How do you compare electric motor power to diesel power?

lmao

that was great Bamby

 

thanks Bamby
Thats what I needed

 

Thanks Bambi, exactly what I had in mind.

-Tom

 

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?

 

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

 

You should always use caution when using more volts.

-Tom

 

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.

 

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?

 

It should be an interesting experiment.

 

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!

 

There is no replacement for horse power. You need a 35hp motor. Don't let anyone tell you otherwise.

Here is a conversion I did on an 2016 Bayliner 175. I replaced the 135hp gas with 147hp electric.

https://electrek.co/2016/08/11/enthusiast-builds-all-electric-speed-boat-tesla-battery-modules/
https://www.youtube.com/watch?v=my0TX3INjSk
https://www.youtube.com/watch?v=BRqcp4Tzzrg
https://www.youtube.com/watch?v=ZcIyKLs5VW0

 

I am going to sound arrogant. Where you are all going wrong. Motors and engines provide power and torque. Propellers use that to do work. What does that mean.
A Lister HRW6 engine provides 50HP and 219 lb.ft of torque at 1200rpm. The propeller it is spinning at 400rpm (my yacht) at that speed needs much less power and torque to do it's work.
Same engine at 2100rpm has 210 lb.ft of torque but the propeller is doing much more work at 700rpm.
Diesel engines have governors which control their output.
This is why a 30HP electric motor with 110lb.ft of torque will spin the same propeller at 400 rpm. The propeller determines the amount of work.

 

That is not correct. If you mean the propeller is more efficient, that depends on many factors. It may be better for your installation. However, work is measured in power/time. If you put less torque there is less power output therefore there will be less work. You are mixing units.

Power is Torque (force) x velocity. If you increase the velocity, the power output increases. Whether the propeller applies the extra power depends on the installation. A propeller with insufficient blade area will cavitate and apply less power.

That is totally wrong. 30HP at the shaft will never be the same power as 50HP at the shaft. Where the power comes from is irrelevant.