Building a Northumbrian Coble in Clinker Ply
Weeks 2 and 3
After I finally finished shaping all the moulds I had to cut the notches for the "hog" (or keelson, as I call it). That's the "backbone" that runs all the way along the bottom of the boat between the lowermost (garboard) planks. Those didn't go as smoothly as I wanted because the wood isn't very good and they intersect the grain at an awkward angle. The end result was a bit messy but more than adequate for this purpose. I just hope that they are the right depth because I need the rabbets to line up properly. It's much harder to get that lined up when the planking meets the keelson at such a shallow angle as it does on most of the frames in this boat. Now, since the plans didn't actually include any information about how wide to make the rabbet I had to use all my wisdom and skill to make it a width that would work and be strong enough. Oh, and it varies all the way along the boat, so really it's a very visual thing rather than choosing a width. To draw it out I had to use a batten (battens again!) which I nailed into a fair curve and traced along. Once I'd laid out one side I had to do the other side exactly the same. I actually managed quite well and it seemed to be well within 1mm the whole way along.
Before I started shaping the keelson I had to cut out the slot for the centerboard. This was challenging since it is a slot over a meter long, 24mm wide, and with nowhere to start the cut with a saw. I had to drill a bunch of holes along the side of the slot and then chisel them flush with the line and then I could get the saw in there and cut it out. I was really impressed with how my new Japanese rip saw worked. I'm still totally amazed that ships as big as a man o' war could have been built with hand tools. I was sawing a 1" thick plank… they were sawing thousands of feet of planks a foot thick.
CB slot cut
piles of shavings
Working the wood for the keelson (sapele - a type of African mahogany) is such a pleasant change from the low quality red cedar of the moulds. This stuff is simply amazing. It planes very smoothly, saws well, is easy to work with a chisel and is incredibly strong (I tested a little bit about 5cm long and 3mm thick and couldn't break it by hand, or really even bend it at all). So far the only problem I've had has been with drilling - it's really hard and you have to give the drill a hefty shove to get it to cut through instead of just spinning and getting hot. As a comparison, it takes me at least 5 seconds to drill through this 24mm board and once I break through the drill goes almost that far again into the 2x4 underneath before I have time to react (of course if I anticipate the end of the board and reduce pressure then it's much less, but still, this is some solid wood).
After shaping the keelson I dismantled the temporary frame I was using to clamp things to while I worked on them. I then started building the base for the strongback. I first made the sides from 2 8' 2x4s each, bolted together with 5/16 x 3" lag bolts. Those connections turned out to be nice and strong so I started doing the ends of the box that forms the base. The first end I tried to be fancy and cut rabbets so the corners interlocked somewhat. I was rushing a bit, so my cuts weren't particularly square, plus there wasn't a whole lot of area for screws, plus I was dumb and didn't make the other end long enough so I used a different technique for the other end. I just screwed blocks onto the end of the crosspiece and then screwed those blocks to the sides of the base. That turned out to be just as strong and was way quicker and easier, so for the other cross-braces I'll use that technique. Next I had to add the uprights that support the moulds. To get the first one in the right place I used Pythagoras to calculate the diagonal distance to the top of the support. I then measured all the rest from that one, since it doesn't matter terribly where they are in relation to the base as long as they are spaced evenly and are square to each other. I was originally going to screw the supports to the base but I ended up buying a bunch of bolts and just bolted them on. That turned out to also be extremely strong although I did have problems with one upright. Most of them didn't move noticeably when I tried pushing them back and forth, but this troublesome one I could move 6" back and forth. I think I had some holes half drilled through the board (leftover from those holes I drilled in the keelson to get the saw in) and one of the bolts just tore through. I redrilled the holes and then it was fine, or so I thought. Turns out that this wood is incredibly mushy and none of the uprights were really strong enough. I could have worked them back and forth half a foot if I tried, so I put a few extra screws in each one and that seemed to improve them quite a bit although they still definitely have some squishiness to them. After I got all the frames mounted up things were much more solid, although for some reason frame "B" was about 2cm out of alignment with the rest of the frames. I moved it up 2cm and everything seemed to be perfect. Running a batten along basically anywhere touched all the frames which is exactly what you want.
"in frame"
steam bending jig
ready to glue
bent strips
steaming box
glued up
After I fine tuned the keelson notches my next step was to glue up the stem. Since I had already cut the strips for this I figured I could just build the jig to clamp them to (to give them the correct curve while the glue is drying) and then glue them up. Alas! Turns out the stem has a much sharper turn than I anticipated and the strips wouldn't make the bend without breaking. I figured that I should try to steam them somehow since you can make wood bend into crazy shapes like that. I actually managed to put together a long box to contain the steam and put a couple of electric kettles under one end. This ended up working amazingly well. I could only fit 3 of the strips in my triangular cross sectioned box, so what I did was steam 3 strips for 15 minutes then clamp them up on the jig and let them cool for an hour. I then did the next 3 strips and clamped those up along with the previous 3. All this went pretty smoothly after leaving them for a day on the jig I unclamped them and they remained bent! They sprung back quite a bit, but they still had plenty of flexibility to bend to the necessary shape so after brushing epoxy over all the contact surfaces I clamped them up. I covered the jig with masking tape and then rubbed wax (ie. a candle) on that to prevent the strips from sticking to it. I also did the same to the clamps for the same reason. I wish I had a couple more 4" clamps just to hold the strips together between the existing clamps but it should be fine as is. Note that at first I had the black clamp clamping the wood to the grey clamp. This was a bad idea. Since the grey clamp was at an angle the round plate thing on the end of the screw on the black clamp got bent and I needed to bend it back to get it to stay on. I replace the clamp with a hardwood block and that worked much better, and gave me an extra clamp which was useful during gluing.
I also had to lay out the transom shape. The shape of the outer face is given on the plans but since the planking meets it and an angle the inner face is bigger, and it depends on how thick the transom is. Mine is 24mm sapele, and so although I couldn't accurately loft it out, I estimated it from the plans to be about 21mm maximum on the garboard plank and about 10mm on the sheer strake (uppermost plank). I gave a 25mm allowance for the lower 3 planks, a 15mm allowance for the sheerstrake, and then for ease of cutting just tapered the remaining plank between those two allowances. It's pretty important that it is symmetric, so I drew out half of it on the back of an old mine plan and then tapped nails through at the important points, flipped it over and aligned it with the centerline and then tapped nails through to mark the points for that side. That worked very well and then I just drew lines connecting the points and sawed it out.
This boat is mostly designed for "stitch and glue" construction in which you have no moulds but just cut out all the planks the right shape, "stitch" them together with copper wire, and then stick everything together with fiberglass and epoxy (and then remove the wires and fill the holes). Needless to say, that is a very messy way of making a boat and although it is quick and easy, I wanted something a bit more refined. Thus I chose to build it in "clinker-ply". The main difference between these two techniques is that clinker-ply builds the boat over a set of moulds and each plank overlaps the next one by a bit and is glued along this joint. This is much more similar to the traditional clinker method except that instead of using rivets the planks are glued and the planks are plywood instead of solid wood. Anyway, since the plan is set out for stitch and glue it has dimensions for accurate plank shapes. I wanted try figure out how I could cut my planks out of 2' wide strips of plywood, so I drew the stitch and glue planks up in autocad, added 40mm to allow for the overlapping planks, and then arranged them. Here are the two layouts. The upper one has a complete set of 10 planks but I managed to rearrange them to get a whole 2x16' piece of plywood spare note: actually, I screwed up. I forgot to draw in the very end of plank 3 and so this doesn't actually work, and the extra sheet is required. (plywood comes in 4x8 sheets; I cut them lengthwise and then glue them together so that they are long enough). The lower layout is the one given in the plans, but with plank 3 flipped 180 degrees..
While waiting for the epoxy to harden I cut out the transom knee. The squareness of this piece is vitally important because if it isn't square then the transom won't be square and that just puts the whole boat out. The transom is quite raked (130 degrees to the keelson) so I was able to get a very good grain direction in the knee. Driving screws along the grain isn't very strong, so in a knee like this you want the grain to run in the direction that doesn't meet either face square on. After marking out and roughly sawing out the knee I had to plane the edges to get them perfectly square and flat. The first side was more difficult because I was planing a bit more across the grain, but eventually after I got it as close to square as I could measure (my 250mm square touched it all the way up). The second side went easier because of the direction of the grain and soon both sides were perfect. I then planed down the remaining side to make it pretty. The next step is screwing it to the keelson and transom and, eventually, epoxying it in place.