In Search of the Unsinkable Boat

While traversing the oceans during two and a half circumnavigations Ive had rather too much time for pondering the consequences of being holed and sunk by a collision - ranging from unseeing ships to sleeping whales to semi-submerged containers or a jagged reef. My concerns were reinforced by a close brush with a group of belligerent whales in the Indian Ocean; again when I ran over an enormous floating tree in the Gulf of Panama, and finally when I T-boned at close to 5 knots a drifting unlit fishing boat near Indonesia that knocked a small hole in Atoms bow just above the waterline. Sailing alone increases the hazards, but even an alert crew on deck has little chance of seeing anything unlit at night.

Instead of trying to protect myself by the usual route of buying boat insurance and a life raft that requires frequent expensive repacking inspections, I turned my attention to strengthening the hull and modifying her systems until I considered the boat itself as its own life raft. This was due partly to budget constraints and partly to my philosophy of maintaining self-sufficiency at sea. To begin with, prior to my second circumnavigation, I installed a watertight collision bulkhead in the bow section under the v-berth. Though not at first seeking outright positive buoyancy for my boat - or even imagining it was possible to achieve - over the years I gradually modified most of my 28-foot Pearson Triton's storage areas into watertight lockers so that if the boat was holed in any of those areas the flooding would be contained.

List of Atom's watertight lockers forward to aft:

Chain locker with two sealed doors and shut-off valve on bilge drain.

V-berth converted to full berth, raised 6 inches above waterline and its five lockers independently sealed to hull and topped with gasket-sealed locking hatches.

Forward water tank is integral to the hull.

Clothes locker face is raised to 11 inches above the waterline.

Toilet is behind a watertight bulkhead 2 inches above the waterline.

Forward bilge is isolated and sealed with a gasketed hatch.

Door between main cabin and forward cabin is sealed with gasket and three latches.

Lockers under main cabin bunks are sealed each side with gasketed hatches.

Main cabin outboard lockers are sealed to 7 inches above waterline.

Lockers under sink and galley are sealed to hull and have gasketed hatch access.

Original icebox locker opposite the galley is converted to storage with a sealed front hatch and above it an open radio locker face 11 inches above the waterline.

Water tank under cockpit floor is integral to the hull.

Three large cockpit lockers are sealed with shut-off valves in their bilge drains.

By converting my existing single fiberglass tank to two larger tanks built integral to the hull I gained several advantages. Integral tanks are more space efficient, take less room and hold more water than any other type tank. They also add massive strength to the hull, and perhaps most importantly, act as a watertight locker in case of collision. A third integral watertank could be installed in the lower aft area of the bilge.


Looking forward at sealed chain locker, integral watertank and raised, sealed lockers.


Looking aft where engine once resided. All cockpit lockers are sealed independently.

Once the majority of lockers were converted to watertight compartments, I wondered if I had incidentally achieved positive buoyancy on a boat with a gross displacement of around 9,700 lbs. By making estimates of the amounts of fiberglass, lead, anchors and chains, other metals, woods, foam deck core, tools and miscellaneous gear, and calculating their respective buoyancy factors (see table), I came up with a figure of 86.5 cu ft of watertight locker volume needed to float the boat with decks awash. (To better picture this imagine cutting up a foam block of 10-foot by 9-foot by 1-foot thick and placing the pieces throughout the boat). More flotation is needed to compensate for the reduced volume of lockers loaded with gear and a safety factor of at least 20 percent should be included.

I then calculated that my watertight lockers which have rubber gasket seals have an approximate volume of 95 cu ft. I have not included any partial watertight bulkheads unsealed at the tops which might be flooded over or the water tanks and the numerous water cans because they may at any time be full of fresh water. I'm considering next to add closed cell bunk cushions well strapped down to create another significant source of buoyancy. Obviously, the more flotation the better. A boat that floats with the cabin top awash is impossible to repair and pump out at sea. At this point I figure Atom will float briefly even if she were holed in one of the few remaining areas of the hull which are not contained by watertight lockers. I say briefly, because although Ive taken care to provide a good seal, I cannot be sure how fast the water will leak past the gaskets into the lockers.

Another consideration is ensuring a level and stable platform after flooding. If all buoyancy chambers are near to or below the waterline, a flooded boat that heeled over could become unstable as the water simply moved to the low side of the boat. Having both chain locker and all cockpit lockers sealed to deck level greatly increases stability of a flooded hull. The ideal situation is to have both ends of the boat sealed up to the deck and a U-shaped area of flotation following the hull shape in the center sections of the boat.

I plan to further improve Atoms flooded stability as well as her insulation by adding sheets of rigid foam in unused areas along the interior sides of the boat above the waterline and under the deck. These blocks can be glued in place, or where easier hull access is needed, they can be attached by straps or Velcro. Another easy inexpensive option is to insert inflatable beach balls into selected storage lockers and inflate and deflate them as needed to fill the spaces not occupied with gear. This unconventional solution may be more practical in some situations than permanently filling space with foam blocks.

While Atom does not yet have an adequate safety factor of reserve buoyancy for me to state conclusively that she will remain afloat when holed, she is certainly far less likely to sink in most flooding situations because of her extensive watertight compartments. Certainly, I sail with less anxiety now when plowing through heaving seas at 6 knots at night in mid-ocean.

To avoid the extensive work involved in fitting watertight compartments, a combination of poured foam and rigid foam sheet and blocks can be used to fill unused sections of the hull and as a liner inside the hull and under the deck. The main problems with using solely foam flotation is retaining maintenance access to all areas of the hull and deck and the considerable loss of storage space. This approach is just not practical for many boats. Other systems available to retrofit your boat for positive buoyancy include inflatable flotation bags which would activate when submerged by water. The problem with active systems such as flotation bags is that they may activate inadvertently, or worse, fail to activate when needed.

Relatively heavy, narrow beamed, low freeboard boats like mine are difficult to retrofit for positive buoyancy. Wooden boats and boats constructed with a cored hull have an obvious advantage in this area. Modern, beamy boats with large interior volume are well-suited for built-in flotation. The most efficient way to achieve this is during the boats design and construction. Most modern production boats have interior fiberglass hull liners that leave large areas between hull and liner inaccessible and unsealed - you have no chance to repair a leak in these areas at sea and adding watertight bulkheads to these boats is nearly impossible. Cored hulls, watertight lockers, and strategically placed rigid foam between liners and hull in new boats should be the rule rather than the exception. The additional cost could be partially offset by lower insurance rates, and the increased safety factor is priceless. Many governments are now or will soon require pleasure craft to have positive buoyancy. Given the loss of life of those who abandon partially flooded boats they felt were in danger of sinking, it is clear that positive buoyancy is highly desirable. Hopefully, boat builders will not wait for bureaucrats to legislate what is an achievable common sense requirement - an unsinkable boat.

Buoyancy factors for common boat materials

Material              Weight in lbs/cu/ft     Lbs. Reserve Buoyancy
Polyurethane Foam                   6                                       58 
Balsa Core                               6-18 (varies)                     54 (avg.)
White Cedar                            23                                     41
Spruce                                     27                                     37
Douglas Fir                              32                                     32
Fir Plywood                             36                                     28
Ash                                          41                                     23
Teak                                        45                                     19 
Oak                                         53                                     11
Fiberglass                                 96                                   -32
Steel                                       490                                 -426
Lead                                       700                                 -636

 Source: Skene‚Äôs Elements of Yacht Design