Deck

The deck can be made from 1/4" or 1/8" (or 3/16") plywood. I calculated that I saved 15 lbs. by using 1/8" rather than 1/4". It does, however, make for a somewhat fragile deck -- step on the wrong place, and you might be up to your shin in tunnel boat.

At this time, cut the openings for four deck plates. These give access to the interior for guiding through control cables, wiring, etc. Also, when you get water in the boat, you can get that last little bit with a sponge, as well as provide extra air circulation while things dry out.

Don't glue on your decks yet. The coaming should come first.

Coaming

The coaming extends from the forward face of the stem to 2" aft of the motor board.

To establish the profile of the bottom edge of the coaming, suspend a sheet of ply above the frames and measure up from the tops of the frame beams.

The coaming rests right down on the tunnel plank from the aft extension of the tunnel plank to a point 3" aft of the beam at frame 4 (see coaming profile drawing). Here an opening is cut for control cables, wires, etc to emerge from under the deck.

From there forward the coaming only touches the frame beam tops, arcing upward between frames to a maximum height just below the carlins. These openings provide access to and ventilation for the under-deck area, while still tying the frames to the carlins, thus strengthening the entire structure.

The first step in determining the outline of the rest of the coaming is to decide how high you want your motorboard to be. The motorboard drawn in these plans is high enough that the propeller on a short-shaft motor will actually be out of the water for part of its rotation. For a prop to operate in this manner it must be "cupped."

Cupping is just a mild curl on the trailing edge of the prop. New props can be ordered with cup; existing props can have it added. Or you can do it yourself -- at least with an aluminum propeller.

For racing, a high-set engine with a surface-piercing prop is a must.

If, however, you want a boat which is a bit easier to drive -- that is, easier to get on plane -- you may want to lower your motorboard a couple inches. Your top speed will suffer considerably. However, you can get that engine height back by using a jack plate -- see page 23, notes on rigging.

Before installing the coaming, make sure the inboard faces of your carlins are vertical.

At the stem, the coaming will eventually taper to nothing. However, leave a little extra there for now. Plane or sand it down after the coaming have been installed, but before gluing down the deck.