A 42" boat with one small outboard probably won't reach scale speeds. What are the specs of the motor you want to use? What voltage will you run? What prop? What are your speed and runtime expectations? I'd hate for you to build a big project like this and not get the performance you want.
I built a slightly larger catamaran airboat rescue craft out of 4" ABS, with a 14" prop it runs about 6 mph or nine feet per second. Built for pushing power, not for speed. It takes a lot of thrust to move a big boat.
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