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There are a lot of things amount the geometry of the cabin top that they don't tell you in books. Reproducing the shape that the designer intended requires at least an awareness of all of them. I wish I had known about them before I started. It would have saved a lot of work. The first lines that someone will see on a boat are the sheer line and the edge and crest of the cabin top. While the crest is harder to define from the side, looking forward from the cockpit, any unfairness will be obvious so the three lines together are critical for the over all impression the boat makes. With a production boat you can fair and fill and jigger the plug before making the mold but with a one off, you have one chance to get it right. It pays to do a lot of thinking before doing any cutting.
 The three things that define the cabin top surface are the curve of the cabin sides, the center line of the top and the camber or curve from one side to the other or "camber. There are several styles of camber. The simplest is a simple circular constant radius that can be visualized as the side of a barrel. This form is not particularly graceful but the curve is simple to lay out. More often the camber is a parabolic shape that has more curve in the center and less at the sides. About the only thing the books cover is how to lay out a camber of this type with the typical diagram at left. You can also calculate the heights with a spread sheet using the formula:
h=H*COS(Pi*d/B)
where h = Height at any point H = Maximum Height of Camber d = Distance from Center B=Beam
What they don't talk about is the relationship between this curve, the curve of the center line and the way it is supported by the cabin sides and that is every bit as important as the camber.
 The center line may have a sheer that echoes the deck and alludes to a more traditional look or it could have negative sheer to provide more head room in the saloon as seen in many "modern" designs. However, these compound curves cannot be developed in plywood. The third option is a straight line, which is what John chose. .This form is actually a segment of a cone cut through at an angle to the axis. Besides giving a contemporary feel to the lines, the surface curves along only one axis so it can be built with sheet goods. Each beam has a slightly different camber and to maintain the straight center line the sides of the cabin have to be defined by the point that it intersects the cone and THAT took a lot of jiggering.
Setting the sheer batten was the big challenge. It is laminated up of two pieces of 3/4"x2 1/4" mahogany that were shaped and tapered to the angle of the top. The cabin sides are two layers of
 okoume. I built the them up in stages in hope of minimizing end grain exposure and improving the strength of the joint between the top and sides. A layer of 1/2" was set first and trimmed to the sheer batten. The beams were notched into the sheer batten and set in epoxy. A Japanese Ryoba saw made this work go a lot faster than I expected.
About right here is where I discovered just how critical the relationship between the cabin sides and the top is. While the sides were fair and measured exactly as the plan showed, the area just forward of the mast where the sides start to turn inward was to low! I had to add 1/2" of mahogany to the top of the batten and fair it into the line and then reset the beams in that area.
 The hatch openings required a little extra work to fit them to the radiused corners of the Bomar SeaBreeze hatches. Corner blocks were the obvious answer so I laminated up a long block of mahogany and ash to match the beams and cut the corners with a hole saw. That worked fine for the two centerline hatches but the camber at the outboard hatches threw them out of vertical so I had to cut some more blocks on the drill press with wedges under them to match the angle of the camber.
The next step was laying the bottom skin of 4 mm okoume. Butt jointing would not be strong enough or look very good so I was facing scarfing 10 sheets of plywood all the way around all four sides. A block plane was clearly not the answer.
 A little imagination, a router with a 2" straight pattern bit and some Plexiglas scraps came to the rescue. I made an auxiliary base for the PC 690 with a long fence set at a 9:1 slope and c-clamped it in place. The 2" bit can scarf up to 6 mm material and the 9:1 slope leaves about half the face ply at the edge so it can be handled without damage and the same depth lip on the high side to help alignment. With a bit of experimenting to get the depth set right, I can cut scarfs on all four sides of a whole sheet in about 5 minutes.
The next problem was how to hold all this area of plywood in place while the epoxy cured. I used 1" ring shank bronze boat nails around the edges to tie it to the sheer batten but holding it against the beams was another problem. Fortunately I have been procrastinating on pouring the keel fin so I hauled 1,000 pounds of lead pigs topside and distributed them over the deck nailing only the corners of each sheet. All the joints came out beautifully even and the skin was tight to the beams.
 The center of the cabin top is Divinicell H-80 with stringers and blocking under hardware locations. What to set all this material in took a bit of experimentation. Mixing the required amount of West 105/205 and silica in small batches would have taken a lot of time and been very expensive. FGCI sells an epoxy adhesive called "Superbond" that is premixed with silica and cost about half of the best discounted price I could find for West. It is so thick that it does not flow so it has to be proportioned 1/.9 by weight. It also does not soak into the foam or wood. I did pre-coat the plywood for water resistance but I tried experiments pre-coating the foam with 105/205 and bare and found no difference in the bond strength. Applied with a 3/16" notched flooring adhesive trowel produces an even 1/16" coat on both mating surfaces.
I found it a lot easier and neater to do this work in a certain order. First the edge stringers were set and glued. Then I chalk lined off the positions of the inside stringers and fit the Divinicell up to the line and set the stringer. Working across the deck this way avoided a lot of fiddling with the foam to fit it between the stringers. The stringers were screwed to the beams but the lead pigs came in handy here to for pressing in the foam.
You will notice the patch of fairing compound on the port side. That was to make a final adjustment to that low spot I mentioned earlier.
At this point I shaped the outside stringers flush to the cabin sides and applied the 6 mm outer layer of plywood. By delaying this layer until now, it covers the edge of the lower skin and the stringer minimizing exposed joints. With all this practice, the top skin was a piece of cake and was installed over one weekend with no incidents.
The final steps will be to sand it all down, give it a couple of coats of epoxy to seal it and then cover the cabin and side decks with 6 oz glass. The next big challange will be laying the teak veneer on the side decks but that will deserve a separate page.
NO! I am not installing portlights in the cabin top. They are just laid up there to egt an idea of the spacing while I build up my nerve to cut into the cabin sides.
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