We’ve been making some proper boat joints this week. Plank scarphs which are what you would use to join planks together if the boat you want to build is longer than the planks of wood that you have, or if you want to replace a rotten section of a plank without replacing the whole thing. They consist of two tapered ends which have to fit together perfectly. That may seem obvious, but it’s easier said than done. ‘How flat is flat?’ is back to haunt me again. The tapers are cut roughly to shape and then planed within a nanometre of their lives. Then I try to match them together and there’s a tiny little wobble. But that just won’t do. The strength of a scarph joint lies in the fact that there’s a large surface area of contact between the two planks. A bit of a wobble means that there isn’t, so the joint will be weak and let in water. So more planing is required. Then I find that I’ve done too much planing and the planks are no longer flush, so I have to cut it off and start again. It’s fun; really. I’ve completed two scarphs so far; a lipped scarph which would be used for a boat with carvel planking and a feather edge scarph which would be used for clinker.
When you’ve mastered your tapering and become a scarph ace, you can’t put them just anywhere. Lloyd’s Register has rules about the use of scarphs in boat construction in order to preserve the integrity of the structure. If you look at the side of a wooden boat, any scarphs along a horizontal line must be at least 1.2m apart and in a vertical line must be separated by at least 3 planks, so you have to plan carefully where they go. They have to be the right way round as well, if the outside lip isn’t on the trailing edge of the joint, then water could be forced inside the joint and open it up.
I can’t claim to be a scarph ace yet, but I have got plenty of scars on the ends of my fingers and thumbs from the chisels. That’s my ailment of the week; at least I can stand up straight again though.
It’s keel scarphs in the next thrilling episode (probably).