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Thread: flotation under floor boards

  1. #1
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    Default flotation under floor boards

    after pondering the best way to add flotation to the flat bottom skiff i'm building, I've thought of the idea of gluing 1/2 inch thick closed cell foam to the floor interspersed with stringers, and laying a slatted wooden floor over it. In an already light plywood glued lapstrake boat that would likely be enough buoyancy for my needs. the thoughts of the forum.?

    thanks

    David

  2. #2
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    Default Re: flotation under floor boards

    Keep in mind stability, on a really bad day - more water in the boat than you ever expected. The foam, plus floorboards, will lift crew, cargo, and sloshing water up; in a small boat you want all this as low and centered as possible.

    Maybe better to add 1 or 2 inches in selected locations, leaving a big, easily bailable footwell and "bilge" in the center of the boat, where water will increase stability. Wet feet provide a reminder to get the water out

  3. #3
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    Default Re: flotation under floor boards

    thanks- that makes great sense.

    David

  4. #4
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    Default Re: flotation under floor boards

    Performed an un-scientific test when I put the 2" thick closed cell foam under the floor of a Jerricho Bay Lobster skiff. Cut a test piece 12" x 12", put it in the pool with a plastic bucket on it and started adding 16 oz. weights until the weight submerged it, forget what the total was for the test piece but it was fairly high. Do remember the total flotation for all that was installed was over 600 lbs. flotation which was greater than the total weight of the boat, motor and equipment.

  5. #5
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    Default flotation under floor boards

    Two thoughts. One, I think low floatation will encourage the boat to turtle when swamped. Two, will the fastenings and floor slats be strong enough to hold against the force of buoyancy the foam they are holding down?


    Kevin


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    There are two kinds of boaters: those who have run aground, and those who lie about it.

  6. #6
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    Default Re: flotation under floor boards

    To allow for the fact that foam weighs more than air, to have a fudge safety factor, and to make for easy in my head calculation, I figure 60# buoyancy for every cubic foot of foam. Foam under floorboards creates stability problems as well as clean out issues. Start with how much you actually need and how it should be located. Like enough at the stern to float the outboard, if there is one.

    A common solution is to strap blocks of foam under seats and thwarts. Or, especially for stern sheets and at the bow, enclose a space and pour in foam. Some foams are flexible enough to glue to the inside of the hull planks protected at the top by the inwale and perhaps dacron glued to the exposed surface. A skiff your size might fit four cubic feet or 240# buoyancy just that way.

    Since the dink is wood, base your buoyancy plan mostly on your crew and gear weights.

    G'luck

  7. #7
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    Default Re: flotation under floor boards

    You can put some of your required flotation under the floorboards. But it is the flotation that peaks above the swamped waterline that will provide waterplane area and stability to keep the boat level. So flotation needs to be placed around the perimeter of the hull. As stated above, place tall flotation blocks in the quarters, amidships at each rail or in the thwart, and in the bow. Or do what I do: Don't use any.

  8. #8
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    Default Re: flotation under floor boards

    There have been thousands of aluminum boats made in the 14-17' range with flotation under the floor and meet CG upright flotation requirements.

  9. #9
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    Default Re: flotation under floor boards

    I have not seen any dimensions as to how big this boat is or mention of its structural components. As a result, I wonder if that which you propose is not overkill on a small boat that will be used in areas that are close to shore where inflatable life jackets can be worn in place of bulky flotation acoutrements within the hull itself unless, the boat as is, does not have positive flotation from its own construction components. Such as what you propose at this time can have the annoying habit of taking up space in a small hull and can also attract humidity, mold and dirt in a small skiff that can be a real pain in the posterior to keep clean!
    Jay
    Last edited by Jay Greer; 03-23-2020 at 12:05 PM.

  10. #10
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    Default Re: flotation under floor boards

    Ideally floatation should be up under the gunnels to insure upright floatation.

  11. #11
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    Default Re: flotation under floor boards

    Look fellas, I'm not here to argue. I'll just make a statement based on a half-century of boating experience, a decade of professional seamanship, a university education in naval architecture, thirty-five years of practicing boat design that specialized in small craft, and a large amount of experience doing stability analysis of boats. Make of it what you will:

    Put floatation as high in the interior of the boat as possible - under the floorboards is better than nothing, but is the worst place in an open boat that you can put it.

    OK; back to your regular programming...
    Last edited by mmd; 03-22-2020 at 02:06 PM.
    Hope for the best, but plan for the worst.

  12. #12
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    Default Re: flotation under floor boards



    Ballast low, flotation high -- them's the rules!
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  13. #13
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    Default Re: flotation under floor boards

    Far be it for me to argue with common sense!
    Jay

  14. #14
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    Default Re: flotation under floor boards

    I have been following this discussion with great interest. I mostly finished my Lightning last Summer, but am still working on flotation.

    I have included the Class Rule below. As you can see, it requires enough flotation to keep the water in a swamped boat 2" below the centerboard trunk cap. The general response by builders has been to locate flotation as low in the boat as possible. For example, the modern fiberglass Lightnings have several inches of foam built into the floor, and the seats also serve as buoyancy tanks.

    The flotation I have in my new Lightning consists of several bags under the aft and forward decks. I will probably get some of the hotdog shaped bags to go under the seats.

    But it sounds like I should add some flotation higher in the hull to help with stability when swamped.

    I have included a couple of pictures to help show what I am dealing with.

    I would appreciate any ideas or feedback very much.

    Mike Seibert


    1. All Lightnings must have sufficient flotation to keep the centerboard trunk cap of a fully rigged swamped Lightning at not less than 50.80 mm (2") above water level while supporting 136.0791 kg (300 lbs.) of additional weight applied above the water.

      DSCN2073.jpgDSCN2079.jpg





  15. #15
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    Default Re: flotation under floor boards

    My 10 ft pram flipped last summer in a vicious squall in Lake Huron (first time in 18 seasons). It has a fore-and-aft seat, not side-to-side, with 1.5" pink foam glued under it, not the floorboards.

    IMG_4478 (2).jpg

    It was unstable when inverted (!) and easy to flip upright. Then I maneuvered it off the stern of Drake, yanked up the bow with all my strength, and a large amount of water gooshed out the back over the transom. I did that twice, then was able to hop in and bail.

    Of course the pool-noodle fenders made another difference to the righting-moment.
    Last edited by Dave Hadfield; 03-26-2020 at 08:02 AM.

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