Tad Roberts

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Alder, 18' Flat-bottomed Displacement Boat

Construction: plywood
Length: 18' 0"
Beam: 6' 8"
Draft: 19.5"
Displacement: 2,450 lbs
Power: 7 HP Volvo Penta MD1
Cruising Speed: 5 Knots
Builder:

Comments

Using our guide of one horsepower per 500 lbs, Alder needs about 5HP to attain her cruising speed. I have drawn in the 290 lb, 35 year old Volvo Penta MD 1 diesel, producing 7HP at 2300 RPM. Why all this engine when a 75 lb 15HP outboard would do just fine? For one thing the Volvo produces 5HP at 1500 RPM, which is so much easier to listen to than the outboards 4500 or so. But the outboard is much smoother you say, this is true, but I can work on the Volvo with a hammer and a couple of metric wrenches. When the outboard throws a fit I must take it to the nearest Factory Trained Technician, besides, the Volvo's slow, steady, thump is appropriate to Alder, and the weight will steady her down.

Economy is important in any design, but when it is the overriding factor, I don't think you can beat this dory/skiff hullform. Another rule of thumb is that the hull is only twenty percent of a boat's cost, and the hull and deck is twenty-five percent. But in a small semi-open boat like Alder there isn't much more than the hull and deck! So economy in the basic structure can go along ways toward making the whole thing affordable. Another area for economy is outfit, no sensible builder will look twice at a design specifying a 35 year old antique engine, he or she wants something available off the shelf. But Alder would be perfectly happy with a new Yanmar, 5 HP Farymann, or an old Lister.

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