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Pro Tunnel Race Boat Plans
Page Nine
Coaming

MY RECOMMENDATION: Cut and fit the coaming next. BUT, before installing it, cut and install the deck. It will be much easier to make the coaming if the deck is not in the way, and easier to fit the deck if the coaming is not in the way.

Before you start fitting the coaming you will want to add some temporary support to the extension of the tunnel plank, which was left hanging when it was installed. I used a length of 2x4 with a nice straight, true edge (shown here at a later stage of construction).

The drawing provided for the coaming is more of a guide than a blueprint. It indicates generally where things are located, but the actual overall length, and the shape of the bottom edge, must be figured out right on the boat.

I used inexpensive plywood to create a pattern for the bottom edge of the coaming. I used two pieces of luan plywood, about 12" wide, which will be joined later with screws and a butt block. I placed the joint just forward of bulkhead 3, with the butt block on the side facing the cockpit so that the pattern could be lifted in and out of the boat without interference from the carlin.

To start, fit the aft half of the pattern to the boat by slipping it into place as best as it will fit at this point. Then mark the locations and dimensions of the bulkhead and transom beams. Remember that the tunnel plank is slightly curved, and you will want the edge of your coaming to follow that curve as closely as possible.

From the transom forward to bulkhead #3, the coaming should extend right down to the tunnel plank, and will be snug up against the outboard edge of the third-from-the-center tunnel batten -- which you have carefully placed right there, just for that purpose.



Now proceed with the forward part of your pattern. Make certain that the pattern is tight against the carlins for an accurate fit. In this part of the boat, the coaming need not fit all the way down to the tunnel plank, but should be notched around each bulkhead beam, and then can be cut away between the bulkheads for access and/or ventilation. (See the "Coaming" drawing.)

At this point you will need to join the two halves of your coaming pattern. Your joint does not have to be perfect, but must be strong enough to allow you to lift the entire pattern out of the boat. BEFORE YOU DO LIFT IT OUT for the last time, mark the Reference Line at each bulkhead so that you can draw it out on the pattern.

Using your pattern and the coaming drawing you can now determine how much plywood each coaming will require and then scarf together the necessary pieces.

Then, with your blanks prepared, you can trace on the bottom profile from your pattern, and the Reference Line. Then locate the corner at the top of the motorboard and the high point at the dashboard and draw a straight line between them.

Forward of the dash, I initially cut the coamings a little high, then set them in place in the boat and measured up from the stem to get just the finished height. (See "Sponson and Cowling Tips" drawing.)

In the picture, the starboard coaming has been trimmed to its final height and length. Note that the coaming extends 3/4" forward of the leading edge of the stem.

The terminal bulkhead between the coamings rests on a bottom piece glued to the tunnel plank. The bottom piece extends about three inches forward of the stem.

Another look at the bottom piece.

The dashboard is placed one inch forward of the high point of the coaming -- or at least I like to do it that way. It is oriented 90 degrees to the downward slope of the coaming.

The dashboard is fastened to the coaming with 1x1 cleats. I glue the cleats to the coaming, but I install my dashboard with screws only so it can easily be removed, or even replaced should that become desirable.

A small bulkhead forward of the dash will carry the forward bearing for the steering shaft. I don't glue this bulkhead in either

An addition bulkhead is installed at the aft edge of the permanently-fixed portion of the front cowling. This bulkhead is glued in place.

To determine the crown of the intermediate bulkheads, use a straightedge long enough to rest on the dash and the terminal bulkhead at the forward end of the coaming. Measure up from the top of the coaming at each bulkhead location to determine their crowns. Then use a thin batten to draw the curve of each bulkhead.

The motorboard is laminated to about 1.75" thick. Just about any plywood will do. In this case I used two pieces of 5/8" CDX construction plywood as a core, and a layer of 5mm luan plywood on each face.

The bracing pieces along the top of the coaming are 3/4" x 3" and extend forward to bulkhead #4.

Also install a cleat at the bottom of the motorboard and cleats where the motorboard meets the coaming.

The 1x3s are notched into the motorboard.

3/4" x 1.5" cleats back up the motorboard. Also note that the aft face of the motorboard is flush with the aft edge of the tunnel plank extension.

Now that the coaming and its associated pieces have been cut and fit, remove all of it and get ready to fit the deck.

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