Gerald, Scarph

TernTern is a 12 foot rowing boat with larch planks. Her original planks are dry, brittle and rotten, but a new boat is slowly emerging. A bit like the Victory, but at a (slightly) faster pace; there won’t be much left of the original but she’ll look like the same boat. It’s a great example of what IBTC can do; rebuilding boats that wouldn’t have a chance anywhere else.

I’ve taken over the clinking planking exercise from Zac, who’s gone off to the real world to earn an honest living. My mission is to make a plank that will run from the stem and will join to one of the existing planks with a feathered scarph joint.

As Pete the tutor was enthusiastically explaining the fiendishly complicated process of making a new plank where there is currently just a gap, the main thought going through my head was “can you just do it and I’ll watch?”, but that’s not how you get to be a boat builder, so I decided to just get on with it one stage at a time and see what happened.

The first stage is to get some idea of the shape of the plank that you’re going to make. For that you need a spiling board, which consists of a few bits of old plywood stuck together in the approximate shape of the required plank, on to which you can mark any lines that might be useful. I drew along the top edge of the plank below to get started, and drew some projection lines to represent some measurements that Zac had taken of the missing plank above.

Then I got a chance at last to use the dummy sticks that we made on the last day in the joinery shop, and were assured at the time that they would be very useful. Dummy sticks are rectangles of plywood of various sizes which are used to transfer lines from one place to another, and now I had a line I could copy it onto the piece of larch selected for the new plank.

Once the plank section had been cut roughly to shape it was time to get out the steamer, which is still a bit magical. The new plank section is wrapped up in a bit of plastic tarpaulin and attached by a tube to a wallpaper steamer. After 30 minutes, the plank can be bent to your will (within reason) and held in place with G-cramps.

It’s amazing how a curved plank has a completely different shape from a flat piece. It’s lucky I allowed a bit spare along the edges as my original lines bore little relation to the shape of the boat once it was steamed.

Feathered scarph

Feathered scarph

Now that the plank section was curved I could join it to the aft end of the plank with a feathered scarph. It’s important to get your scarph the right way round, the forward plank should overlap the aft plank so that the water flows over it rather that scooping it up and  gradually opening up the joint.


Gerald at the stem

Now comes a long succession of clamping the plank on to the boat to check the fit and taking it off again to make appropriate adjustments. It has to be shaped along its length, it needs to sit flush on the brow of the plank below, and at the stem and transom, which are where another devilish joint comes in, known as a ‘gerald’. A gerald slopes in two directions: along the length of the plank and across it, but at any point along its length it has to be flat so that the plank above sits flat on it to ensure a watertight joint (rather important in a boat). In Boat Building Techniques Illustrated, Richard Birmingham says:

Because of the changing angles, this is not an easy joint to cut; it is achieved with a rebate plane, chisels and care. It also improves with practice.


I used to think this was a warning to take your time, but now I see it as an aspiration

I used to think this was a warning to take my time, but now I see it as an aspiration.

It’s the details that really take the time in boat building; I got the plank to shape quite quickly, but have spent days working on the contact points with the rest of the boat. It’s also got to be symmetrical with the plank that Alan is making for the other side before we get the sign off to nail and rove them in place. The final hurdle is Pete’s unfailing critical eye which homes in on every error and omission and makes you wonder how you ever missed them. Wishful thinking maybe.





Double jointed

It fits!

The last week was mainly a tale of 2 joints: scarphs and dovetails. We’ve been working on two types of scarph that could be used in a boat’s keel; the table scarph and the hooked scarph. These joints are used as they have longitudinal strength in compression and tension, which is essential for joints along the centreline of the boat. The table scarph is essentially a lipped scarph with a channel cut across it in which is fitted a small piece of wood of square section (called a table) which gives the joint it’s longitudinal strength. The table is craftily placed with the grain running across the joint, which has the advantage that if it gets wet, it expands along the channel, wedging itself even more firmly in. I found it harder to make the hooked scarph as it can’t be planed very easily in the rebated section above the hook, so it involved lots of chiselling, which is harder work and it looked less tidy when it was done; bearing the scars of a few near misses with the chisel.

I thought it would be plain sailing when it came to making dovetail joints as I’ve done a few before for various projects at home. Unfortunately, this previous experience doesn’t seem to have counted for much, I still spent hours on end fine tuning my tails so that they fitted together precisely enough. “I presume it goes in at a right angle” said Jon the tutor of one attempt; followed by “It’s quite a flexible joint”. I don’t think that was a compliment. Anyway, it’s all good preparation for the first big project, a mahogany tool chest. Hopefully I’ll be able to start on that in the next day or so. That will keep me busy for a few weeks.

An inexpertly chiselled hooked scarph

A more successfully planed table scarph

Scarph Ace

A lipped scarph (it doesn’t wobble, honest)

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).

A feather edge scarph