Keeping her afloat

Until we grow gills, keeping the water on the outside of the boat is a pretty good way to ensure longevity. But one day you could find yourself sloshing around in shin-deep water, especially if you spend any time fishing the rips.


Getting the water back over the side as quickly as possible is important for maintaining stability. One of the attractions of our 22-foot Swamp Yankee project boat is its self-bailing cockpit and big scuppers. The Sisu 22 has two oversized scuppers that are almost 6 inches by about 2-1/4 inches. “Big mothers” is how shipwright Charlie Koller describes them. The idea is to shed any water sloshing around on the deck in a hurry in order to minimize the free-surface effect.


Charlie enlarged the original scuppers to remove a lip on the inside edge of the drains, bringing the scuppers flush with the deck and preventing water from puddling inside the boat. We think we’ve got a quick-draining cockpit; time will tell.

The other thing we’ve done to increase safety is to add flotation to Swamp Yankee in the form of closed-cell foam cut to fit the forward and aft bilges. I wanted the boat to stay afloat under the worst-case scenarios — decks awash to the gunwales, a hull breach, a capsize.


Something to keep in mind: There are no federal regulations or standards for flotation in boats 20 feet and greater, even though a handful or two of companies do add flotation to their boats.

The Coast Guard requires all monohull outboard boats under 20 feet to have level flotation — in other words, they must have enough flotation distributed symmetrically so the boat will float relatively level when swamped.

I hope I never find out whether Swamp Yankee floats level, but Charlie and I are confident she will float. He packed the forward bilge, which is about 3 feet at her deepest, with foam cut with a handsaw to fit the space. The aft compartment is right next to the transom — the ideal area to place flotation to support the outboard.


Altogether, Charlie estimates we used seven 10-by-20-by-96-inch foam billets, each with a buoyancy force of about 610 pounds. “That’s a lot of flotation,” Charlie says. “Any more and that boat would float right out of the water.”

Charlie knows a little something about flotation. In the 1970s he worked at the Coast Guard R&D Center in Groton, Conn., when the agency was testing flotation in small boats. And during the 1990s he rebuilt a lot of floating docks as associate harbormaster in San Francisco.


“You build docks long enough, you get to know foam and flotation,” he says.

If you want to delve more deeply, visit the American Boat & Yacht Council website and try a free five-day trial of its standards. The buoyancy project is H-8.

* * *

Shifting gears. When you do a boat refit project, you get as many opinions about the right way to do something as you have people who stop by to see the work. The three deck panels on our Sisu project boat are cored with balsa, and each has a solid-glass flange about 3/8-inch thick and about 4 inches wide. The panels are heavy.

A mechanic friend thought each panel should be supported by an additional bulkhead once in place to keep them from flexing. Charlie laughed and said they were stiff enough to carry a VW without flexing.


In this photo you see Charlie and me standing on a panel supported by sawhorses while trying to get the piece to bend. (He humored me.) That’s roughly 400 pounds in our work boots.

No give. Stiff as a post.

10 thoughts on “Keeping her afloat

  1. I like the idea of the large scuppers to drain the cockpit, but I also see it as a good source of losing equipment. If the cockpit fills, your tools, parts and supplies are going to easily wash overboard. So, if your deck is not clear beforehand, it will be after that wave breaks over the gunwales. Though it may defeat the purpose somewhat, would a grate of some kind be useful?

  2. I thought that Placing the floatation below the water line would cause the boat in a decent sea condition to turn upside down?

    That was the reason for gunnel floatation…

    Thank you for your article it raises questions which is good for the learning condition…

    • For starters, putting foam below the decks won’t cause a boat to be any more or less prone to capsizing, but not adding foam will likely ensure it sinks if the hull is breached. How much foam is used and how it is distributed will also have a big impact on whether the boat floats level or capsizes if her hull is breached.

      One thing is for sure: A boat with no compartmentation or foam in the bilge will sink like a stone with a hull breach that the bilge pumps can’t keep up with. So the only question is whether it sinks on an even keel or, more likely, goes down stern-first and maybe rolling at the same time, which I suppose you could call capsizing if you like. So just having basic flotation is a good start, since the alternative is for your boat to end up sitting on the bottom.

      Having foam is much better than not, in part because foam injected into hull voids can also act as a barrier to water ingress.

      Many boats, even those built to level flotation standards, are susceptible to capsizing in a seaway due to free surface effect — that’s what water sloshing around is called — in the bilge. One excellent way to reliably prevent capsizing, besides keeping the water out of the boat in the first place, is by having lots of buoyancy up high (the RIB being the ultimate design) or by greatly reducing free surface by either filling the bilge chocker-block full of foam and/or by dividing the bilge into many small compartments longitudinally. (The impact of free surface is a function of the square of the beam/width of the compartment in which it is free to slosh about.)

      This also argues for having no bilge, and no free surface, with foam completely filling the space below the main deck, with the exception of fish and storage boxes, which would be so small that free surface would be negligible. With no free surface, the boat would not be any more susceptible to capsizing than with no hull breach — a hull breach in such a boat being a non-event, since there would be no empty space below the damaged waterline for the water to fill.

      If the bilges of Swamp Yankee are divided into compartments and most of the area below decks is filled with foam, then there would be little room for water to slosh around, so the boat would basically be swamp-proof. However, any boat, even one with an intact hull and no flooding, can capsize if the waves are large and steep enough.

      When you see a photo of a scuttled Boston Whaler with 30 or 40 people in it waving at the camera, note that the boat is in flat water and those smiling employees are standing stock still. Once any swamped boat encounters significant wakes and waves, it would likely quickly capsize due to free surface reducing the righting energy. This is especially true if the deck is awash (see comment about beam squared above).

      The reason a RIB with intact collars is such a stable and safe design is that there is so much positive buoyancy up high that free surface alone has almost no chance of capsizing the boat.

      Bottom line is that the foam will keep the boat from sinking; that meeting level flotation standards should keep it floating at something close to static trim (even if capsized), while basic flotation will likely hold the bow out of the water; and a bow is a whole lot easier to spot from a helicopter than your head is. If it is very calm, foam may even keep the boat floating upright, but don’t assume your level-flotation boat will stay upright in a seaway.

      Eric Sorensen is Soundings’ technical expert. He will be giving a seminar about unsinkable boats at the International BoatBuilders’ Exhibition & Conference in Louisville, Ky., Sept. 17-19.

  3. In the photo you are standing on the panel with one foot of each person directly above the supporting sawhorse…..Not a good or true test.

    • You’re right. Climbing up on the deck pieces was spontaneous and done more for a bit of levity at the end of a long day than as a true test. The deck panels are in place and there’s no flex.

    • We considered building up the transom and going with an outboard bracket but in the end felt comfortable enough with the traditional cutout. It’s not perfect and one of those compromises you have to keep an eye on when drifting in a steep chop.

  4. The nice thing about doing your own boat, you are the one who has to suffer the consequences of any
    screw-ups. I like terms such as over-built, will support a VW, quick draining, balanced, etc.. If the decking
    is not to your liking, you can replace it rather easily. Last but not least, every boat is a compromise. The
    inexperienced and uninitiated do not realize the dangerous situations that you can get yourself into with
    a boat even when you are doing everything right – thanks to Mother Nature.

  5. I think I am looking at some spacers under the console to add height? If thats so I would have built up a composite box for the console to sit on that would have raised the “water tight” hatch and inspection port up with it to get those items above nominal deck level and out of the water plane. Water leaking down on to the tank and in the compartment is never good even with the drain going aft. I would also put some scupper flaps on to keep water from coming in those nice drains.

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