Have been researching info on my Perkins m50 as I belive my yacht has the wrong Prop.
Interesting to see what RPM other yachts cruise at. Many over 2000RPM , I cruise at 1500 to 1750 @ 5 to 6 knots, the
forums.ybw.com/index.php?threads/cruising-range-fuel-consumption.264029/
recommend stats for M50 is 2200RPM ! 
And apparently it's good to run them at 2800RPM!
that would be screaming on my yacht and just wasting fuel/noise/cavitatio/vibration!
or do I have the wrong prop?
boatdiesel.com/Engines/Perkins/Perkins.cfm
you will need to login but you should find data for the M50 in there
www.cruisersforum.com/forums/f54/perkins-4-108-power-curve-52046.html
there's a original sale brochure half way down the page also in pdf
My Perkins 65hp chugs along at 1500 rpm 5knots in slack water - very similar to your motor.
2.3 lph fuel usage - I'm pretty happy with that!
I wound it out to 2000rpm the other day and was doing a touch over 8 knots.
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)
The prop cavitates going into any wind or current. I have to increase slowly from stop to 5 knots , if I throttle up too fast I can here it cavitate air. Yes ho2 I googled those posts as well, seems they like wasting fuel!
Select to expand quoteAusCan 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.
If the prop cavitates when you apply too much power, I'd be guessing that the diameter is too low, or it's swinging too close to the skeg, rudder or other object. With cavitation, I don't think you can draw any conclusions about pitch.
