Enclosed buoyancy compartments on wood/plywood boats

Discussion in 'Boat Design' started by laukejas, Jun 27, 2019.

  1. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    Alright, thanks for the specifics, but why do you put that foam in the first place? Since it doesn't adhere to plywood, that means the water can still get around in these compartments, and drying them out with the foam is much harder, because it blocks ventilation, isn't that so?
  2. Ike
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    Ike Senior Member

    Actually the answer is simple, an overabundance of caution. On the 12 foot boat I really didn't need to put foam in the air chambers. On the next boat I built, an 8 foot sailing dinghy, I used just air chambers. But on the 12 footer my thinking was, it's going to have a 2hp outboard so I should put some flotation under the rear seat to float the engine, in the case the boat was swamped. In the original configuration that seat had a hatch cover, and like you I worried about water getting in. But I never used the compartment for storage and in fact was usually sitting on the hatch cover. So I just sealed it all up, but left the foam in there. It was glued in place with epoxy and fiberglass and would have been a real mess to remove. But the compartment under the middle seat is open and used for storage and has about 2 cubic feet of foam under the seat, but it doesn't protrude into the storage part of the compartment. The bow seat was the same as the rear seat. It has about 3/4 of a cubic foot of foam in it, which left room for storage. But again it was never used for that, so when I sealed up the rear seat I also sealed up the bow seat.
  3. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    You can always have inspection plates on all your floatation chambers. This is common practice in the Puddle Duck community. When the boat isn't being used, remove the plates.
  4. tlouth7
    Joined: Jun 2013
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    tlouth7 Senior Member

    A few days a year I would guess. It spent several weeks afloat for a few summers. Summers spent in a dinghy park with a cover on, winters in an open garage. 50 years is not terribly old, with a bit of care.
  5. rnlock
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    rnlock Senior Member

    If you were really paranoid, and there wasn't anything subject to corrosion in or near the compartments, you could throw in some salt to prevent rot.

    I imagine a solar powered fan over an open inspection port would dry things out pretty well if there was another open inspection port. Or you could stick a hose a couple of feet into the compartment and attach it to an aquarium air pump. If solar powered, you might need an inverter. You could just leave it like that, maybe.
  6. phillysailor
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    phillysailor Junior Member

    My Va’a motu, an outrigger canoe, has four cockpit watertight compartments and bow & stern compartments.

    I use Armstrong hatches, as they are the only ones approaching watertight performance. No need for a ring, but only works on flat surfaces.

    Epoxied and painted all compartment surfaces used for storage, bow and stern compartments have a small inspection hatch and are just epoxied.

    In summer, they seem ok, over winter I put a Damp-Rid container in each compartment. Refills are $5
  7. rnlock
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    rnlock Senior Member

    I understand that some cat litter is based on silica gel, which might be cheaper if you have a bunch of compartments,to deal with. The regular is made out of a kind of clay that works as a resistant. If I remember correctly, silica gel absorbs a lot of water, but clay can absorb down to a lower humidity level. Both can be baked to restore, but I don't know if cat litter has extra stuff in it that might cause problems.
    Gore Tex made a product that would keep liquid water out but would allow gases through. That probably includes water vapor, though. I think the stuff I remember is more robust than what they sell for clothes. We used a little patch of it to prevent a sealed medical gadget from popping off a stick-on membrane switch when transported on airplanes.
    When I was a kid, we had an outboard boat that sunk at the dock several times when filled with splashed water that came over the dock. I recall it had foam under the floor, but apparently not enough or the wrong kind.

  8. John Perry
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    John Perry Senior Member

    Our plywood sailing dinghy was first launched in 1978 and is still sound and in use today. It has probably had more than average use for a sailing dinghy - a number of passages across the middle and wide end of the English channel, cruises of several weeks duration in various parts of Europe using the overboom tent at night, numerous weekend trips. It has a number of compartments low down in the boat which were filled with foam and sealed up when the boat was built. These compartments do not have any inspection hatches or drain holes and from the outside at least, there is no sign of degredation. About 20 years ago I did cut a small hole in one of these compartments and found that the foam was completely dry, I then sealed up the hole with epoxy resin. Another time I am not sure that I would bother with the foam - it is not necessary for bouyancy since even if the boat were damaged in a colision enough compartments would almost certainly remain bouyant. In large volumes, the foam is significant weight, for our boat I actually saw this as an advantage since it is low in the boat and can be considered as part of the ballast, however if you are aiming for a minimum weight boat this is definitely not an advantage. One point in favour of the foam is that it does support the hull skin and the floor above, it makes the floor in the boat very firm to walk on. We do also have compartments higher in the boat which are not foam filled and which have inspection hatches. These inspection hatches leak slightly and these compartments have to be dried out with a sponge after sailing in rough conditions. At least the inspection hatches are close to the low points in the compartments which makes drying with a sponge possible. Another time I would be tempted to just seal everything with epoxy resin and forget about it!
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