I rigged a few with 20' Seafari's and SF's with 85 hp Johnson's and inline 4 cyl 85 Mercs back in the early 80's. They would cruise at 23-25 mph, and top out in the 33-34 mph range. With Bennett trim tabs, they even got out of the hole reasonably well, and could stay on plane down in the 12-13 mph range.
I did rig just one with an inline 3 cyl 75 hp Johnson "Stinger" in '83. Customer bought a used 20' SF hull with twin electric-shift 55 hp Evinrudes and brought me his old boat so I could de-rig his 3 yr old 75 hp motor off his old boat to put on the SeaCraft.
It was slow to plane, but actually topped out at 34 mph with a single person on board, the same top speed it got with the twin 55 hp's.
(I remember he asked for extra-wide 9"x18" trim tabs. He was another of those crazy SeaCraft nuts who took his boat to the Bahamas regularly, and he put a SeaGull 3.5 hp kicker right on the transom next to his main engine).
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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