My Westerbeke 30 hp in my 1974 Pilot. Turns out this is a British Leyland engine, the same one in the London taxicabs (I'm told)! Westerbeke marinized it, as they do with most of the engines they sell.
A recent discussion with one of the engineers at Westerbeke revealed that the original W-30 engine was designed to have a full throttle RPM of 2350 and that the reduction for best 'power' efficiency is at 2115 RPM.
I admit that even as experienced as I may be in my theory, I am guilty of rarely running my own Westerbeke at more than about 2000 RPM for what 'Sounds' and 'Feels' to be its comfort zone when powering upwind in a bit of a sea. Generally, if just wandering towards the next waypoint, the tachometer rarely sees more than about 1800 RPM and my Pilot clocks out a steady 5.9 knots, consuming about .5 gallons per hour. Fuel efficiency certainly suffers measurably for the sake of another quarter of a knot.
The Westerbeke 30 diesel in my boat spends far more time at 12 - 1500 RPM charging batteries that it does pushing the presumed 2200 pound (dry), cat displacement hull at 85% of the theoretical hull speed (of 6.8 knots).
