Prop Power Consumption

[Divan]

Good day,

I am trying to work out how much power a 9" prop with 9" pitch at 1200 rmp will consume.

I am unable to find any formulas or calculators to work this out any help will be appreciated.

 

[jehardiman]

How many blades and what is the blade area ratio?

 

[Divan]

How many blades and what is the blade area ratio?

It is a 3 blade prop and the BAR 0.53

 

[baeckmo]

It will vary with boat speed as well.

 

[Divan]

It will vary with boat speed as well.

It is a very large and heavy boat so top speed at 1200 rmp will probably be around 5 to 6 km/h

 

[Mr Efficiency]

Perhaps some more information would help. Where are you getting this figure of 1200 rpm from ? In any case, you are talking about an engine of less than 10hp

 

[jehardiman]

Just for a benchmark, the Honda 7.5 4-cycle OB on my Catalina 22 has a 9x9 prop and drives the boat at about 5 knots at half throttle (? rpm ... WO is ~2000 rpm i.e. ~5k shaft/2.5 final). I could work the B3-35 Bp-delta chart, but as Mr Efficiency says it's not much hp.

 

[Divan]

Perhaps some more information would help. Where are you getting this figure of 1200 rpm from ? In any case, you are talking about an engine of less than 10hp

The 1200 rpm is the maximum rated speed of my electric motor, I am trying to figure out if I will have enough power onboard to run the motor at full speed with out installing it on the boat since it will take a lot of effort and time replacing it with my current motor.

 

[Mr Efficiency]

What is the boat ? Length ?

 

[Divan]

The 1200 rpm is the maximum rated speed of my electric motor, I am trying to figure out if I will have enough power onboard to run the motor at full speed with out installing it on the boat since it will take a lot of effort and time replacing it with my current motor.

It is an catamaran and 6 meters in length

 

[gonzo]

The maximum rated power of the motor is the amount of deliverable power you need onboard. It is pretty straightforward.

 

[Mr Efficiency]

Is there the ability to fit a larger diameter propeller ? Sounds a good idea if possible

 

[Divan]

Is there the ability to fit a larger diameter propeller ? Sounds a good idea if possible

It is possible but would prefer to keep the prop as small as possible due to areas with shallow water. I am aware in my situation that an bigger prop will be a lot more efficient and I might be able to go with a bit bigger prop but for now it will have to do.

 

[Divan]

The maximum rated power of the motor is the amount of deliverable power you need onboard. It is pretty straightforward.

True but I am unsure if the motor will be strong enough thus I want to calculate the amount of power needed to turn the prop at the rated speed of the motor and if it doesn't have enough power then to be able to see what size motor I need. It should be an fairly strait forward formula. Lest assume you have a prop that is 9" in diameter with 3 blades and a pitch of 9" and an BAR of 0.54 how much energy is required to rotate the prop at 1200rpm in a body of stationary water? This is what I want to calculate.

 

[jehardiman]

True but I am unsure if the motor will be strong enough thus I want to calculate the amount of power needed to turn the prop at the rated speed of the motor and if it doesn't have enough power then to be able to see what size motor I need. It should be an fairly strait forward formula. Lest assume you have a prop that is 9" in diameter with 3 blades and a pitch of 9" and an BAR of 0.54 how much energy is required to rotate the prop at 1200rpm in a body of stationary water? This is what I want to calculate.

A 9" 1.0 P/D B series 3-50 requires 11.328 ft-lbs of torque at a J=0 and n=20 (20 rps=1200 rpm). Since hp = Torque * rpm/5252, you would need a motor of at least 2.59 rated shp @ 1200 rpm.