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Sport C Tunnel Race Boat Plans
Page Three
Tunnel Battens

Before we get started, a cautionary tale:

After bending on all the tunnel battens, I couldn't help but notice 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.

Granted, my beam at #2 was narrower than what is called for in the plans. BUT, you too 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 thirteen 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, somewhere between Stations 3 and 4, they are reduced to 1/2" thick. I did this in the original Sport C for weight savings, and considering my problems with beam #2, it seemed prudent to make it part of the plans. 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 add an additional 1/4" to create the thicker section.

Two of the battens are located 10" either side of the Centerline (see Transom drawing). The coaming will attach to this batten 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.

A cleat is installed on the aft face of the bow beam, 1/2" from the edge, for attachment of the forward ends of the tunnel battens.

Those battens that extend beyond the transom beam must be notched 1/4" deep so that the face of the battens will be flush with the beam.

For each group of four "extended" battens the two outermost ones extend to a point 3/4" short of the end of the tunnel side. The two in the middle of each group are 1/2" shorter than the outer ones.

The reasons for this will be explained later in the building process.

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

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

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.

So that water does not get trapped between the battens, limbers are cut into the battens just forward of the transom beam.

I used a router with 5/8" corebox bit.

At the tunnel sides, the plywood is also drilled so that water will drain into the sponsons.

I screwed the plwood at the bow beam.

The remaining was fastened with 1/2" staples shot from a manual staple gun and placed 4"-5" apart. The batten locations will have to be marked on the plywood to that you know where to staple. Remove the staples afterward by first getting under them with a screwdriver, then pulling them out with needle-nose plyers. It's tedious, but the holes left to be filled are minimal.

Extend the tunnel plank all the way back to 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.

A look at the trimmed plywood, as seen after the hull is turned over.

There is a reason it is shaped this way.

Later in the building process, the motorboard is going to be installed on that part of the tunnel plank that extends 1.75 inches aft of the transom beam. However, this assumes you will be mounting your motor on a jackplate with six inches of setback so that the motor ends up in its designed location.

If you already possess a jackplate with less than six inches setback, you may want to adjust the way you cut the tunnel plank in order to accomodate it.

See UPDATE below.

In any case you definitely should use a jackplate. It is simply impossible to know the best height of your motor until you get out on the water and test.

If you need a jackplate, I can suggest one.

UPDATE, July 2013: In my continued quest for better performance from my Sport C, I increased the setback of my motor by 4.5 inches.

August 2013: I pushed it back a little bit more, now a full six inches aft of its original location. Gains have been modest, but real, and this was clearly the proper move to make.

You may want to consider placing your motorboard farther aft, even all that way back to the length of the tunnel extension.

The angle 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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