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Pro Tunnel Race Boat Plans
Page Six
Outer Transom, Tunnel Plank and Spray Rails

The first step in planking your boat is the outer transom. The photo shown is from another project; for the Pro Tunnel, my outer transom consisted of two pieces which cover the aft faces of the sponsons only. I did not carry it across the aft edge of the beam as is shown in the photo. You can do it either way.

Unless you are working with extra-long plywood, the tunnel plank will have a scarf joint. I elected to place the joint near the stern. It could just as easily be placed near the bow, and if you have any doubts as to your ability to make a good joint you may want to do yours that way. The bow generally receives less punishment when the going gets rough.

You can join the two pieces of your plank before installing them on the boat, or you can elect to join them during installation, as I did.

I used a foam roller to completely coat the plywood face with epoxy. The tunnel battens are also coated with epoxy (rolled or brushed on) and then followed by thickened epoxy brushed or dabbed on with a foam brush.

I installed the main section of plank using a batten and clamps at the stem, battens and screws at each bulkhead, and weights wherever the plywood did not want to lie down on the tunnel battens. Use of thickened epoxy on the tunnel battens will ensure that any gaps that remain are bridged.

A batten is also screwed down along the very edge of the plank at the scarf joint.

It is a good idea to cover the undersides of all your glue battens with plastic packing tape (or something similar) to ensure they do not end up glued to the boat.

The same batten used on the scarf joint when installing the main part of the tunnel plank, can used again to join the remaining piece.

The tunnel plank extends approximately six inches beyond the transom beam.

The spray rails fit into the intersection of the tunnel plank and tunnel side. These structures also greatly strengthen this part of the boat. I used lumber about one inch thick, but anything from 3/4" to 1.25" will work.

I made mine in two pieces, as shown, joined with a simple butt joint. Even if you have lumber long enough to make this in one piece, it may still be simpler and more efficient to do in two.

In this photo the rail has been fit to the curve of the tunnel plank.

At the transom and at bulkhead #2 the width of the spray rail is equal to half the depth of the tunnel, and is cut in a straight line between those two points. Forward of bulkhead #2, the rail is cut to a fair curve which fades to nothing at the stem. Use a batten to draw the curve.

Use plenty of thickened epoxy when installing, especially at the butt joint, but also anywhere the rail does not perfectly match the curve of the tunnel plank.

The aft ends of the tunnel spray rails extend all the way to the end of the tunnel planking. You may leave it square, or round it off, or cut it to any shape you choose.

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