Wednesday 10 September 2014

Finished stringers, steaming and inner gunwale scarf failure

We finished the stringers today. It's really starting to come together. Here's a side view, taken from the vantage point of my bench:

It was actually pretty easy - we used 12mm x 20mm Tasmanian oak for the stringers, and there was no need for steam - they just went straight in. The bow is really rigid now.

There were a few difficult bits. A couple of the frames were not quite spot on for fair stringers. I found the lowest stringer had to come up about 3mm from plan on bulkhead 3, and the top stringer had to come down about 6mm on bulkhead 6. I'm vacillating about dropping the middle stringer on bulkhead 6 a couple of mm too. Here's the detail for the stringer join on bulkhead 6. Note the spacer that I've made that sits above the stringer.

Once the notches are adjusted, the line of the stringer is really lovely.

We experimented with steaming on a deck reinforcing piece - I added some 18mm square Tasmanian oak half way between the inner gunwale and coaming, from bulkhead 1 back to bulkhead 4. The plan has a fair bit of unsupported deck - I thought it would be useful to add something. Anyway, we used a technique I've seen on youtube for steaming the timber, where you stick it in some tubular bag material and pump steam through. After twenty minutes we just stuck it in its slots while it was still in the bag - easy peasy.

We tried to repeat the exercise with 20mm x 30mm Tasmanian oak for the inner gunwale, but had rather less success. The process was a bit of a comedy routine - the bag came off the steamer just as we were ready to put the gunwale on the boat, then George's mobile rang causing much juggling. I tried to bung it on the boat but found I couldn't tie knots with oven mitts on. Then finally the scarf join half way along the gunwale came apart, and that was the end of that. Here's the scarf join before we started. I'd have said it was a real good one.

And here's the sorry mess afterwards. It was definitely a failure of the epoxy. Both sides look much the same. I'm thinking epoxy and steaming are fundamentally incompatible. I'll either have to scarf after steaming, use timber the full length without scarf joins (not available locally) or else build the gunwale up from multiple thinner pieces that don't need steaming.

1 comment:

Wayne said...


Epoxy and steaming are generally not compatible methods. Epoxy does not like high moisture content, which can occur if you steam before gluing. Epoxy also weakens very badly at elevated temperature, as happened in this case.

Probably the only way you could glue steamed materials is if you steamed it, then clamped it into place and allowed it to completely dry before using epoxy.

On a lighter note, I was able to build Good Enough with no steaming whatsoever. I also used meranti ply, which is less flexible than okoume. I did do the gunwale stringer in two 20x20 pieces.