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This post was updated on .
The classic formula for the theoretical speed (in knots) of a displacement hull is 1.34 * SQRT of the LWL. Narrower hulls will earn a higher multiple. Fatter ones, the converse. Thus, even a little, 8' rowboat that puts 81” on its design waterline offers a theoretical speed of 3.48 knots, or a respectable 4 MPH.
If LWL is doubled, hull speed isn't, for achieving just 4.92 knots, or a mere 41% gain, though boat weight and cost are doubled. Thus, speed is expensive, due effort needing to be cubed.
So the design question isn't "How fast do you want to get there?", but "How much will you pay to get there faster?"
Regrettably, sheets of marine plywood longer than 8' are either unavailable or very expensive. Using scarfs to make longer planks is possible, and it has to be done to build most canoes, kayaks, and pirogues. But decent, sub-8' rowboats can be built, especially for petite, single users, for around $150 and under 50 pounds, even when framed with walnut.
Why avoid scarfing? Because the planking to either side is the joint cannot be matched and creates ugliness if the boat is to be finished bright. Possible solution? Lots of planks and scarfs, so that the mis-matches "hide in plain sight".
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