Expanding foam for buoyancy tanks

  • 26 May 2021 21:43
    Reply # 10559267 on 10472007


    In my proa Nixe I used two component foam in some parts where I had no or did not want no access. These spaces were sealed with epoxy. What I did was pouring about half of what was needed to fill and waited to get it cured and cooled down. Then it shrinked. I poured some more into the gaps on the side and on top and again and again until the space was filled. Then I cut off what was protruding and closed with epoxy sealed plywood.

    After 5 years I realised that there was a gap where water poured in in two of these foam filled spaces. I had bought it as closed cell foam but it seems it was not. Foam on the lower part had become soft, filled with water and was there in crumbs. The plywood was well sealed so it did not suffer but there was a lot of water inside.

    The idea to fill spaces with foam was to get safety against running aground or hitting something in the water at higher speed. I would not do it again but look for another solution.


  • 15 May 2021 11:15
    Reply # 10506996 on 10472007

    Perhaps it is a lot better than it used to be.

    The only time I used it, some years ago, it definitely shrunk after a while.

    Also the supposed close cell foam on my catamaran was quite waterlogged, though after about 35 years.

    Curious thing about fizzy drinks bottles is that you will find after a year or two they have shrunk quite a bit.  

    The best option I heard of was to cut up extruded polystyrene into cubes, first having calculated how many cube needed to float your boat, then stuff them into all available spaces.

  • 14 May 2021 15:41
    Reply # 10502872 on 10472007

    Be careful with the pour-in-place foam.  If you miscalculate the volume it is capable of generating pretty tremendous forces and causing significant distortion. If you have good open access, then multiple pours might be a good way to avoid distortion.

    I've only used the two-part foam on three occasions and never enjoyed it.  It expands so quickly after mixing and puts so much force on any semi-enclosed space, I find it a very persnickety material.

  • 13 May 2021 03:29
    Reply # 10475409 on 10472007

    I got some "damaged" pink foam from the hardware store at a discount. Would have coast me a lot to use the two part poor in foam for the whole lot. I just put two pieces of pink foam in then poor the two part on ether side to fill the gaps. The top piece and plywood cover I just spread a thin layer of two part on it and closed it up really fast. 

    Second David with the plastic. It's a hot mess in my cabin. 

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  • 12 May 2021 16:28
    Reply # 10473322 on 10472007

    Use metal casting techniques, with a runner in the centre of the top face, and risers right out in each corner. Put a large cardboard funnel in the runner so that you can pour quickly, and tape down some plastic sheet over the whole area so that having calculated for a small excess of volume, it will ooze out harmlessly and can be lifted off when set.

  • 12 May 2021 15:03
    Reply # 10472895 on 10472007

    I don't know what the potential for polyurethane foam to become waterlogged is if any.  Some folks simply fill empty spaces with air filled containers.... empty plastic bottles, ping pong balls, etc.   Pour in foam or spray in foam are a mess if you have to take it out again.

  • 12 May 2021 13:37
    Reply # 10472464 on 10472007

    Thank you David. That's what I will do, then.

    I presume you closed off the tanks first, leaving a couple of holes in the top for pouring and venting? Any particular way of ensuring no voids?

  • 12 May 2021 13:10
    Reply # 10472332 on 10472007

    I put fore and aft buoyancy tanks into my dinghy Dorothy, with the inner faces coated with epoxy. I totally filled them with pour-in polyurethane foam, and capped the pouring and venting holes permanently.

    Good as new, 32 years later. If there are no voids, there can be no water ingress.

  • 12 May 2021 12:10
    Message # 10472007

    I am putting some buoyancy compartments into this Golden Bay conversion - the two main tanks will have hatches so they can be inspected inside, ventilated  - and also used for a bit of light weight storage.

    There are some other tanks which will be just for buoyancy only, and I am reluctant to close them in completely - also reluctant to add yet more inspection ports. (Part of the problem is retrofitting, one of the downsides of modifying an existing boat rather than starting from the beginning with an integrated design).

    I have been wondering about a couple of coats of epoxy inside, then filling them with pour-in polyurethane foam and closing them off with no drainage or ventilation. Or, perhaps, leaving a cavity in the bottom and providing a drain plug. I haven't used expanding foam before, wondering if this is a good way to provide extra buoyancy - is this good practice with plywood?

    Is anyone able to offer advice?

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