# Prop Power Consumption

Discussion in 'Props' started by Divan, Sep 6, 2019.

Tags:
1. Joined: Sep 2019
Posts: 7
Likes: 0, Points: 1
Location: South Africa

### DivanJunior Member

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.

2. Joined: Aug 2004
Posts: 2,922
Likes: 457, Points: 83, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

3. Joined: Sep 2019
Posts: 7
Likes: 0, Points: 1
Location: South Africa

### DivanJunior Member

It is a 3 blade prop and the BAR 0.53

4. Joined: Jun 2009
Posts: 1,241
Likes: 207, Points: 63, Legacy Rep: 1165
Location: Sweden

### baeckmoHydrodynamics

It will vary with boat speed as well.

5. Joined: Sep 2019
Posts: 7
Likes: 0, Points: 1
Location: South Africa

### DivanJunior Member

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

6. Joined: Oct 2010
Posts: 9,607
Likes: 822, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

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

Last edited: Sep 6, 2019
7. Joined: Aug 2004
Posts: 2,922
Likes: 457, Points: 83, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

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.

8. Joined: Sep 2019
Posts: 7
Likes: 0, Points: 1
Location: South Africa

### DivanJunior Member

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.

9. Joined: Oct 2010
Posts: 9,607
Likes: 822, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

What is the boat ? Length ?

10. Joined: Sep 2019
Posts: 7
Likes: 0, Points: 1
Location: South Africa

### DivanJunior Member

It is an catamaran and 6 meters in length

11. Joined: Aug 2002
Posts: 15,129
Likes: 901, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

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

12. Joined: Oct 2010
Posts: 9,607
Likes: 822, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

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

13. Joined: Sep 2019
Posts: 7
Likes: 0, Points: 1
Location: South Africa

### DivanJunior Member

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.

14. Joined: Sep 2019
Posts: 7
Likes: 0, Points: 1
Location: South Africa

### DivanJunior Member

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.

15. Joined: Aug 2004
Posts: 2,922
Likes: 457, Points: 83, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

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.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.