Inboard Diesel for 30' Trawler

TOO much power.

I have a larger and heavier 30' trawler. A Willard 30' Nomad. 30' overall, 16,000lbs, 10.5' beam, 27.5' WLL and 3.5' draft.

I repowered from a Perkins making 33hp to a Klassen Mitsubishi 37hp. The 37hp is an industrial rating so probably equivalent to 40hp .. or so. A 107 cu in engine at any rate. Using a 2.57-1 gear and an 18" wheel there is more power than I have ever needed. I cruise at 6.15 knots at 2300 rpm and am propped to the engine's rated 3000rpm. I sometimes (not often) cruise at 2500rpm and 6.4 knots. In rough water I've always decreased rpm. Never needed more power.

All these ideas of 60 to 150hp are way off the mark. I could do everything I've done in the last 9 years w a 32hp engine. And the OPs boat is smaller and lighter than mine. A 3GM30 Yanmar (27hp) and slightly bigger should do him fine. Or a Vetus 33hp. That's a shorter stroke engine like mine and of course also a Mitsubishi. Lots of other engines are in the 27 to 35hp range and big brand names have little if any advantage. All the above applies only if the OP actually has a FD hull.

West Van,
The Willard needs about 18hp for 6 knots. One gph.

Tim,
You could probably make 5 knots on 8 to 10hp.

All the above is probably rubbish. It occurred to me while waking up this morning that Tim probably was talking about a SD boat.
Tim stated in the OP that his boat was "displacement". That probably meant displacement like a NT, GB or IG. Really Semi-disp boats. So the power numbers I talked about probably don't apply. 1.5 to 2 times as much power would be required depending on how close to a planing hull Tim's boat is.