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Building a Sport C Class Outboard Powerboat

This twelve-foot long tunnel boat is being developed and built for racing in the Sport C class. Sport C utilizes the two-cylinder, 40 horsepower Nissan or Tohatsu to achieve speeds approaching 60 mph. At these speeds a tunnel boat can literally fly.

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December 31, 2006

Cutting out bulkheads.

Bulkhead for the Dillon Sport C

Most of the bulkheads have been cut out. Some, as you can see, have been cut from AC plywood. I had some on hand, so I used it. Technically it might add a tiny bit of weight to the boat, but nothing significant.

The okoume marine plywood for the hull has been ordered, and as soon as it arrives construction can begin in earnest.

full set of bulkheads, almost ready

The plywood arrived and I jumped right in. Here I'm scarfing together some 4mm ply for the tunnel sides.

Scarfing the tunnel sides

There's actually more here than the tunnel sides, as these pieces extend all the way up to the underside of the deck, forming two longitudinal bulkheads whose shape is a complete cross-section of the hull.

Tunnel sides cut out.

The first dry assembly, mating the bulkheads with the tunnel sides. The joints are half-slots.

Assembling the framework.

Another dry assembly after all the "doughnut holes" have been cut out. I eliminated about four lbs. in this step, which may or may not be significant, only time will tell.

The two halves of the first three bulkheads are joined together with temporary braces. When the braces are removed later, the cockpit will drop down into this area.

Assembly turned into Swiss cheese.

The joints between bulkheads and tunnel sides are secured with a fillet of thickened epoxy and a layer of four-inch fiberglass tape. All four corners at each joint are so joined.

fillet and tape seam

Upside-down for the next phase....

and over she goes

The tunnel stringers ready for installation.

stringers ready to install

The tunnel plank is installed, clamped down at bow and stern and pressed down in between with various lead weights, plus such professional boatbuilding tools as cans of paint, jugs of epoxy and a propeller.

Using weights instead of screws eliminates the need to fill and sand dozens of screw holes.

tunnel plank

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