Clipper's Vera Cruise Build

Clipper's Vera Cruise Build

Sunday, March 15, 2015

First Bottom Skin Fitting

The front bottom skins on this design are one of the most difficult areas to work with.The skin not only has to curve around to the front of the boat, but it also has to curve over to the keel. Plywood does not do this too well so it can be challenging.

One way to ease this a bit is to get the panel as close to the final shape as possible before bending. As more material is removed, it gets a bit easier to bend . Another technique is to work from the center outwards. This also aids in keeping the panel in place while you work on it.

But first you have to get some idea of what you are working with. Here, I have made the initial rough cut on the keel edge (farthest away) so the panel is beginning to take on a triangular shape.

Because the side skins are already in place, one edge of the bottom skin needs to match up to the side skin. Since this area is curved, it can't be a simple straight line (or straight edge). Fortunately the center of the panel at the edge is fairly straight. But the remaining edge must either match up or overhang (so it can be trimmed back).

Trying to get this lines up just right while allowing for the bend over the hull (with a panel that is very stiff to boot) caused me some difficulties at first. But eventually I got the panel fastened in a few places and curved around to the stem. There was some overhang at the aft end and quite a bit more at the front.

At this point I started studying the panel to see what could be done next. The front section laid up against the keel quite nicely and I could see there wouldn't be any problems there.

But the aft section was quite a ways from the keel and had to bend over the frame and two of the battens in order to reach the keel. These are the same battens that I had to pull down with the ratchet straps when they were installed.

Pulling down on that panel, I could tell right away that it would probably break if I forced it. I was also worried about the screws along the opposite edge pulling out or the edge breaking. So I decided that I was going to need some "persuasion" in the form of boiling water applied to the skin before bending.

Before proceeding further, I removed the panel and trimmed off the overhang on the chine edge where it matched up against the side skin.  Once the panel was back on I needed to do something to keep the boiling water on the hull long enough to have an effect on the plywood. To keep the water in place and apply the heat better, I laid towels over the area that needed to be bent.

To better protect the edge that was screwed down by the side skins, I added additional screws and also a plywood strip near the aft end where the stress would be the greatest. Then boiling water was applied .

I began clamping the panel down from the front and worked my way back to the aft edge. From the outside everything looked good.  But when I started examining the inside, I saw that there was a sizable gap between the skin and the inboard most batten. There was also a gap by frame 5.

Further investigation showed that the main problem was the stiffness of the battens at the ends. They were pressing up against the skin and forcing it away from the frame. It also appeared that because I had clamped the ends down but had not applied any pressure in the center at the battens, that the skin had arced up in this area. I reasoned that I could relieve the batten stiffness by kerfing them (more in a moment) and by applying screws at the battens before completing the bend at the keel. This way the skin can be "walked " over to the keel and conform better.

I then let the panel dry out for two days while clamped in position. This would aid me later when I had to re-attach the panel and re-curve it around the hull.

After the panel had dried but before before removing it, I traced all the internal structure on the inside of the skin. This would give me references were to drill the screw holes and later apply the epoxy. I also marked off a straight edge on the aft end of the panel. This is where it will eventually butt up against the next skin.

Back when I was installing the bottom battens, I had hoped that I could get away without kerfing the ends of the battens. The reader will perhaps remember that kerfing means to make a cut across the end of the batten and cut back to close to the mounting point on the frame. The idea here is to make it easier for the batten to flex. When the panel is later glued into position, epoxy will need to be applied in the slit to glue the wood back together.

The easiest way to do this is to use a band saw before installing the batten. I do not have a band saw and considered using my table saw. But I wasn't convinced at the time that kerfing would help and I was concerned that I would mess up the battens by trying to cut the slit on the table saw.

Well, because I did, after all, need to kerf the batten ends, I had to do it by hand using my Japanese pull saw. This was not easy and it took me over an hour to do both battens. I'll still need to do the same two battens on the port side when I get to that skin.

However, the kerfing paid off and my idea to "walk" the panel down using screws also worked out well.  The panel was first trimmed to near final size at the keel edge. The aft edge was sanded straight. I also trimmed a minor amount off of the chine edge where it still had been interfering with the side skin.

I performed all the "walking" from the front moving aft, applying screws first in the center of the skin at the battens and then at the keel edge. I worked back and forth gradually moving aft until I reached the aft edge.  The skin fits much better now and the gap I saw earlier is no longer there.

The panel is basically ready to install now. I will probably wait a few days until I have sufficient time to accomplish this. I suspect it will take two or three hours to install so doing it after work probably won't be possible. What I will most likely do instead is to do the preliminary fitting and "boiled water" bending of the other skin. Since there will be nothing to clamp to once the starboard skin is on, it will be easier to do the port skin pre-bend first, when I can still clamp to the structure.

So that's it for now. I am anxious to get past this point in the build. Although it has turned out to be less of a problem than I imagined, the worrying about it over the last month has slowed me down and it will be good to get to something less worrisome, like the aft skins. Take care.

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