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Sport C Tunnel Race Boat Plans
Page One
Tunnel Sides

NOTE: If you are planning to build the Dillon Composite Cockpit for your Sport C, you may want to build that first so that you know exactly how wide it turns out and can adjust your boat's bulkheads where necessary.

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Construction begins with the tunnel sides.

Since the tunnel sides are longer than a single sheet of (typical) plywood, a panel must be constructed.

The joint can be scarfed. Or, as I prefer, you may use a butt block. The butt block must be on the plywood face that will end up in the interior of the sponson. Here the butt block is held in place with weights. Alternatively, you can use screws.

Your panels should be approximately 22 inches wide and 144 inches long, with the bottom long side perfectly straight.

You will want to make sure that you butt blocks do not end up on a station line (see below) where they would interfere with bulkhead installation.

Suggested layout on a 22" x 144" plywood panel. Station lines are at right angles to the long sides, while the
reference line angles across the panel and intersects the short sides at the locations indicated --
but only if your panel is actually 144 inches long.

Locate the station lines from the drawing Tunnel Sides -- Sheet One, then draw the lines across the panel, and carefully continue them over to the other side of the panel. Also carry the Ref Line to the other side.

At each station locate the indicated points (see Tunnel Sides -- Sheet Two) above and below the Reference Line. Note that the bottom edge of the tunnels sides are a straight line from Bulkhead #3 to the transom -- measure down from the Ref Line at transom and #3 then draw a straight line between. If the indicated measurements at Bulkheads #4 and #5 are more than slightly off, check your work. (If the error seems to be mine, please let me know.)

Draw the curved lines using a batten approximately 3/4" wide and 1/4"-3/8" thick. The batten should ideally be longer than the line you are drawing, but a shorter one can be used as well. More information on this subject is included in the Construction Notes for the EZ Tunnel.

Tunnel Sides -- Sheet Three shows the locations where the bulkhead beams pass through the tunnel sides. Note that the beams at Station 1 and 2 and the Transom lie forward of their station lines, while the remainder lie aft of their station lines.

Tunnel Sides -- Sheet One shows two suggestions for cutting openings for ventalation and weight reduction. The top example is quite radical (it's what I did for my own Sport C), and only makes sense if you plan to race your boat. It had to be laid out very carefully to make sure I still had enough plywood left to attach the bulkheads. For the non-racer, a series of holes approximately 3" to 6" diameter will do very nicely.

NOTE: Small differences in weight will not really effect the top speed you can achieve with your boat, but will impact holeshot and acceleration -- attributes usually more important to racers than non-racers.

Even if weight is of little concern, I highly recommend some cutouts in the tunnel sides, bulkheads and coaming to provide ventalation for the inaccessable parts of your boat.

After laying out the Ref Line and Sta Lines on the second panel, lay the first one on top, carefully align the Sta and Ref lines and then trace the outline and the cutouts.

After cutting out, screw the two tunnel sides together and trim the edges so that they are exactly the same, paying particular attention to the straight line from Bulkhead #3 to the Transom.

The tunnel sides, all cut out.

I cut a vent hole in the forward end of each tunnel side.

In the event of a capsize, vents will allow air trapped in the hull to escape and the the stern to sink, which simplifies boat recovery (and possibly driver rescue). Vent holes have been required by APBA, not sure if they still are.

I find they also make good handholds when pulling my boat that last few inches onto the trailer.

Glue a 4"-diameter reinforcement to the inside surface of the tunnel side. Then cut a two inch vent hole with a hole saw.

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