In an effort to understand how the engine performs I took some data and plotted some graphs. The Yanmar 4JH3e has a max of 56 hp. Recommended max continuous RPM of 3650 at 50 hp. The recommended continuous RPM for small boat owners is 85% of max continuous which would be 3100RPM.
I measured boat speed under good conditions. i.e. Freshly painted bottom, tanks full, gear on the light side, 2 adults, no wind and calm seas. The prop is a 3 blade folding Martec Slipstream 16RH10.
At 2000 the boat moves along at 6.4 kts at .6 gph or 10.7 mpg
At 2800 the boat moves along at 8.0 kts at 1.26 gph or 6.3 mpg
At 3100 the boat moves along at 8.2 kts at 1.7 gph or 4.8 mpg
The interesting plot below is the red (delta) curve which shows the return in speed for an increase in RPM. The dashed red show projected results. My results showed decreasing improvement in boat speed with corresponding increase in RPM. In actual measuring I saw no boat speed increase from 2900 to 3100 although some fractional increase would be expected.
The sweet spot for my boat seems to be 2800, even though this is under the recommended continuous rating. According to the experts, Continuous running below 80%, or 2920 can cause glazing of cylinder walls. This apparently can be countered by running at 3650 for 10 to 20 minutes after several hours at a lower RPM. Not sure how much of an issue this is. It is more of a problem for people using the engine to charge batteries or heat water at very low RPM.
As another point of reference. With old sails Santosha will hit over 8.5 kts on a broad reach in 14 kts true wind. I am guessing healing over a bit reduces hull drag as it seems like the stern squats (is too low in the water) while motoring.
I will be monitoring this over time to see if I can get more data and better understand the motoring characteristics.