Electric boat prop sizing
#1
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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
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
#2
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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.
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.
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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.
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.
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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.
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.
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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
Thanks for feedback, makes sense.
What do you suggest is a optimal RPM for a prop that is 2-4" diameter?
Thanks
150-200 Watts is a lot of power for a boat
Tony Fitzpatrick