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Old Dec 05, 2010, 11:42 PM
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Joined Dec 2010
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Props and motor help!!!

Hi everyone, my name is Nery.

i'm designing an aquatic robot and i need to calculate the prop and the motor for my project.

I've read sth about drag and thrust but I need to justify the calculations of every part of the structure.

I've done the calculations for the stability and flotation but i don't know how can i start with the propulsion system.

So, can anyone help me with the propeller and motor?

I just need the equations or formulas you use in this cases.

For example i have a ship of "x" mass and volume, and i want to move it at certain speed.

How can i determinate the diameter or pitch of the propeller and the power and rpm of the motor?

Just in variables. It will help me a lot.

Thanks in advance
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Old Dec 06, 2010, 12:30 AM
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Bucharest, RO
Joined May 2009
1,990 Posts
The problem is that the formulae we use (in a software called FE calc) will only give data on ESC, battery, motor and given prop from an electrical point of view. These calcs are based on the assumption that you already have the right hull for those components. Most of us will use a prop based on our or others experience...
I am sure there are some calcs out there involving mass, drag, etc, but I doubt these are used around here
In theory, for a given mass and physical size you need a certain amount of power to move it. If you could get to this stage, then it's easy: power gives the motor. Based on the speed you want, you need to find a balance of prop/rpm that will suit your application: you can have a bigger prop at lower rpm doing the same thing as a smaller one at a higher rpm (but with a different load on the motor).

Best thing is give some examples, this way some of the guys around can also give their 2c...
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