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Author Topic: Testing trolling motors help  (Read 1116 times)

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Offline amarquez87Topic starter

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Testing trolling motors help
« on: March 09, 2011, 01:40:52 AM »
Hi,
For my project I am doing an autonomous boat and i will be using a trolling motor for the propulsion system. I am wanting to get the rpm specs on it while its in 'load', meaning in water or having some type of resistance on the motor/propeller so that i could later determine speed, torque. Does anyone know how i could do this? i have already got the rpm values for when its just operating in air but those would be different if it were in water so that's what i'm needing to know.

Thanks!!

Offline raptorwes

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Re: Testing trolling motors help
« Reply #1 on: March 09, 2011, 05:42:12 AM »
you could try to run the engine in a 55 gallon drum or large garbage can full of water. It could make a mess though if the motor is too powerful. I have done this with Gas boat motors but only at idle   

Offline garrettg84

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Re: Testing trolling motors help
« Reply #2 on: March 09, 2011, 07:41:02 AM »
I second the 55 gallon drum idea. I would say that a decent size trashcan may be a good alternative for a normal trolling motor though since trolling motors are much smaller and most people have access to a trashcan or two.
-garrett

Offline amarquez87Topic starter

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Re: Testing trolling motors help
« Reply #3 on: March 09, 2011, 02:31:56 PM »
Yes i was thinking that as well, but my only problem is how to measure the rpm while in the trash can?

Offline Aberg098

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Re: Testing trolling motors help
« Reply #4 on: March 09, 2011, 02:43:56 PM »
55Gallon drums are a good idea. I have done this with small gas engines as well, at wide open throttle for short bursts. My recommendation is to build some kind of test stand out of wood and hook up your motor to it. It is possible to substitute the drum for a large size garbage can, plastic seems to work best. Fill it with your typical garden hose and you should be fine. Alternatively, set up your test rig on the side of an INGROUND pool. Safety warning, please make sure to keep anything clear from the propeller, as it will hurt.

Notice I also said inground pool, above ground types don't have that much lateral strength.

I'm not sure how you would measure RPM below water, perhaps optically, using a video camera. I would try to paint one of the blades white and count how many times the white blade appears over a given period of time.

Although you have not stated your end goal with rpm measurements, I suspect you intend to determine the rpm of the propeller in order to determine your boat's speed. Please remember that this is NOT as straightforward as it seems. Propellers (screws) don't behave exactly like wheels. You cannot determine vehicle speed as a function of rpm quite so simply. There are fairly complicated fluid dynamics at play. Boat shape, displacement and environmental factors (waves, current, wind, etc.) also play a role.

Having said that, it may turn out to be a worthwhile approximation for speed to measure rpm. I'm not trying to discourage, just inform!

Offline amarquez87Topic starter

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Re: Testing trolling motors help
« Reply #5 on: March 09, 2011, 03:03:38 PM »
Thanks, and you were right I am trying to find speed using rpm and i will take into account any other factors

Offline raptorwes

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Re: Testing trolling motors help
« Reply #6 on: March 09, 2011, 03:49:36 PM »
Aberg089 is correct. Measuring speed by propeller rpm alone would be very difficult . You would have to take into account  propeller slippage ( its not 100% efficient), propeller cupping, pitch, and blade size. All of that would still only give you the speed the motor alone would go in the water, you also have hull design ,hull drag, vehicle weight and water conditions to add into the equation.
« Last Edit: March 09, 2011, 06:44:43 PM by raptorwes »

 


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