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Pro Tunnel Race Boat Plans
Page Four
Tunnel Battens, Sheer Clamps and Sponson Keels

Tunnel Battens

Before proceeding with the tunnel battens, make sure your structure is square and the tunnel sides are perfectly aligned with each other. Then adjust everything laterally to make sure the centerlines on the stem, bulkheads and transom line up perfectly down the center.

I used long, narrow scraps of plywood to hold everything in line. Once about half the battens were in place the structure was stable and the plywood was removed to make way for the remaining battens.

Please note that although generally speaking the tunnel battens are spaced evenly across the width of the tunnel, you will want to pay particular attention to the third one out from the centerline on either side of the hull, especially where they cross bulkheads 3 and 4. Later, the coamings will be joined to these battens, so they must be properly located 1/4" inboard from the bulkheads' inner edges. (See "Construction" drawing)

The forward ends of the battens simply butt up against the stem. They are held in place with scraps of wood that hang over the aft edge of the stem just enough for the battens to slip underneath. Cover the scraps with tape so that they don't end up glued to the boat.

Use epoxy thickened to peanut butter consistency to glue the battens in place.

A slight bevel to the forward end of the batten provides a better fit.

The aft ends of the batten slip into the slots cut into the transom beam.

Note that at the time I made my transom beam, I was intending to install battens 3/4" wide. Later, I increased that width to 1", so I had to trim the ends of the battens to fit into my slots.

Also note the limbers cut into some of the battens. Ultimately this will allow water that would otherwise be trapped between the battens (and beneath the deck where it could not be accessed for removal) to flow into the sponsons, which will be provided with drain plugs. We will revisit this issue after the boat is rightside up.

Sheer Clamps

This image shows the locations of the sheer clamps, along the outer margin of the hull, and the sponson keels, along the bottom edges of the sponson sides.

NOTE: To avoid introducing uneven stresses to your boat, it is a good idea to work on both sides of the boat at essentially the same time. For instance, install the sheer clamp on one side, then install the sheer clamp on the other side. Then install the sponson keels on each side in succession. Then the chines, etc.

Each sheer clamp consists of two layers, each 3/8" x 1.5". The layers are installed simultaneously. They are glued to the sponson end with a glue block, preferably cut to an appropriate angle.

And they are fastened to the bulkheads and the transom with similar glue blocks. I allowed the excess length to run out beyond the transom, then later trimmed them with a handsaw and completed the job with a belt sander.

Sponson Keels

The sponson keels are cut down the middle from the sponson tip back to bulkhead #2.

A sponson keel being glued into place.

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