In Search of the Unsinkable Boat - SailNet Community
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In Search of the Unsinkable Boat

Long-distance sailors never know what will loom in their paths, which is the principle reason behind the author's quest for an unsinkable boat.
While traversing the oceans during two-and-a-half circumnavigations I've often pondered the consequences of being holed and sunk by a collision—ranging from unseen ships to sleeping whales to semi-submerged containers. These concerns have been reinforced by a close brush with a group of belligerent whales in the Indian Ocean; and again when I ran over an enormous floating tree in the Gulf of Panama, and most recently when I T-boned a drifting, unlit fishing boat near Indonesia at five knots, knocking a hole in Atom's bow just above the waterline. Sailing alone increases the hazards, but even an alert crew on deck has little chance of seeing anything that's unlit at night.

Instead of trying to protect myself by beefing up my boat insurance and owning life rafts, I concentrated my efforts on strengthening the hull and modifying her systems until I could consider the boat 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 I wasn't at first seeking outright positive buoyancy for my boat—or even imagining it was possible to achieve that—over the years I gradually modified most of my 28-foot Pearson Triton's storage areas into watertight lockers so that if the boat is ever holed in any of those areas, the flooding will be contained.

Working from the bow aft, here's a list of the watertight lockers that I've created on board Atom:

  • Chain locker with two sealed doors and shut-off valve on bilge drain.
  • V-berth converted to full berth, raised six 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 two 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 seven 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 sits an open radio locker sealed 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.

    While improving the positive buoyancy of his boat, the author isolated the forwardmost compartment of the bilge (top), and later sealed it with a gasketed hatch.
    By converting my existing single fiberglass tank to two larger tanks built integrally into the hull I gained several advantages. Integral tanks are more space efficient and hold more water than any other type of tank. They also add massive strength to the hull, and perhaps most importantly, act as a watertight locker in case of collision.

    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 around 9,700 pounds. 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 below), I came up with a figure of 86.5 cubic feet of watertight locker volume needed to float the boat with the decks awash. (Imagine cutting up a foam block 10-feet by nine-feet by one-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 cubic feet. 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 freshwater. I plan 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.

    Looking forward from the companionway aboard Atom, several watertight compartments are in view though you wouldn't know it. One sits beneath the V-berth, and one forward of it. The cabin door itself is gasketed and seals with three latches. Beneath the settees in the main saloon, lockers on either side are sealed with gasketed hatches.

    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 I've 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 the stability of a flooded hull. The ideal situation is to have both ends of the boat sealed and a U-shaped area of flotation following the hull shape in the center sections of the boat.

    I plan to further improve Atom's 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. 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 six knots at night in mid-ocean.

    Looking aft from the main cabin past where the engine once resided, there is no longer any access to the cockpit lockers. Those areas are now sealed off to create a more watertight interior and additional positive buoyancy.
    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 problem with using only foam flotation is in 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 that can activate when submerged by water. However, there are problems with active systems such as flotation bags, principally 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 boat's design and construction. Cored hulls, watertight lockers, and strategically placed rigid foam 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 around the world 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 a common sense requirement—an unsinkable boat.

    Suggested Reading:

    Cockpit Confessions by John Kretschmer

    Avoiding Collisions at Sea, a Proactive Approach by Mark Matthews

    Giving Your Boat Some Legs by James Baldwin

    SailNet Store Section: Harnesses and Tethers

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