Question about completely foam filled hulls

Discussion in 'Boatbuilding' started by Jmooredesigns, Dec 25, 2016.

  1. upchurchmr
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    upchurchmr Senior Member

    You really need to do a design and see.
    It is really of no value to speculate and fool lots of people into believing your wishful thinking.

    I participated in this same design study in aircraft using honeycomb instead of foam.
    Inevitably the filled cavity was the heaviest option.

    There are always other very good reasons to choose a design besides weight.
    I'm not objecting to foam filled due to weight, you just need to know what the reality is, and make all the other choices.
     
  2. JamesG123
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    JamesG123 Junior Member

    It depends on if you can fool them in to giving you money. ;)

    The difference being the cell size and how much they will flood when the skin or filled space is ruptured. While in aircraft the analysis usually stops at the structural, in the nautical, you then have to consider if your now broken structure is going to sink...
     
  3. upchurchmr
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    upchurchmr Senior Member

    Lets not be silly.

    An aircraft is going to fall out of the sky, if the damage is enough.

    A boat doesn't have to sink and it doesn't need foam to stop it. Closed cavities can provide just as much flotation - actually, with them you don't need the weight of the foam, so there is more net flotation.
     
  4. JamesG123
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    JamesG123 Junior Member

    Depends, if you breach the skin on a coarsely filled (ie: "honeycomb") structure enough that it threatens positive buoyancy, then... its going to sink. The benefit of heavy closed cell foams is that they are very damage tolerant and will retain buoyancy even after extreme insult.

    There is a reason (beyond just cost) why no one uses aerospace honeycomb in boats...
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    It depends. I know a fella who hit something very hard, travelling at night in a glass power cat, the amount of damage was such that both sides were shattered, for a goodly length, and he'd have gone down if not for the large blocks of polystyrene foam packed in there when it was built.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    JamesG123 beat me to it, by one minute !
     
  7. upchurchmr
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    upchurchmr Senior Member

    It certainly does depend.
    There are always circumstances which defeat almost any safety feature.

    Now if a boat is hit which has honeycomb any place not having the facesheet punctured will retain its flotation. "it depends" upon the weight of the boat and the total flotation left if the boat will sink. If you sail a lead mine you will have less chance of not sinking.

    You guys are not listening. I only said that foam filled is typically heavier than another construction.
    I then said you had to then move on to "other" considerations to see if you still want all that foam.
    I never said it was bad, just that there were other options.

    Gee whiz, all those boats in the 1800's must have been unacceptable - right?

    Oh, I didn't suggest aerospace core in boats, I just used that example because that was my background. Although Stiletto catamarans seem to work well.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    There are certainly jurisdictions that mandate foam buoyancy in recreational boats, and lots of people owe their lives to it. I would not put to sea in any boat without it, failing that, a self-inflating life raft on board. But I can see the problems with PU foam are a deterrent.
     
  9. jorgepease
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    jorgepease Senior Member

    the newer foam core sandwich boats may have enough flotation as it is, mine does. I think the regs are for boats 20' and under. I dislike fill foam I have seen too many boats with foam turned to slush, it holds water against the glass and starts to rot. Much better would be some blocks of foam not attached to the hull. Also, most foam flotation, including in that pic is going to cause the boat to turtle, don't really see how you can prevent it in that style boat. As a structure, foam is friable, less so in the higher densities but then you have lot's of extra weight.
     
  10. TANSL
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    TANSL Senior Member

    I would never use foam as a structural element. In any case, it would be necessary to study the loads that the foam must withstand and to determine if the mechanical properties of the foam are sufficient for that. For example, wood works very well to compression and not so well to bending. The foam becomes a material whose properties are not homogeneous throughout its volume, depending on its density, how it has been applied and many other variables that are difficult to determine. In my opinion, as a structural element, it is not reliable at all.
    By the way, the photo in post #7 shows a hull that probably, with longitudinal girders, would not have broken in the way it has.
     
  11. upchurchmr
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    upchurchmr Senior Member

    Tansl,

    That hull was cut in half with a chainsaw as a promotional gimmick.

    Wood doesn't work well in bending? Tell that to the trees.
     
  12. TANSL
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    TANSL Senior Member

    Thank you for your explanations.
    Correct: cutting it would not have been so easy if there were longitudinal girders
    Can you re-read what I've written? I mean that the wood works better to compression than to bending. No need to consult the trees and, in any case, I would not know how to interpret their answer.
     
  13. upchurchmr
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    upchurchmr Senior Member

    Longitudinal girders would not make a difference, unless they were steel.

    A chainsaw was used to remove wings from a small bomber at my company to start a rewing job. (Long ago).

    The wing was aluminum with multiple full depth girders (spars).
    I was told it took only minutes to cut off each wing.
     
  14. TANSL
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    TANSL Senior Member

    upchurchmr,
    I see that you have very accurate and up-to-date information.
    Currently cutting, even granite, can take only a few minutes.
    But, as you probably know, all this has nothing to do with the longitudinal strength of a ship, that is what I thought we were talking about
     

  15. upchurchmr
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    upchurchmr Senior Member

    Last response:

    Originally Posted by upchurchmr View Post
    That hull was cut in half with a chainsaw as a promotional gimmick.

    Correct: cutting it would not have been so easy if there were longitudinal girders

    The only time we were talking about cutting something was the foam filled boat.
    Personally I don't call that a "ship".
    The boat we were talking about had foam instead of stringers.
    Why did you introduce girders?

    Waste of time nitpicking this.
    My apologies to everyone else.
     
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