Espritmodel.com Telemetry Radio
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Old Aug 03, 2005, 06:14 PM
Scratch Builder
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Birmingham, England
Joined Sep 2004
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Big or small prop?

Hi,

Is it best to have a small prop at high speed, or a large prop at low speed?

Thanks

Michael
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Old Aug 03, 2005, 08:51 PM
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Well, in real subs, you want the screw to move slow enough to reduce cavitation and noise. As for model subs, I'd imagine it depended on the motor and ESC you were using.
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Old Aug 04, 2005, 06:06 AM
Scratch Builder
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Birmingham, England
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But in models, if you have to choose a motor and esc and prop, and your building a custom sub (not scale), is it best to use a large prop, at low speed, or a small one at high speed?
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Old Aug 04, 2005, 09:31 AM
RC sub dude
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Montreal, Quebec, CANADA
Joined Nov 2004
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Well, assuming that you have a non-proportional radio and a sub with a fixed speed, a large prop will stir a larger amount of water as it is spinning than a smaller prop, which may slow down the motor a little and making it harder to run. If you're using an ESC, which causes the motors speed to alter when you wish, this theory still applies and it depends again on the motor type (you can't couple a 4" diameter prop to a small 3 volt motor, stick it in water and expect it to run at full capacity with little friction can you? ). My suggestion, don't go too big or too small, choose a size that you feel most comfortable with. Remember, there are always other factors that go with your choice, sub's size, ESC, battery run time, prop's pitch angle, only to name a few...
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Old Aug 04, 2005, 10:26 AM
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that's some good advice. just find a prop that looks good, then adjust your motor, esc, and gearing choice accordingly
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Old Aug 04, 2005, 11:36 AM
Man from Atlantis
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London
Joined Nov 2003
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Couple of things to bear in mind.

1. Speed you want the boat to travel.
2. Metacentric height of sub.

A large prop will provide large amounts of thrust at fairly low RPM. However as the boat travels faster, the large prop will create drag, so if you want a very fast sub, a smaller prop rotating at a higher speed is a better option.

Most subs have a fairly low metacentric height. A large single propped sub is going to give torque roll problems, unless corrected by a stator or some other kind of vanes.

Andy
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Old Aug 04, 2005, 10:22 PM
RC sub dude
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Montreal, Quebec, CANADA
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For another way to counter torque, you could always use counter-rotating props, they're a bit complex to install, but it's a neat system to consider.
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