathed
with 6-ounce fiberglass cloth inside
and out and filleted with epoxy/microballoons and silica. Compared
with metal tanks, these were economical, light, apparently long-lasting, suffered no electrolysis problems, and
could be custom-built for any application with common woodworking tools.
We began to consider them a viable
alternative to metal tankage and, as
they could hold water out as well as in,
a possible source of flotation.
We made sealed air compartments
for a couple of rowing dories and before
long had built a pontoon boat entirely
of wood. Each of the pontoons had a
capacity of over 100 cubic feet, was
lighter than a steel float, and would
never have electrolysis problems.
We designed and built flotation
chambers for an off-shore sailboat,
which were incorporated into the furniture under the settees below the waterline. Each had a capacity of 20 cubic
feet, was made to fit an odd-shaped
hole, and was very lightweight, being
of 1/2-inch ply with cornerposts and a
permanently sealed top. At some point in the
future, they could convert to stowage
by simply cutting hatches in the tops.
The last tanks we built took the idea
one step further. They had removable
tops that could be sealed, either secured
with thumbscrews over rubber gaskets,
or sealed with silicone. These tanks
could now be used for stowage as well
as flotation. In some cases they also
served as furniture and hull reinforcement for cold-molded or glass boats.
The tanks could be used with water,
then sealed for flotation once the
water's gone. Sealed tanks could hold
seldom-used gear while still offering a
degree of flotation.
The tank had now become a true
multi-purpose device: keeping water
in, keeping water out, keeping gear dry,
serving as furniture, stiffening the hull,
and, of course, providing positive flotation.
If you wish to avoid adding flotation
altogether, yet still have some margin
for safety, you can divide and subdivide your boat into a number of
separate compartments, each with it's
own pumping system. This isolates
potential hull damage and flooding to
a specific area. If it works as planned,
the compartmentalization allows time
to think and gather your wits — time
to deal with the problem. This is the
very opposite of the traditional limber
hole system which drains leakage to
the lowest spot in the bilge and the
Submarine-type doors and watertight
bulkheads are a fine way to divide a
boat into compartments. More than a
few craft have limped into harbor with
a sealed, flooded compartment. The
vulnerable forepeak is particularly well
suited to a watertight door and bulkhead. A large safety factor is achieved
with only the one division.
Effective watertight bulkheads are
harder to achieve in traditional plankon-frame construction, but coldmolded, steel, aluminum, and fiberglass hulls readily lend themselves to
this treatment. Decent watertight doors
can be built fairly lightweight with
plywood, epoxy, and common sense.
There are also beautiful aluminum
versions commonly used on fishing
Along with the idea of compartmentalization, try to raise the bottoms of
hatches and entryways as far above the
waterline as possible. This way flooding in one area won't spill over into the
adjacent compartment, even if they are
not sealed off from one another.
If some of the ballast can be easily
and quickly removed from your boat in
an emergency, its flotation requirements can be drastically reduced, and
what flotation there is will float the
boat higher. Removable ballast could
also help get you off when grounded.
Offload the ballast into the dinghy,
then reload after the keel comes free.
Movable ballast could also be used to
trim for peculiar or extreme weather
conditions or for long passages on the
same tack. It can compensate for uneven loading, and a portion could be
left ashore for an overloaded cruise.
Movable ballast must be arranged in
weights small enough to handle in a
moving boat. It must be well secured
when in place, yet capable of quick
removal. These requirements seem to
conflict, and we've been through a
number of trials and errors looking for
the best system.
We tried bolting lead pigs in series
to the deadwood or an internal ballast
casting, using bolts tapped in place.
Although these are both very secure,
removing such ballast is tedious, and
both systems take up lots of good storage room in the bottom of the bilge
where we like to keep heavy canned
goods. And 50-pound pigs, even with
handles cast into them, are hard to grip
and too heavy to maneuver easily in a
Our current choice is lead shot in
25- or 30-pound bags. Lead doesn't rust
like boiler punchings, and doesn't stain
the bilge or end up in a solid glob of
oxidation. Lead is cheaper as shot than
as bars, ingots, or strap. It conforms to
the odd nook and cranny, settling into
places where a pickle jar wouldn't fit.
The lighter weight allows the smallest
crew member to handle them easily. A
number of bags nestle together, and
you can put more weight than you can
lift in a small bucket. For clean installations, the shot can be sealed in plastic bags, then plopped into canvas, burlap, or tarp bags and tied with a strong
cord, leaving a hank of bagtop for a
grip. You can scrounge wheel weights
and swap them to a metals dealer for
shot. If you have a band saw with an
old blade, you can clamp lead wheel
weights in a pair of vise grips and cut
them into marble-sized chunks yourself.
But make no mistake: these bags,
though they stow well and seem to cling to the corners, must be well
secured! Of course, it's a simpler job
than with ingots, and bags will cause a
lot less damage than a 50-pound bar
hurtling through the main salon.
In a client's boat, we built a reinforced box in the appropriate space
atop the keel casting, but still under
the sole. With a latch on top made
available by flipping back the cabin
sole, the bag box was instantly accessible. We also favor containing the ballast under heavy netting with 7/16-or
1/2-inch shock cord sewn into the perimeter. Hold the shock cord in place
with heavy hooks or eyelets. Half-inch
shock cord is stout stuff and will take
considerable weight, yet stay flexible.
You could achieve considerable flotation by stuffing your Avon into the
cabin and pulling the inflation cord.
We're not advocating that you try that,
but it demonstrates what we have in
mind. The possibilities are endless in
this approach, but simplicity and reliability are paramount. This is not the
place for Rube Goldberg prototypes.
Visualize sausages of multi-celled
raft material, or something similar,
deflated and rolled into a tube, Velcro strapped alongside the sheer clamp,
behind the settees, and against the
bulkheads. When inflated, they can be
secured with trampoline netting or
grommets and line. Restraining gear is
rigged beforehand during tests. Such a
set-up would have to be inspected frequently, and drills in its use conducted
often — especially in the dead of night.
Air bags could provide the low-lying
volume necessary to prevent a great
weight of water from entering the boat
before the inside level was up to the
outside water line, thus stopping the
leak after a fashion. If all other methods
failed — sealed compartments damaged and lockers breached — the second
line of defense could fall to air bags
higher up in the hull, possibly up
against the cabin top, in a last ditch
attempt to stay afloat. This is when
you break out the flare guns, survival
suits, and EPIRB.
To develop confidence in your positive flotation, rely on more than
one flotation "device." Have as many
back-ups and contingency plans as you
can create. Prepare yourself for the
first rule of Murphy's Law, since it applies so frequently to those who venture off dry land.
Become proficient with epoxy and
skilled in crafting good epoxy fillets; it
takes a little practice to get the knack
of it. Make everything watertight. Start
with major bulkheads and work down
to smaller lockers and crannies. Epoxy
bonds perfectly to polyester resin and glass, providing it's clean, dry, sanded,
and the gelcoat is removed.
Consider enclosing your throughhull fittings in sealed compartments
extending well above the waterline. That way, if they do let go, they'll only
fill their own compartment to the
Arrange flotation logically fore and
aft, port and starboard. Don't stick it
all in one place unless you plan on
clinging to the rudder or stem for
Overhead hatches, constructed airtight with sealing gaskets, can form air
traps in flooded compartments. These
help float the boat. Inverted trimarans
float indefinitely on trapped air alone.
Even with a swamped boat, you're
better off than in a raft. You have
access to gear and stores. Possibly the
dinghy can be tethered alongside in
calm weather. Maybe there's a survival
suit and EPIRB aboard. The Fastnet
race demonstrated the fallacy of leaving a floating $80,000 boat for a $2,000
Positive flotation won't save you
from tankers, Moby Dick, or the Whirlpool of Charybdis, but well-planned, it will keep you afloat indefinitely.