AusCan said..
I normally cruise at about 1800-1900 rpm with my 42 hp Vetus which pushes me just under 6 knots. At 1850 rpm, the engine achieves maximum torque, and this tends to be an efficient speed to run diesel engines at.
The engine is good for 3000 rpm, but why double the fuel usage, noise and engine stress for maybe one extra knot.
Southace - why do you think you have the wrong prop? If it's sized properly, at full throttle you should just be able to achieve 2800 rpm, but there is very little gain in speed after you reach your theoretical maximum hull speed of about 6.5 knots. (square root of your water line length X 1.34)
For most engines the most fuel efficient engine speed (RPM) is that at which maximum torque occurs. If you nurse an engine at low load speeds the bore will become glazed.
So, if at that RPM you are achieving maximum hull speed, the propellor is correctly matched to your hull and engine. If you are not reaching hull speed, you may need a longer pitch on the prop. If you are reaching hull speed and getting cavitation and or black exhaust smoke, the engine is overloaded and you may need a shorter pitch on the prop.
Remember that the formula for hull speed is using waterline length in FEET.
