How do you compare electric motor power to diesel power?

[Boston]

lmao

that was great Bamby

[Yobarnacle]

thanks Bamby
Thats what I needed

[Submarine Tom]

Thanks Bambi, exactly what I had in mind.

-Tom

[Yobarnacle]

I finally found the pitch for Minn Kota trolling motors OEM props. It's 2 bladed 10.5 inch dia X 4 inch pitch. No wonder I'm getting more prop wash, thrust, and speed from a 5 bladed, 11 inch dia X 10 inch pitch, kort nozzled prop. Now I'm worried I'll burn these motors up!
Also, as I suspected, I discovered indeed Minn Kota motors are brushed Permanent Magnet motors. The controller is digital PWM. That's good.
Next time I am with and can launch boat, I'm going to try feeding 36 volts and maybe 48 volts to these motors. Should lower drawn amps.
Read where someone else ran a Minn Kota 12 volt motor off of 24 volts and no harm, just used less amps and got more RPM. That would be nice. Found a source for buying replacement brushes, new encapsulated magnets in a new shell housing and new rotors. Just in case I fry one or both motors.
Anybody cautioning against trying more volts?

[hoytedow]

Check max rpm ratings. You don't want to have it blow apart.

[Submarine Tom]

You should always use caution when using more volts.

-Tom

[Yellowjacket]

Both of the above are correct. If you use more volts you will have to either spin the motor faster or you will increase the amount of current in the motor. Neither is a good idea. Go too far and you will either spin the guts out of the motor or you will slag down the windings.

Think of the motor as having a back emf (voltage) that is proportional to the speed. If the motor spins faster it takes more input voltage to drive the same current through the motor. Power is equal to voltage time amperage. so if you put more voltage at the same amperage you are getting more power through the motor.

But losses are I squared, so higher current increases the heat in the windings and that can slag down the motor.

If you keep the same prop and drive the boat with the sail, then up the voltage the motor is going to spin faster but, since you are not asking for as much power it won't require as much current and you won't over heat the windings. Keep an eye on your current draw and if it isn't high you may get away with it. Since you are primarily overdriving the motor speed wise the limt comes when the guts get spun out of the motor.

[Yobarnacle]

thankyou one and all. I was thinking getting the same RPM, still only adding about 2 kts to my sail speed by motorsailing. As I've stated before, the intent is to have prop power around docks and other boats, but not suffer heavy drag penalties underway sailing. Even the simplest 2 blade feathering prop is abought $2 grand. Unless I use folding model airplane props. I considered this. Still the Kort nozzles and motor housings cause significant drag.
So, using more volts for same RPM, would reduce amps and heat? Thus motors last longer spinning these relatively large props (compared to OEM)? I hope?

[hoytedow]

It should be an interesting experiment.

[Yobarnacle]

I might invite you to sail with me on that experiment, Hoyt.
Someone to play devil's advocate and advise caution, would be helpfull considering my natural exuberance. And someone to call "stroke" if I have to row back!