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Keeping Her Afloat
Positive Flotation for your boat
by Paul Butler
illustrations by Marya Butler

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
anchors from.

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 floating one.

To get an idea of the flotation requirements for an individual boat, divide its 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 ballasted boats.

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 poured foam.

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 I 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. Eureka!

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.

Sealed Tanks

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. (see our article on tanks)

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 its 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 main pump.

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 plank-on-frame construction, but cold-molded, 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 boats.

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
pitching hull.

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 lighterweight 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.

Air Bags

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 through-hull 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 waterline.

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 added visibility.

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 raft.

Positive flotation won't save you from tankers, Moby Dick, or the Whirlpool of Charybdis, but well-planned, it will keep you afloat.-indefinitely.