III. Lofting, Shaping, and Stitching
Building the Core Sound 20
March 2, 2008
NOTE: This article is divided into chapters. Click here for the Table of Contents.
It's been a long week of hard work, but the Core Sound 20 is finally taking shape. Here, I'll describe the sequence of events that took our boat from a pile of plywood and lumber to the 3D skeleton of a boat.
1. Scarf and Join the Panels
First we roughed out and scarfed (joined) our plywood panels together to create larger panels of the necessary length (20 - 22 feet long). I built a jig that would hold each panel just 8 degrees off of true vertical so I could run it through my table saw to create the scarf. (A scarf is an angled cut that allows the sheets to be glued together almost seamlessly.) It was a strange experience balancing 8-foot tall plywood sheets on my table saw, but the results were superb. Just don't try this on a windy day!
The Core Sound plans suggest "butt joining" the panels (no scarf) using fiberglass tape to reinforce the joint, but scarf joints provide a much wider gluing surface, producing stronger, better-flexing panels without the flat spots that sometimes show up along the seam of butt joints. When scarfing, you must remember to adjust your measurements to compensate for the extra few inches of overlap that each scarf consumes (about 3 inches per seam). In our case, we cut it dangerously close: our rough cut panels ended up being just 1/16-inch wider than we needed for the bottom hull panels! If they had been too narrow, it would have been a very expensive mistake to correct.
2. Loft and Cut the Curves
Next, we took measurements to loft the curves onto the scarfed panels. The curves define the shape of the parts that will become the hull and sides. The measurements establish a series of fixed points. After placing a nail at each of these points, we laid a batten (flexible stick) against the nails to establish a smooth curve. It is important that the batten be cut from a nice, straight-grained board with no warpage or else the resulting lines will not come out "fair" (even, symmetrical, and smooth-flowing). This is the stage to check, re-check, and triple-check your measurements. Mistakes here can have a profound effect on the quality of the boat that results.
Once the lines are drawn, they must be cut out. The first cuts are nerve-wracking because marine plywood is expensive and any mistakes here will be very costly. Since circular and jig saws are prone to "wander" while cutting, we decided to cut approximately 1/8" outside the pencil line using power tools, then we switched to a (more accurate) block plane to shave the wood down to the pencil line by hand. The result was a very accurate set of panels that looked like they had been laser cut.
3. Attach Sides to Bottoms
The Core Sound has a beautiful V-shaped bow section that is produced by first pre-attaching each side to one half of the bottom. Later, the panels are "folded" into shape, which forces the front panels to bend and curve into the V-shape. This is one of my favorite features of the Core Sound's design and a key component in its impressive sailing performance. As we would soon discover, the ease with which the panels pull into shape is a tribute to the designer's skill and ingenuity.
For this part, scarfing is not an option, so we butt-joined the sides to the bottoms with thickened epoxy and two layers of 6 oz. glass on each side of the joint. We did all of the joints at once so we could make sure that both "halves" of the boat remained a perfect mirror-image of each other. Then we clamped the whole pile down tightly to ensure the fiberglass tape cured flat and smooth. It worked like a charm.
4. Stitch the Centerline
When each side has been attached to one half of the bottom, the panels are stacked and holes are drilled along the centerline every 6 inches. In our case, we drilled the holes before attaching the seat stringer to ensure the panels (and holes) were perfectly aligned. A day later, after pre-attaching the seat stringers to the inside faces of each side, we inserted 16-gauge wire through each set of holes to "stitch" the centerline together. The wires must be relatively tight at the bow, but relatively loose along the rest of the centerline to allow for the panels to be folded out.
With everything stitched and ready to go, we unfolded the panels. The two halves of the bottom spread apart. Meanwhile, the sides get pulled down and in to follow the curve of the bottom panels, and bulkheads are inserted to help establish the shape. In less than a minute, the boat transforms from a pile of flat panels to an impressive, three-dimensional shape with beautiful lines. There is no doubt that this will be a sleek, able sailboat. It is also surprisingly large for a 20-foot boat. There will be plenty of room for family members aboard this craft.
© 2008, Wesley Kisting