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Laker SC12 Tunnel Race Boat Plans
Page Three
Tunnel Battens

Before you begin installing tunnel battens, it is critical that the structure be straightened.

I used plywood scraps that include a straight edge. After a few battens are installed, these braces can be removed.

Before we go any further, a cautionary tale:

After bending on all the tunnel battens on my Dillon Sport C, I realized that the beam at Sta. 2 had taken on a distinct curve. Although the battens are only 1/2" thick at this point, and quite flexible, the accumulated pressure was too much.

I had to free the ends of the battens (the glue hadn't dried yet) and shore up the beam with a length of 2x4. I removed the 2x4 after turning over the hull.

The bend on the Laker SC12 is gentler than on the Sport C, but it would be a wise to assume the same thing will happen. You should shore up this beam -- with a piece of lumber or even a scrap of plywood (at least, say, 10" wide), anything that will withstand the strain of seventeen battens, will not interfere with continued construction of the boat, and can be removed later without too much fuss.

The following is an illustration of the a tunnel batten with the vertical dimension greatly exagerated for illustration purposes. The battens are 3/4" thick for about half their length. Then, between Stations 3 and 4, they are reduced to 1/2" thick. You will find similar battens are used for the sponsons.

An alternative would be to cut out battens 1/2" thick the full length of the tunnel, then laminate an additional 1/4" to create the thicker section.

Two of the battens are located 10" either side of the Centerline (see Construction drawing). The coaming will attach to these battens in the aft part of the boat, so locate them accurately.

The two outermost battens are located right against the tunnel sides.

The remaining battens need only be evenly placed between these four.

The forward ends of the Tunnel Battens can be clamped to the angled cleat, or held in place with small pieces of plywood temporariy screwed to the beam. You could possibly screw the Battens in place -- do it carefully to avoid splitting either the Batten or the cleat.

Once all your Tunnel Battens are in place, your stern will look something like this, EXCEPT that your Laker SC12 will have a Tunnel Extension Bulkhead attached to the tunnel sides. Unfortunately, none of my other tunnelboat designs have this bulkhead so I don't have one to show you. This pic is from the Laker 14.

See the drawing Stern Details.

The Tunnel Battens that lie between the coamings are cut short to leave room for the Motorboard. The space that remains between the Motorboard and the Battens will be filled later in the build.

The three middle Tunnel Battens between Coaming and Tunnel Side are cut just short of the Tunnel Extension Bulkhead so that water can pass by and exit the drain shown on the right side of the drawing.

Glue and clamp the battens to each of the beams. The forward ends can be held in place with small scraps of plywood. Put plastic packing tape, or masking tape, on the scraps so they do not end up glued to the boat.

Bilgewater will naturally flow toward the stern, so you need to provide places for it to go. Specifically, provide limbers that carry it to the sponsons where drain holes will be provided to drain the boat.

I used a router with a 1/2" corebox bit to cut 1/4" deep coves just forward of the beam. A fence clamped to the battens guides the router.

The router cannot get to the outmost batten. Instead, clamp a small block to the batten and, from the outboard side of the Tunnel Side, drill a 1/2" hole through the tunnel side and through the batten and block.

Remove the block and you have a neat cove in the batten, and a hole through the tunnel side leading into the sponson.

The hole as seen from outboard of the tunnel. A long drill bit is called for here.

Water that finds its way aft of the transom beam will have to be drained directly out of the boat -- it can't get to the sponson from here.

I routed limbers in the battens as shown here. Only the battens that lie between the coamings, and the ones just outside of that group need limbers.

I fastened the tunnel plank to the Bow Beam with screws which were later removed.

The remaining Plank can also be fastened with screws. I have also used staples driven with a manual staple gun. Staple removal is rather tedious, but they leave very small holes to fill.

Most recently, I have fastened plywood with a manual staple gun that also drives 9/16" brads. I countersink the brads with a nail set, leaving very small holes to be filled.

Extend the tunnel plank all the way back to the aft ends of tunnel sides.

The joint in the plywood can be scarfed or butt-blocked.

I used plywood butt blocks about 6" long, one between each pair of tunnel battens -- as seen here after the hull has been turned over.

The inside corner between the sponson sides and tunnel plank should be filled to assure a watertight joint.

I used a molding, attached with glue and short nails. The nails can be either removed or countersunk.

Alternatively, a fillet of silica-thickened epoxy can be applied.

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