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MIni Tunnel (Redesigned Version) Boat Plans
Page Five
Sponson Sides and Pads

Install the outer transom.

Hold a suitably sized piece of plywood against the inner transom and trace the shape. There are no stringer notches in the outer transom.

Install it with glue and clamps.

Carefully bevel the edge of the Shear Clamp and the Chine Log.

Installing the lower sides is straight-forward. Virtually the entire piece can be held in place with clamps. If you don't have enough clamps, then brads or staples or screws with do the job.

You can scarf or butt block to get the required length.

Fill in between the sponson battens to provide good gluing surface for the sponson pads.

The inner-most block is extra large. Your drain holes will be drilled through these larger blocks.

Where the sponson battens meet these blocks, cut or rout some limbers as was done for the tunnel battens just forward of the transom beam (see earlier page). These prevent water from being trapped between battens, unable to find its way to the drain.

The lower sides get spray rails, much like the tunnel sides. These also extend from the Transom to Bulkhead 3. As with the tunnel spray rails, install these with screws driven through the plwood into the rail.

These rails also serve to strengthen the lower sides, which may be subject to considerable stress in a turn.

After trimming the edges of the lower sides, install the Sponson Pads. You can scarf the plywood, or you can use butt blocks to get the required length.

The outer edge of the pad should be left square, and sharp. Likewise, the inner edge and the trailing edge of the pad should be sharp.

Sharp edges inspire water to fly right off the boats surfaces rather than trying to crawl right around the edge and up the side or transom.

Maintain the square outer edge up to about station three, then blend into a beveled edge that matches the side.

The inner edges of the sponson pads should also be sharp up to about station three. Round-over this edge from station three forward. In a turn, it is this edge that grabs the water and allows a tunnel boat to turn sharper than just about any vehicle, on or off the water. However, if this grip is maintained too far forward it can pull the bow down, possibly with disastrous results. If your boat exhibits this kind of behavior, the rounded edge may have to be carried farther aft.

Now it's time to flatten those pads, again concentrating on the aft-most few feet. Your tools are a straight edge, and belt sander and a long board. A long board is just a long, flat sanding block. I use a couple of feet of 4" wide aluminum channel with 100 grit glued to it. Longer would be even better. (More on this on Page Fifteen)

The level of perfection needed here is up to you. Racers pay attention to a lot of small improvements, not because any one of them will make a big difference, but because the little things add up in the end. However, those little things might not matter to a guy who just wants to go fast down the lake.

You may choose to finish the bottom at this point to avoid having to turn over again later. Racers generally prefer a clear finish. A few coats of epoxy will do it, and sanding and perfecting the pads between coats can be part of the process.

Paint will work just fine as well.

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