Drying out Foam Core

Discussion in 'Fiberglass and Composite Boat Building' started by keith66, May 17, 2021.

  1. keith66
    Joined: Sep 2007
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    keith66 Senior Member

    My Son & I are building a Proa out of a recycled Cat, The cat in question is we think a Kelsall design? about 40ft long, Originally built around 1976 the hulls were very well laid up using a combination of vacuum bagged flat panels with constant camber sections on the lower parts of the hull to give a round bilge.
    The owner invested a lot of time & money into the project but gave up when all the major components had been made. The hulls sat upside down until 3 years ago when they were cut into 3 & dumped.
    We recently rescued the bow sections & have cut & shut them into a proa hull.
    Overall condition of the bits is pretty good but a couple of areas have got fresh water into the core caused by puddling in areas where kerfs & joins hadnt been glassed over on the inside.
    Core is 15mm Divinylcell with roving & csm inner & outer skins, Not sure what resin was used Probably Iso polyester.
    We will be running tests with a moisture meter & are thinking to drill a lot of holes in the thinner inside skin & see how it dries. May try vacuum bag & heat lamp if its bad. As its never been afloat salt isnt a problem!
    Im wondering how long drying out will take anyone done similar?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    How do you know water has got inside ? If you know the point of ingress, maybe hang it up with that at the bottom and let gravity do it. Perhaps you can inject air in to an opening(s) and see if that pushes it out, alternatively.
     
  3. keith66
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    keith66 Senior Member

    We noticed one panel that we had cut out for the Ama was noticeably heavier, took a core sample through the inner skin & core with a tank cutter, it was obvious that water was in there as the dust from the hole saw was damp. Popped the sample out & squashed it in the vice, It was well wet. Never fails to amaze me how supposedly closed cell foam can soak up water over the years. with the ama its not going to be critical as we may well just use it as a plug for a new foam sandwich ama. It wil be down to time & how much grinding which way we jump.
    With the main hull we want it to be reasonably sound to start with. Hoping for some hot sunny weather before long to dry the thing out a bit!
    Its not a fancy project & very much being done on a shoestring with scrap materials & leftovers.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Are you sure this is divinycell ? 1976 seems early
     
  5. keith66
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    keith66 Senior Member

    I was told by a guy that helped the owner lay it up that that is what they used, he may have been mistaken? But it is a PVC foam core, heavier green grade in the bottom & lighter grade in topsides, Considering the hulls were standing outside for nearly 45 years (upside down) they have stood up remarkably well, core is still flexible & doesnt seem to be friable at all.
     
  6. fallguy
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    fallguy Senior Member

    I used a heat lamp to dry a transom on a Boston Whaler Sport and it was an exercise in how to create blistering. The steam migrated to the areas of the hull adjoining and now there are blisters there. The pattern of blisters perfectly resembles where the steam would have gone..I needed more escape holes, but didn't want to punch the hull, now to get the blisters, I need like 100 holes and vac.

    If you can find a way to use vacuum; it'd beat heat unless the steam isn't going to migrate to some other location. Or some combo of vac and heat may work. Might need a water catch pot in the line.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Definitely not PU foam ? If it is spongy wet, seems unlikely to be divinycell or similar. I can't think of a PVC that was green
     
  8. fallguy
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    fallguy Senior Member

    corecell pvc is green, but was it around then as atc?
     
  9. ondarvr
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    ondarvr Senior Member

    Wet foam can be dried with time and effort, but being dry again doesn't make it functional.

    Over time the cell walls begin to breakdown, being wet can speed that process.

    So what was once a closed cell foam now resembles a sponge.

    Freeze and thaw cycles accelerate the degradation even quicker.

    So just having the foam dry again frequently doesn't help you at all.
     
  10. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Remove the wet foam.

    It will take decades in the Sahara to dry.

    And then it will still be structurally compromised.

    Removal will be the least problematic route to travel.
     
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  11. keith66
    Joined: Sep 2007
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    keith66 Senior Member

    You are right. Took some readings with a moisture meter today, one end of the ama is very wet. We will use it as a plug for a new one, Got some Pet foam left over from an old job, Wrap it round the outside & fasten on with small cable ties, then glass over. Its not worth doing all that grinding.
    As for the main hull the wet bits are very localised so not too bad to deal with!
     
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  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Is there some other device besides a moisture meter that might "see" what is inside ? Some sort of ultrasound device perhaps ? It is strange that a boat that has never been in the water had become waterlogged.
     
  13. upchurchmr
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    upchurchmr Senior Member

    Forget drying, cut out the foam.
    Redo it.

    Drying is a fools errand.
     
  14. keith66
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    keith66 Senior Member

    The hulls were stored ok for a long time but then sawn into three to destroy them & make them easier to move. They were then just left in the open on their sides, the panels that got water in were kerfed in places on their inner skin & were a pond for some years. Rest of the hulls that had no laying water are bone dry.
     

  15. keith66
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    keith66 Senior Member

    Update, went over the main hull with moisture meter, luckily damp areas were quite localised & in non critical areas. Drilled a 1" grid pattern of 5mm holes on the inner skin in those areas & left it open to the sun, in two days the moisture content had halved. Will pull a core sample or two to be sure. Rest of it is very dry indeed. New ama is ready to lay up.
     
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