EZ Tunnel Race Boat Plans
Drawing Curves

You will need two battens for this project, each of them at least seven feet long and 3/4 inch wide.

The thinner of the two battens should be about 1/4 to 5/16 inch (6-7mm) thick.

The thicker batten should be about 1/2 inch (12mm) thick.

Rip the battens from the edge of a clear, straight-grained piece of lumber, being careful to maintain an even thickness along the entire length. Variations in thickness will cause uneven bending leading to poorly rendered curves.

The diagram above depicts a sheet of plywood with a small piece added on to achieve the desired length. The part of the boat we are making (the "girder" in this example) is depicted below just to illustrate what the goal is.

At this point, we have drawn lines on the plywood corresponding to the two ends of the girder, and the four stations (where the bulkheads will be located). Two additional lines toward the forward end of the girder are included to better define the sharper part of the curve.

Note that none of these lines falls on or near the joint between the full sheet and the addition. You do not want this joint to interfere with the bulkheads that will be attached to the girder.

The added piece is joined to the main sheet with a butt block, here being glued on with clamps and weights.

Additions have been made to both sides of this sheet, for the two girders. The sheet has been turned over so that the butt blocks are on the underside and won't interfere with our drawing.

Using measurements from the plans, points corresponding to the curved line have been measured from the edge of the plywood and marked.

The aft-most part of the curve is rather gentle, so we'll use the stiffer batten and bend through as many points as we reasonably can. Locate the batten just off the points so that when the line is traced the pencil line falls right on the the points.

Hold the batten while you trace so the pencil does not push it out of place.

The thinner batten is used for the sharper part of the curve, and is bent around far enough to overlap the first tracing.

Hold the batten in place with clamps and/or weights. Here I've used weights on the outside of the curve and short lengths of 1x1 held by clamps on the inside of the curve. Each weight or 1x1 is located right at one of the points we're going to trace through.

Once a batten is in place, study it carefully to make sure the curve it describes is "fair." A fair curve does not have any flat spots or humps, but is smooth along its entire length.

If you do have a problem with your curve, check your measurements to make sure there are no errors. If you have to move your batten slightly off one or two points to achieve a fair curve, do so. The points at either end of the girder (or other stucture) should be used as stated in the plans, but the others are subject to subtle correction. Trust your eye.

Our curve is drawn and our basic girder is ready to be cut out.

The second girder starts out the same as this one -- lines drawn on the sheet corresponding to stations, etc. But you can then use the first batten as a template to trace the second. After the second one is cut out, make sure they are as close to idencical as you can make them, trimming one or the other as necessary.