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Electric boat prop sizing

Old 03-20-2009, 11:55 AM
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pilotj
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Default Electric boat prop sizing

Can anyone point me in the right direction to size a prop for an electric boat?

I want to size a prop for a Speed Gem 9TD Cryptonite motor (I figure it puts out about 150-200 watts).
I need high torque, low speed for the boat to pull a payload at only 2-3MPH.

Can anyone suggest what size prop I should begin with or how to analyze this requirement?

Thanks
Old 03-20-2009, 02:20 PM
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mfr02
 
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Default RE: Electric boat prop sizing

This really doesn't sound like the right type of motor for the job you describe. I would look at the size of prop (to scale) for the size and shape of hull, then gear to match the motor. Or look for a more appropriate motor and drive direct. A prop that is too large, or having too much pitch is a fast way to go nowhere and flatten the battery. A widely accepted rule of thumb is to restrict the prop diameter to no more than that of the motor.
The object of the prop is to move water backwards - for the boat to move forward by its own length, the prop has to move AT LEAST the volume of water that the hull displaces. A slow, heavy hull needs a large prop to move a wide stream slowly, a fast boat requires a narrow stream going at AT LEAST the intended speed forward, i.e. a small fast rotating prop.
Old 03-20-2009, 05:39 PM
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pompebled
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Default RE: Electric boat prop sizing

Hi pilotj,

mfr02 is right, your motor is a high revving car motor, which has to be geared down substantually to be able to turn a prop large enough for your low speed purpose. Even then, this motor is unsuitable for such an endeavour

How big, heavy is your boat (going to be)?

A low rpm, high torque motor, 700 size (depending on the boat at hand) would work much better (and longer without skimming and brush renewal).

Regards, Jan.

Old 03-21-2009, 10:21 PM
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pilotj
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Default RE: Electric boat prop sizing

Thanks for feedback, makes sense.

What do you suggest is a optimal RPM for a prop that is 2-4" diameter?

Thanks
Old 03-22-2009, 07:40 AM
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mfr02
 
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Default RE: Electric boat prop sizing

It depends. On number of blades, shape of blades, profile of blades. You are firmly in the "black art" area here.
Best starter is to find out what RPM your prototype has, and multiply that by the square root of the scale.
Unless you are looking for the ultimate in efficiency, bear in mind that a prop is a really good torque converter/slipping clutch.
Have a look at http://www.towboatjoe.com/ and navigate the "shafts" for a masterclass in home made props. If you do a home build, you will be able to experiment at relatively low cost.
Old 03-23-2009, 07:12 AM
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Tony Fitzpatrick
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Default RE: Electric boat prop sizing


ORIGINAL: pilotj

Thanks for feedback, makes sense.

What do you suggest is a optimal RPM for a prop that is 2-4" diameter?

Thanks
Power = a constant x RPM x Prop load so to answer your question you could give us an idea of the size/shape and speed of the boat and a power figure could be arrived at.

150-200 Watts is a lot of power for a boat

Tony Fitzpatrick

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