Hello

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.

Alex

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.

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.

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?

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?