I once had a 20' Bertram that was a very heavy boat after I rebuilt it with a full enclosure and large fuel capacity - probably 4,500 lbs of very deep-V. It originally came with a 250cid L-6 Chevrolet engine coupled to an old Mercruiser drive. I replaced the engine with a long stroke 292cid L-6 (truck and bus engine) and modified the mounting and made a custom scatter shield and exhaust to fit it to a Gen1 Alpha I. I have forgotten the diameter prop I used, but after getting too much slippage with a 13.75" I replaced it with at least a 14" diameter (I think it was a 16" diameter) and it made a world of difference in the ability to use the huge amount of torque in that engine. Top end was slightly sacrificed, but the boat would cruise offshore in a heavy sea almost effortlessly at whatever rpm I wanted and with zero bog in following seas. The Alpha I is tougher than you might think. I had 1.65 gears in that one and always wanted to change it to 1.50 to see if it would run faster with even more prop pitch and still have the same guts - I think it would have been perfect because I never thought I was using all of the torque, but I never got around to making the gear change. You might want to look into your gear ratio, too.
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