GRP | scantlings

Dave conducting impact research

On a cold morning last week, IBTC class 111 gathered outside the boat shed in white suits and solemnly took it in turns to smash glass fibre hulls to pieces with a hammer. It wasn’t a wooden boat builders’ anti-plastic-boat ritual; it was all in the name of educational research and definitely not at all for fun.

To rewind a bit, we had spent the week under the guidance of Rob, trying out glass reinforced plastic (GRP) hull manufacture and repair; just in case we’re ever tempted to work with boats that aren’t completely made of wood. The first thing to learn about GRP construction is that everything is done backwards: first you wax it, then you apply the outer finish, then you lay on the construction materials.

Coming out of the mould

We started with a two part mould of the bow section of a boat that we polished with mold release (so that our bow sections would drop out of the moulds with ease), then applied two layers of gel coat (it does seem strange to paint something that doesn’t exist yet). Then we applied alternate coats of polyester resin, and chopped strand mat until the layer cake seemed quite substantial. Then we had to leave it for the night. The next morning the bow sections popped out of the mould and looked perfect until they had their first appointment with a hammer. The structure of gel coat, polyester resin and chopped strand mat can take quite a hammering, but if there’s an air gap between the gel coat and the mat, the gel coat just flakes away, and it doesn’t look so pretty.

Preparation for an external repair

The damage had only just begun; the next step was to cut some holes in our (not quite so perfect) work so that we could try out some repairs. In one repair we were allowed access to the inside of the hull and in the other we were not, so we had to do the whole repair from the outside (quite likely in a boat repair when the insides haven’t been stripped out). The principles of both repairs were the same: make a negative of the hull shape required, tidy up the edges of the damaged area, file the edges to a razor sharp edge, and then build up the layers of resin, chopped strand mat and gel coat. The difficulty most of us faced was getting the depth of the repaired area to match that of the original structure, so when we started sanding it smooth, we lost too much gel coat and the resin showed through.

Rob did a great job presenting the course, and the advantages of working with GRP were clear: strong, light structures with a perfect(ish) finish are quick to produce and easily scalable for mass production. One of the downsides is the requirement to work with a heady mix of chemicals with scary looking warning labels on them. I have to say that on balance I’d rather stick with wood.

By the way, the result of the impact testing was that the external repair was generally the first to cave in, usually because the backing plate wasn’t attached strongly enough, but even that took an impressive beating before giving in.