Clipper's Vera Cruise Build

Clipper's Vera Cruise Build

Sunday, March 29, 2015

Bottom Skin Installation Continued

After fitting the first bottom skin, I knew that I wanted to apply a pre-bend to the second forward bottom skin (port side) before gluing the starboard skin in place. By pre-bending the second skin first, I could take advantage of the still open framework for clamping.

I decided to try a slightly different approach to fitting the chine edges of the skin. On the starboard side I basically laid the flat edge of the plywood on the flattest area of the side skin edge and then trimmed the ends after bending and determining how much overlap there was. On the port side I overlapped the entire chine edge by a small amount. My thinking was that I could use measurements on the side skin to determine a line on the bottom skin and then connect the measured points for a more accurate edge.

So I made measurements about every six inches along the side skin edge. I measured down three inches and made a cross mark and repeated this along the length of the skin. Then I temporarily installed the full plywood sheet with the slight overlap mentioned previously. It was held in place by three screws, one at each end and one in the middle. The panel was bent around to the stem as well.

At each of the measurement points I measured back up the full three inches which gave me a reference mark for the side skin edge. Unfortunately, I only took one picture of this process.

After making these reference marks every six inches on the plywood, I connected the reference points together with lines using a yardstick. This represented the side skin edge transferred to the bottom skin. I wasn't totally confident with this approach so I decided I would cut short of the mark and sand it. Then test fit and sand more off if necessary.

While the plywood was in place, I roughly marked the excess area that I would remove by the stem and keel. This was a very rough line and drawn well enough away from the final dimension. I was primarily interested in getting rid of excess material so that the panel was lighter and easier to bend on the next fit up.

My garage is quite cramped and finding a place to cut large pieces of plywood is difficult. Usually, I have to pull out several pieces of yard equipment and then set up a temporary workbench so that I can make the cuts. This time I decided to improvise and use the boat to make the rough cut.

 I cut off the excess and trimmed the edge where I had made the measurement marks. The flat area of the boat bottom served well for this purpose although it was a bit high to work from. The skin was reinstalled to check the fit of the edge. It still overlapped some but it was close enough now that I felt I could pre-bend the panel. The panel fit about the same as the starboard skin panel and would require "persuasion" from the hot water  as well.

This time I avoided burning my foot when putting the boiling water on the panel. After a day of drying, I marked the stem/keel excess a little closer to the final line and placed a perpendicular line at the aft edge where I wanted to butt the next skin up to. The skin was then removed and I began preparations for installing the starboard skin.

It had been nearly a week since I had removed the starboard bottom skin and I was a bit afraid that it was going to flatten out and negate the pre-bend I had performed on it. So I was anxious to get it installed, but had to wait several more days until I had enough time to perform the task.

In the meantime, I started doing clean up on the inner side skin surfaces. There was plenty of this work to do and I have not touched any of it in over a month, mostly because it is a pain in the rear to get inside the boat, but also because I am not a big fan of sanding.

Anyway, yesterday morning I started the starboard skin installation. I used the same approach for glue up that I have used throughout the build, namely pre-wet the bonding surfaces with un-thickened epoxy, then re-coat with thickened epoxy, install the panel and screw down with steel screws and plywood washers using wax paper between the plywood washers and the skin. I installed the screws into the panel in the same order I had done when fitting so that I insured that it fit correctly.

It took about two hours from start to finish which surprised me as I expected it to take three hours.

One thing I did which I hadn't originally planned on doing was to use three shorter bronze screws to hold the kerfed batten together in the area aft of the panel. Readers may remember that I cut slits in the battens (kerfed them) back to about 1/2 inch from the frame. However, the panel ends about 8 inches from the frame. I had to put epoxy in the kerf slits during the glue up. But there was no pressure on the exposed 8 inch section of the batten so the screws were added to pull the two kerfed pieces together. Since I will eventually be placing other screws in this area of the batten when I install the next set of skins, I made sure to stagger these "pull down" screws off to the sides.

A few hours later, I removed the plywood washers and replaced the steel screws with silicon bronze screws. The panel was now in position. I also took the opportunity to fill in the screw holes and the seams between the skins with more thickened epoxy.

All of this was sanded down this morning.

The only remaining task for this skin was to sand back the excess material where the skin overlapped the stem and keel. Like the side skin installation, the port skin will eventually overlap the edge of the starboard skin. However, as you move aft on the stem, the angle between the two bottom skins flattens out to a point where it is not practical to overlap one over the other. Instead they will need to butt up against each other. This required a transition joint. I am planning on using a similar technique on the next set of skins where they change from overlapping the side skins to butting up against the side skins. But I only realized I needed a transition on the bottom this morning.

This transition is somewhat hard to visualize but the following pictures should clarify what I am doing. First, here is the way the skin is sanded back on the front of the stem. This is where the port skin will overlap the starboard skin.

You can see more of this overlap in the next photo, but them you will also see the transition to where the skins will butt up against each other.

Here is a closer shot of the joint.

At the bottom of the picture is where the skins overlap. Then the skins changed to where they butt in the middle over the keel center line. The reason the skins need to butt rather than overlap is because the shallower angle between the skins means that the overlapped skin edge would have too much material removed and the overlapping skin would have too much edge material exposed.

I will show more of this when I get the second skin installed and post on that next time. I am on vacation this week, so I am hoping to make some good headway on finishing the skinning.

At this point, I am ready to reinstall the port forward skin and do the final fitting before installation. I should have it installed within the next day or two. Then I will be moving on to the next set of skins aft . These will require but joint backing plates like the side skins did, however, the plates will be shorter in length because they will be installed between the battens. I will cover this in a future posting as well.

Until next time, take care.

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.