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Old 23-06-2015, 10:11   #1
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Prop rpm

Good morning,

I have searched for info to answer this question but cant find it. I am converting a 1975 Johnson outboard to a electric power head.

I am looking for the correct Max rpm at the prop pushing a 1965 twin keel kingfisher max hull speed 6 knots.

I chose a high torque 4 blade prop 10 x 5 and I am using the foot from a 1975 johnson / evenrude with a 12:29 gear ratio.

If anyone can help I appreciate it. The only variable I can change is the rpm at the electric motor.

Thanks
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Old 23-06-2015, 10:48   #2
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Re: Prop rpm

What kind of motor are you using?? If its a brushed DC, the motor will run at the rpm where its torque matches the load. The original gas engine was probably designed to run at 4-5,000 rpm max while golf cart DC motors are happier at half that, so a bigger prop that the orignal would help.

If its a brushless motor, its more complicated.
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Old 23-06-2015, 10:52   #3
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Re: Prop rpm

I put the data into a prop calc spreadsheet and working backwards, came up with 4820 rpm; (2000 rpm at the prop). I assumed a a vessel displacement of 2600 pounds and a water line length of 18.5 feet for the calculations.
Also assumed the electric power head could deliver 6hp (15 ft lb torque)
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Old 23-06-2015, 14:01   #4
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Re: Prop rpm

Quote:
Originally Posted by Kokanee View Post
I put the data into a prop calc spreadsheet and working backwards, came up with 4820 rpm; (2000 rpm at the prop). I assumed a a vessel displacement of 2600 pounds and a water line length of 18.5 feet for the calculations.
Also assumed the electric power head could deliver 6hp (15 ft lb torque)
Yes those numbers look about right the motor nominal is 6hp peak at ten 100 % at 4000 rpm


Thank you for your time
Bobby
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Old 23-06-2015, 14:26   #5
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Re: Prop rpm

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Originally Posted by donradcliffe View Post
What kind of motor are you using?? If its a brushed DC, the motor will run at the rpm where its torque matches the load. The original gas engine was probably designed to run at 4-5,000 rpm max while golf cart DC motors are happier at half that, so a bigger prop that the orignal would help.

If its a brush-less motor its more complicated.
Its is going to be brushed. The prop I chose is a high torque for heavier boats. Its maybe 1/2 inch larger with a extra blade.

My thought process was:
I can choose a motor about 100 percent eff at 4000 or one that is the same eff at 2000 rpm . If a 2000 rpm motor will achieve hull speed in flat conditions then the excessive rpm might not be needed and would be wasting power. I was going by the motors data chart from kelly motors or the manta 2 motor.
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