Hi, need some help on this one. I am fitting a new
motor, output is 32hp @ 3000rpm. torque 83.5N.m (I believe this is the continuous rating).
Displacement is 16,200lb but I want to allow for a loaded weight of up to 18,200lb. WLL is 25.5ft, beam 9.6ft. Cruising speed I want is 5 - 6kn running the
engine at 2-2500rpm where it will produce 24 - 30hp Prop is 16x10. 2 shaft bearings. Changing the prop is not realistic as there is no slip available
The
engine I am replacing was 35hp @ 2800rpm intermittent, (30hp cont) with a 2.4:1
gearbox which gives a shaft speed of 1167. Performance was excellent. I checked the figures with a prop calculator and it agreed that this setup should be right and gave a max speed of 6.78kn which is about right, possibly a little conservative.
The new engine is 100cc bigger but slightly lower torque but is 3cylinder so shorter and taller. The old one barely fitted the engine room and left no room for servicing. This one fits much better
So working out the new setup it should mean that with a 16x13 prop and a 2.74:1
gear ratio at 3000rpm I get a shaft speed of 1095rpm. Assuming the same prop efficiency this should give a speed of 6.3kn. I am fine loosing 1/2kn at top speed as I never
motor above 6kn anyway.
So that all looked good until I ran it through the same prop calculator. the result gives a
power requirement of 26hp at 6.3kn for standard
displacement and 29hp for the additional load. Max speed is given as 6.34, great but the prop recommendation is now for 18.9x11.3. scaling that to 13" means a
pitch of 15.8 - 17.3.
I don't understand, I have only changed the revs by 72rpm or 6% how come I get such a drastic change for prop recommendations. Even if I play with the
power number I still can’t get anything approaching my
current prop. Am I missing something or does a few revs really make that much difference?
PS even stranger, just ran the prop calc again using 32hp and 2.5:1 gearing, Gives a shaft speed of 1200rpm, 33rpm HIGHER but a
boat speed of 6.34kn which is 0.34kn LOWER (even though the shaft speed has gone up) but now comes up with a Prop recommendation of 16x13.3-14.1 which puts me a fraction under propped.