Gearing for new engine

[roland stockham]

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.

[SailsWithFists]

How many blades does your prop have?

A 2:1 transmission will get you little, and you will be over propped with your current 2 or 3 blade 16 x 10.

2:1 trans asks for 16.4 x 8.3 Two blade or 15.6 x 8.3 Three blade


Me? I would keep the current 2.4:1 transmission, run it, then change the prop later to the recommended 17.4 x 10 three blade.

[roland stockham]

Originally Posted by SailsWithFists

How many blades does your prop have?

A 2:1 transmission will get you little, and you will be over propped with your current 2 or 3 blade 16 x 10.

2:1 trans asks for 16.4 x 8.3 Two blade or 15.6 x 8.3 Three blade


Me? I would keep the current 2.4:1 transmission, run it, then change the prop later to the recommended 17.4 x 10 three blade.

Sorry I missed that bit, its a 3 blade prop. Engine was flooded and current gearbox is toast, it was full of seawater and the case corroded through. new gear was either 2.74:1 or 2.5:1 not 2:1 but I just noticed that I mistakenly put the prop size as 16x10 should read 16x13

[roland stockham]

Please note error in original message - prop size should be 16x13 not 16x10, 3 blade

Originally Posted by roland stockham

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.

[SailsWithFists]

Originally Posted by roland stockham

Sorry I missed that bit, its a 3 blade prop. Engine was flooded and current gearbox is toast, it was full of seawater and the case corroded through. new gear was either 2.74:1 or 2.5:1 not 2:1

I was trying to find a ratio that worked best with your current propeller.

2.1:1 requires 16.1 x 8.7 3-blade
2.2:1 requires 16.5 x 9.1 3-blade
2.3:1 requires 17 x 9.5 3-blade
2.4:1 requires 17.4 x 9.9 3-blade
2.5:1 requires 17.9 x 10.1 3-blade
2.6:1 requires 18.3 x 10.7 3-blade
2.75:1 requires 18.9 x 11.4 3-blade

Pick your poison.

[roland stockham]

Originally Posted by SailsWithFists

I was trying to find a ratio that worked best with your current propeller.

2.1:1 requires 16.1 x 8.7 3-blade
2.2:1 requires 16.5 x 9.1 3-blade
2.3:1 requires 17 x 9.5 3-blade
2.4:1 requires 17.4 x 9.9 3-blade
2.5:1 requires 17.9 x 10.1 3-blade
2.6:1 requires 18.3 x 10.7 3-blade
2.75:1 requires 18.9 x 11.4 3-blade

Pick your poison.

Working on the rule of thu10.1mb that 1" diameter = 1.5 - 2" pitch the 17.9x10.1 is close to 16x13. that does appear to work but it seems counter intuitive because it speeds the shaft up to deliver less power so I wondered if I had it wrong. Or if it was right then why?

[captlloyd]

Originally Posted by roland stockham

Working on the rule of thu10.1mb that 1" diameter = 1.5 - 2" pitch the 17.9x10.1 is close to 16x13. that does appear to work but it seems counter intuitive because it speeds the shaft up to deliver less power so I wondered if I had it wrong. Or if it was right then why?

As you are going to need a new or different gearbox anyway why not ask a gearbox distributor? They do this stuff everyday.

[roland stockham]

Originally Posted by captlloyd

As you are going to need a new or different gearbox anyway why not ask a gearbox distributor? They do this stuff everyday.

I will probably run it by the local prop shop. I was more curios as to why it worked out this way and it is the sort of question some of the tech guys on here often have great answer to. Prop calc's are a bit of a mystery to me beyond being able to use the formulas.

[Scaramanga F25]

If you have the space go, with extra diameter every time. But approx 1 in. dia. for 2 in. of pitch.

[Cadence]

Originally Posted by roland stockham

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.

Sounds like your problem is using calculators. You can pretty much figure your slippage on your prop. from your history. You might use that and do your figuring.