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EZ Tunnel Race Boat Plans
Page Four
Sponson Pads

As your tunnelboat flys across the water, supported by a cushion of air in the tunnel, and the aft-most inches of your sponsons, it is the sponson pads that take the bulk of the punishment from the waves. The pads need good support to make sure they can't flex -- flexing leads to failure, of the joints or of the plywood itself.

Of particular concern are the aft-most 48 inches of the pads, the space spanning bulkhead three, four and the transom.

Two plywood reinforcements are attached with glue blocks between bulkhead four and the transom, two more between bulkheads three and four, and a single one between bulkheads two and three.

Be sure to notch the aft corners of all these plywood reinforcements so that water does not become trapped and can flow toward the tunnel side on it's way to the transom.

Add an addtional tranverse reinforcement between bulkheads one and two (red arrow).

* * * * *

In the picture, the glue blocks extending from bulkhead two to the transom have have been installed on the tunnel sides and the hull sides.

Remember that these glue block are not square, but have an angled side corresponding to the slope of the sponson pads. That angle is approximately six degrees from bulkhead two to the transom.

Because it curves in two planes, small blocks, about 1.5 inches long, are attached to the hull sides from bulkhead one to the bow.

I also used small blocks along the tunnel side in this area. But it would have been simpler to cut longer curved pieces just as was done along the curved edge of the tunnel plank.

Again, there is an angled side to these blocks, starting at six degrees at bulkhead one and increasing as you go forward.

After the blocks are glued in place, they need to be trimmed smooth and to their proper angle. Use a plane for best results, but a belt sander will also do the job.

Before installing the sponson pads, take some time to seal those parts of the sponsons that will be difficult or impossible to access later on.

I used latex paint, one coat primer and one top coat. Epoxy resin, oil-based paint or spar varnish also work very well.

This is also a good time to add drain holes to the sponsons.

I start by drilling a one inch diameter hole, then back-filling with thickened epoxy.

Once the epoxy is cured, drill a 3/4" hole through the middle and install a lever-type plug.

With this method you avoid exposing any bare wood to the elements.

Glue blocks have been added to the bulkheads and the reinforcement pieces. Note that the glue blocks on the bulkheads are not square, but are cut at a two-degree angle sloping downward toward the stern.

Also, the glue block on the transom should be 1.5 inches wide. A drain hole will be added later to each sponson and will be drilled through this glue block.

* * * * *

Lying in the tunnel is one of the sponson pads. The pads are of course longer than a sheet of plywood so there is a joint to be made. I located this joint near the stern so that it does not end up in the curved part of the sponson.

The butt blocks can be seen in this pad. Note that they are spaced to fit between the plywood reinforcements in the sponson.

The sponson pad glued and screwed in place.

Tunnelboats turn sharper than anything else on the water -- or on land or in the air for that matter. When a tunnelboat turns, the tunnel side on the inside of the turn grabs onto the water, and the boat turns as if on rails.

Early tests (and my first races) of the EZ Tunnel showed that the tunnel side was grabbing too much -- specifically too much near the bow. The result was a dangerous tendancy for the bow to dive into the water, threatening to cause the boat to "stuff" or barrel roll.

The remedy turned out to be simple: round over the inner edge of the sponson pad, thereby loosening its grip.

I rounded over this edge from a few inches aft of the bow to a little behond bulkhead two.

I only rounded the edge on the left sponson -- in racing, we only turn left. For general use, you may elect to treat both sponsons.

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