TECH BIGHTS
Small Craft Advisor|July - August 2021
Some fiberglass boats, like Boston Whaler skiffs and MacGregor power-sailers, are built with integral flotation foam between the inner and outer laminations. Roger MacGregor was willing to flood his water-ballasted boats just to prove that they would not sink—and you can still find his old sales videos if you want to see for yourself. (youtube.com/ watch?v=hemNdJmzQBo)
JERRY CULIK

Other boats have their positive buoyancy added on, or they have a combination of built-in and added flotation. For example, a Potter 15 sailboat has three parts. The easiest-to-see pieces, assuming they’re still there, are two large blocks of styrofoam located underneath the cockpit seats. Each of these blocks provides three cubic feet or about 190 pounds of flotation (that is, 3 cu-ftx 62 pounds per cubic foot for freshwater, or 64 for seawater). Since the empty weight of a Potter is around 500 pounds, the foam blocks alone are not quite enough to keep the boat afloat if it floods.

The rest is harder to find because it’s underneath the cabin floor. The space forward of the mast support post is filled with foam flotation. If the foam is the older, “open-cell” type, it can slowly absorb water. Unfortunately, without removing the floor or cutting an inspection port, there is no way to inspect it. Fortunately, that area on a Potter is not sealed and any water that gets into the boat can drain back to the cabin, where you can get at it. I estimate the volume of the under-floor foam is 4 to 6 cubic feet, adding another 300 or so pounds of flotation. Combine that with what’s in the stern and you’ve got more than enough —700 pounds of positive buoyancy—to keep the boat from heading to the bottom, even if it’s completely flooded and loaded with gear. There are photos showing a young woman standing in a scuttled Potter full of holes drilled through the hull. I noticed that there were no waves —it’s nice calm water for that particular demonstration. Not quite the same kind of conditions we experience out sailing...

Enough positive buoyancy is necessary, but not sufficient, to keep you safe. The specific location of the flotation is the other part of the equation. How many runabouts have we seen where the unfortunate occupants end up clinging to a capsized hull? When most of the flotation is located under the floorboards, a big enough wave can roll a flooded hull, and it will “turtle”—with the flotation now on the high side. So, the best flotation is located up around the gunnels —to reduce the tendency of the boat to roll over. Since half of the flotation of a Potter (and many other small sailboats) is also fairly low in the hull, they have the same tendency to turtle, especially if the centerboard is not locked down or is lost, and the mast and sails fill with water. The situation is applicable to a lot of production and home-built small craft. Be forewarned—and make sure your centerboard does not become the problem in a knockdown.

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