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Laker 14 Tunnel Boat Plans
Page Ten
Motorboard ("Racing" Configuration)

My motorboard is laminated from one layer of 1/2" AC plywood; two layers of 5/8" CDX plywood; and two layers of 5mm luan plywood for a total thickness of 2-1/8".

Approximately 2" thick is what you are aiming for.

The rough demension of this blank is 22" x 24".

Carefully cut the width of the motorboard to cleanly fit between the coamings. Bevel the bottom edge to match the tunnel plank.

I cut my motorboard 21" tall, measured up from the tunnel plank along the forward face of the 'board. For my longshaft (20") Merc 700, that appears to leave room to eventually jack the motor up about one more inch. Actually, the gains from that last inch or so are probably not very important in a non-racing boat. I expect to mount the motor right on the motorboard, as is. Should be plenty high, and plenty fast.

If you expect to use a shortshaft motor, such as the OMC SST 60 motor, or a Stinger, then lower the transom height by 5" -- or measure your motor to determine where your 'board needs to be.

Note that the coamings extend 2 inches aft of the motorboard. Cleats made from 1/2" to 3/4" lumber (clamped into place temporarily at this point) will be installed here to support the motorboard.

I am dismayed to see some of my customers neglect this step, simply cutting off the coaming flush with the aft face of the 'board. Maybe you can get away with that with a 25hp motor, but a big 3-cylinder needs some serious support.

A filler strip is added to fill the space between the motorboard and the ends of the tunnel stringers.

Note that the half-cove limbers cut earlier will allow water to flow laterally, exiting the cockpit on its way to the sponsons. I trimmed away the ends of the filler strip to avoid blocking the limber through the coaming.

To further reinforce the motorboard, add a knee along each coaming, tying the motorboard to the transom beam. Here I am making a template with a scrap of plywood.

Width of the knees is 3 inches.

I used 3/4" thick oak for my knees. If you use softwood, 1.5" thickness is recommended.

Notches will need to be cut into the motorboard to receive the knees.

Secure the upper (aft) end of the knee to the 'board with lag bolts driven through the coaming and knee and into the 'board.

The lower end will be secured to the transom beam using brackets cut from steel or aluminum angle. Use lag bolts to secure the bracket to the the transom beam. Bolt the bracket to the the knee and coaming.

Of course, the knees will also be glued to the coaming.

Notching the motorboard: I cut the outer margins of the notch with a handsaw, then removed most of the waste with a chisel.

You could also do this with a circular saw, cutting the margins and then cutting away most of the waste with multiple cuts.

I finished up the notch with a router.

A knee in its final position with a steel angle bracket ready to help tie it in.

The bracket is lag-bolted into the transom beam, and bolted through the knee and coaming.

I also drove lag bolts through the coaming and knees and into the motorboard.

Leave room for the coaming trim, shown here temporarily clamped into place.

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