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Pro Tunnel Race Boat Plans
Page Seven
Lower Sides and Sponson Pads

Bevel the sheer clamps, chines and sponson keels in preparation for the sides and pads. Pay close attention here; you want good contact between the plywood and the framework. You also want your sponson pads to be good and flat, especially from bulkhead #3 to the transom, and absolutely from #4 to the transom.

The lower sides go on first, screwed and/or clamped.

The spray rails extend from the transom to approximately bulkhead #3. They are located about 2" above the chine. Install them with screws driven from inside the sponson.

Finally the sponson pads. Leave the outer edges of the pads square, NOT beveled to match the lower sides. Sharp, square edges act like a spray rail, encouraging spray to to exit the hull. For the same reasons, a sharp, square edge is desired at the trailing edge of the sponson pad, at the transom.

You may, however, bevel the edge of the sponson pad forward of Bulkhead #2.

Again, the sponson pads must be flat from about bulkhead #3 to the transom, and really, really flat from bulkhead #4 to the transom. In the image, one of my pads was a little hollow and needed some filling. The other was a bit high in nearly the same areas.

A longboard of some sort is good a tool to have here. I use a 24" length of 2" x 4" aluminum channel with 100 grit sandpaper glued to the face. About 20 lbs. of weight placed on top makes the sanding fairly easy; just push/pull it back and forth. And keep checking your results with a straightedge.

Again, you will want all your edges to be sharp, the outer edge of the sponson pad and at the transon. Sharp edges encourage spray to quickly exit your hull, while rouned edges tempt it to follow the curve around and stay on your hull that much longer. Spray on your hull equals drag.

Now's a good time to drill the drain plugs in the sponsons (picture borrowed from the Dillon Mini Tunnel). For a 3/4" rubber plug, drill a one-inch hole, then fill it with thickened epoxy. Then drill a 3/4" hole through the hardened epoxy.

You may choose to finish the bottom at this point. If so, skip ahead to the finishing section.

Otherwise, turn her over and admire your progress.

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