Re: USCG Max Capacity for 1979 18' Seacraft
The formula for USCG recommended maximum horsepower is simple: for a sterndrive, or a 20" or 25" transom outboard, (LxWX2)-90=rounded to the next higher 5hp increment max recommended hp . (For a 30" transom it is (LxWX2.5)-90=max HP) Length and width are LOA and WOA, and are calculated to next higher foot measurement. The 18' SF is actually 18'2" , and the beam (including rub rail) is 7'5", giving you 19' and 8' respectively
So the calculation is 19x8x2-90=215 hp maximum. Your real limitation isn't HP, it's weight.
If you raised your transom to 30", your max HP rating would be 290 hp.
According to the USCG, LOA includes permanent fixtures such as bow pulpit, dive platform, anchor rollers, engine drives, etcetera. It does not include OB motor . So if you have a swim or dive platform and an anchor roller your 18 could easily be 21' or 22' by USCG specs.
My 20' Seafari has both swim Platform and anchor roller, giving me an actual LOA of 22'2". So for my Seafari I would calculate (23x8x2)-90= 280hp maximum. I actually run a 90hp Optimax.
This fits with the max power originally offered with the '75 Seafari being a 255hp Mercruiser sterndrive, with a max hp rating of 265 sterndrive hp.
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Fr. Frank says:
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Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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