Now this may need to be redirected to the engine gurus, but here goes. In a carburated engine, at wot, when we see rpms decrease due to the increase in drop pitch, do we see a decrease in gph? I'm trying to go for fuel econ. in a prop selection. Thanks for replies.
Lug the engine and you use more fuel and the engine doesn't like it at all. Prop the motor for the top of the recommended wide open throttle RPM band with normal load. Best for economy and engine longevity. This is not a car where lower engine rpm = better mileage. Outboards cannot be lugged.
Yes, it does, but picture it this way...If you run a 25 pitch prop and at WOT (butterflies wide open) and engine is turning say 4000RPM, you have a high manifold pressure/low RPM situation.OK, let's say now we are running a 19 pitch prop at 5800RPM...we have less manifold pressure/high RPM.OK, now lets back down to 25MPH, the 25P prop is requiring the butterflies to stay at 3/4-full open to maintain that speed because it is way below the torque curve..the 19P, on the other hand, shows the butterflies to be at 1/3-1/2 open because we are in the middle (or close to) the torque range, but still at 25MPH. We have more RPM with the 19, but lower manifold pressure (some will say vacume) so we are doing less work requiring less fuel.
Yamaha has some very nice fuel consumption data on their web site in the Performance Test section. Choose any motor (2 or 4 stroke) and any boat type that they've tested. View those tests and it should become apparent how fuel economy/RPM/load/prop pitch interact.