I*owned a 78 Mainship for 14 years. Are you running the original Perkins T6.354 at 160 hp?
Assuming that engine, your OEM propeller should have been a 4 blade 20 x 20.* That would be with either the BW at 1.91 to 1 or 2.10 to 1.* You should be able to reach a minumum of 2450 rpm at WOT (with a calibrated tachometer).
I had a very heavily loaded boat and the 2.1 tranny ratio. I could get about 10.5 knots at WOT (2500 rpm) with the original engine.*
Fairing the keel (by adding fairing blocks ahead of the prop) I gained 250 rpm so then added one inch of pitch to my prop then could get 11 knots at 2550 rpm. Installing Big A$$ trim tabs (42 x 12) will get the arse end of the boat out of the water at 7.5 knots and really help the boat.
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