Typically it takes only 2 to 2.5 h.p. per long ton (2240 lbs.) to drive a vessel to hull speed
in flat water
. But then you have to add in all of the contingencies that rear their ugly heads....things like:
Fouled bottoms and props
High output alternators
winds and adverse currents
So if you look at what manufacturers are doing today you will see that they are installing 4 or more h.p. per ton, and hulls are more easily driven than ever. Go figure..
What you can do is look at the way you have been using the Perkins
and compare that to the prop shaft h.p. curve for the engine and estimate how much power you typically used. For instance, the 4-154 had 62 h.p. @3000rpm (crankshaft hp) Assuming the boat was propped so that it could achieve somewhere close to the 3000 rpm
, if you cruised at 2000 rpm
you would have used about 20 hp. At 2400 rpm you used about 33 hp. At 2600 rpm, 42 hp. 2800 rpm, 50hp. Not only that, but if you had the old Borg Warner 71 or 72 series transmissions, you had more losses than you will have today with a more modern transmission
Bottom line is that a 55 hp engine will probably do the job for you. Remember also, that you should use the transmission
with the most reduction and largest prop that you can get under the boat.