double skin boats

Discussion in 'Boat Design' started by busumark, Jan 8, 2009.

  1. busumark
    Joined: Dec 2008
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    busumark Junior Member

    hello. can someone please tell me if a double skin boat sinks if there is a hole in the bottom? the boat does not have expanding foam. thanks
     
  2. messabout
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    messabout Senior Member

    Let us assume that you are referring to double planked hulls or perhaps laminated hulls. In that case the boat, with a sizable hole in the bottom, is likely to sink. The sinking conclusion supposes that the boat is normally loaded and that there is not enough skin structure to provide a displacement figure larger than the weight of the boat and its' contents. That would require a very thick skin of some material, wood perhaps, that has a specific weight that is less than the specific weight of water.

    If the boat is constructed in such a way that the inner skin is seperated from the outer skin by some distance, then there is an air chamber between the two skins. That construction is used often on various fiberglass boats. If the air chambers have watertight cells, then they will constitute a very useful bouyancy component. Such a construction might very well stay afloat, even when the bottom is holed. The key words are: "might stay afloat" Many hulls have double bottoms of that type but not double topsides. In that case the boat may tend to invert itself, bottom up.

    The answer to your question can not be stated with certainty because of the many variables that may be involved. In any case it is obviously better and safer to avoid holing the bottom in the first place.
     
  3. busumark
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    busumark Junior Member

    ok thanks it is explained well. so to be safe its better to do a boat with expanding foam
     
  4. robherc
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    robherc Designer/Hobbyist

    That's what I'd recommend. For expanding foam to pass USCG testing, it must keep its flotation with VERY minimal waterlogging for over 18hrs! I'd much rather trust my life to foam that'll hold water out for 18 hrs even AFTER the boat's been punctured, than to air chambers that are compromised the instant the puncture happened. :)
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    And if the hull have been punctured in an earlier event you know nothing about it. Wetting foam just sink waterline a bit...
    So foam if used got to be either totally waterlogging for extended periods (meaning years), be placed or protected in a way that protects it from water or used in a dinghy size boat when you can notice the weight of excess water (and buy a new one)..
     

  6. robherc
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    robherc Designer/Hobbyist

    You make a good point, but I would HOPE that I'd know enough to check the boat's hull after any event that likely damaged the outer skin to that point. Otherwise, there's really nothing to save you from bad boatsmanship...you'll sink immediately with the air chambers, or lose your boat days later with foam...either way, not checking your hull after you hang it on some rocks/coral is pure stupidity in action.
    Oh yeah, if only 1-2 "well segmented" air chambers are filled...then "flooding airchambers just sink the waterline a bit" too. Not trying to disrespect you, but your post didn't make much sense from a practical standpoint there...pessimistic of good, safe design, and excusing of reckless operation & lack of maintenance.
     
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