Under Deck Foam Flotation?

Discussion in 'Wooden Boat Building and Restoration' started by CapeCodAngler, Mar 7, 2009.

  1. CapeCodAngler
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    CapeCodAngler Junior Member

    I am restoring a 19' fiberglass skiff. The previous owner removed the rotten fiberglass decking and replaced it with a plywood deck over PT stringers. The deck is "watertight". I am concerned that there is no flotation in the boat. Should I add foam flotation in the bilge area? The original design had this, but it was removed when the old deck was replaced. I would appreciate your recommendations. Thank-you
     

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  2. PAR
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    PAR Yacht Designer/Builder

    It appears you have a freshly painted interior. It would be a sin to have to redo this for some foam of questionably value. Your deck space, seems to small to offer much by way of underdeck area to help much with floatation. Frankly, on a boat like yours, with a huge, wide open cockpit, there aren't many options in the event of a swamping. The enclosed spaces below the sole (cockpit decking) will be a buoyancy chamber and just as effective as if foam filled, plus they'll support slightly more boat, without the weight of the foam.

    Fit her out and move on to bigger worries, like how to convince the other half you truly need that new 90 HP 4 stroke.
     
  3. rasorinc
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    rasorinc Senior Member

    I'm guessing that you could fit about 300 lbs of flotation far up in the bow in the enclosed space. Also, I'm thinking of inflatable flotation there and a series of small tire tubes tied to the sides that act as dock bumpers. Just a thought for you to consider. Low expense, guaranteed to float and hold up a lot of weight. At the end of the day protect them from the sun and they should last several seasons. Only use enough pressure to fully inflate them NOT 35 lbs. PSI. In the bow if you can get measurments you can glue up a shaped bladder and put an auto filler nozzel on it. Water won't rust this form of floation and if protected from the sun it will not degrade for many years. deflate them when not is use.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Unless you can devise a way to hide in the neighborhood of 32 cubic feet of flotation (yep, that's a whole lot of dead, unusable space) adding buoyancy bags or foam or whatever is a fruitless operation. I've estimated (on the light side) about what your boat would displace, full up with a crew load, stores, equipment, engine, tanks, gear, coolers full of beer and Pokie, the wonder dog. This is what you have to keep from sinking, plus a reasonable safety margin to keep the gunnels above water. In most small powerboats you sacrifice a huge amount of space to do this. Since you're under sole areas are sealed up, have limited under deck space, you're best just running with what you've got.

    I suspect the under sole areas will be enough to keep, if lightly loaded, boat awash, but afloat in the event of a problem. In these conditions, you'll want to be able to tow your boat to shore and make repairs. I'm reasonably confident she'll do just this if the need should arise. Buy Pokie some doggie treats and save yourself the effort, moving on to other issues.
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    The bottom edge of the bulkhead is flat but it's sides suggest a Vee bottom so it probably continues down past the visible floor. Also the bilges are rounded but the floor is flat. Both these suggest a false floor. If so there is already floatation space between them. Measure the bottom thickness from inside to outside to verify.
     
  6. PAR
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    PAR Yacht Designer/Builder

    That's what I saw Terry, but my tip offs were the scuppers in the transom, several inches above the bottom of the boat. In the forward portions of the hull this will be deeper, if typically shaped, which she appears to be. It's disturbing there is no visible access the bilge nor a drain for it. This might be a future task for the owner.
     
  7. CapeCodAngler
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    CapeCodAngler Junior Member

    Restoration

    Thanks Gentleman for your interest. I purchased the Midland Skiff in November and brought it down here from NH. The previous owner is a lobsterman and restored the boat for use in his work. He raised the deck and installed the scuppers. It has not had its "sea trials" since the work was done.

    I plan to add a small center console and swingback seat. I will add access hatches for ventilation and for running the cables and gas line. I'd like to set up the fuel like the classic Boston Whaler Montauk: two portable tanks under the seat open on the back side.

    I believe there is good depth to the bilge towards the front. I have yet to hear a strong case being made for adding flotation under the deck.

    I would appreciate any thoughts you or other readers may have.
     
  8. Ike
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    Ike Senior Member

    Well let me chime in here. I hate to see a small boat without flotation but you are kind of stuck. To provide adequate flotation to accomplish anything would require major work. Just to float the outboard would require several cubic feet placed on either side of the boat at the transom. Then there would have to be flotation to float the boat weight and flotation for the persons weight.

    You could stuff some foam up under the gunwales but that would probably only float the boat. A 5 foot pool noodle will float about 12 lbs ( about 1/5 cubic foot) So you would need a lot of noodles. To know how much I would need a lot more info on the boat though.

    PS, if that deck is watertight and there are no limber holes in the frames and stringers under the deck then you have a great big airchamber that will provide flotation. But the boat would turn turle and float upside down.
     
  9. CapeCodAngler
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    CapeCodAngler Junior Member

    Turtle? Photos?

    Are you suggesting that should the boat get "swamped" and unable to make headway, it might turtle? Yikes! My primary concern is what might happen if I run into something and the hull gets punctured.

    Here are a couple more photos of the boat.
     

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  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    "I have yet to hear a strong case being made for adding flotation under the deck"
    "... you have a great big airchamber that will provide flotation. But the boat would turn turle and float upside down."

    Agree with both in spades! If she floats fairly level level at least you will have a place to wait for rescue if she ever flips over. However, with the weight of the outboard she will want to stay upside down and will float bow up. Adding floatation in the bow would make it worse. With that much floatation low down I don't think you have much chance of making her self-righting, but by the looks of those holes low on the transom you have a self-draining cockpit if the motor isn't too heavy.
    __________________
     
  11. thudpucker
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    thudpucker Senior Member

    Well let me muddy up the waters here with a question.
    You can see his topsides are roomy beneath!
    What if you filled that space with Foam all the way around?
    Would that be enough to float it 'Gunn'l's up' and keep it up at the surface.

    I'm thinking of a Capsize, as in he got caught where he shouldnt have been and was rolled over.
    With an outboard, and Battery etc that would be heavy, but Gasoline that would float, would the Foam or whatever under the Gunn'ls keep the Gunn'ls on the surface?
     
  12. Ike
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    Ike Senior Member

    That is why I suggested he put flotation under the gunwales. That would help to keep the boat upright. And, yes it would turtle. Did you see the photos of the boat off Florida with the NFL players. One guy survived out of four and he was sitting on the over turned boat. SO it would float level upside down. I have tested several hundred boats for level flotation and those with lots of flotation under the deck do exactly that. You need some flotation up higher and in the bow to keep it upright. The other problem is the outboards. O/Bs have a high center of gravity and tend to turn a swamped boat over. So usually there is a box in each stern quarter with flotation (it can be an airchamber) to float the engine in an upright manner.
     
  13. thudpucker
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    thudpucker Senior Member

    Do you think the Gunnl's in question would provide enough floatation by themsleves, to keep the boat on the surface?

    I gues it wouldnt matter where the floatation is, as long as it kept the boat on the surface.

    In CCAnglers case, By himself, he may have to sit on the overturned boat, but with some help you might be able to turn the boat back upright.
    Then it would stay upright and on the surface, if it had some floatation up in those Gunnl's.

    In the NFL players case, the Bimini was just too heavy to ever get the boat upright without a lift.
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Weel, you could add a bouyant mast to stop it turtling and a fin keel that you could stand on to right it, but then it would be well on the way to turning into a sailboat.
     

  15. PAR
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    PAR Yacht Designer/Builder

    The stability of a hull shaped like that, unfortunately is quite stable upside down..

    Floatation under the side decks is the logical location and yes it matters where you place the floatation.
     
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