How do you compare electric motor power to diesel power?

At any given operating point:
Power and Torque are not independent. At a given operating point:l
Power = Torque x Rotational Speed (RPM)
Torque = Power / Rotational Speed (RPM)
These are fundamentals whether it's a diesel engine, an electric motor, a steam engine, or a person pedeling.

Maximum power/torque of a motors or engine varies with speed. Also the power/torque level that an engine can be run at continously is frequently less than the maxiumum it can deliver for a short time. And rating methodologies differ.

 

I agree as far as your explanation goes. And it completely covers combustion engines.
It falls short in covering steam and electric which do something combustion engines don't.
Both electric and steam can apply torque when stalled at 0 RPM. Or if you don't call that torque, what would you call the rotation force they exert on the imovable load that stalled them?
In this case, definition by formulae "Power=torque times RPM" is useles, but power/force/stress is being applied and may eventually cause the load to begin to rotate.

 

The formulas still hold at 0 RPM. Torque can be non-zero at 0 RPM. No contradiction of anything. Power is 0 at 0 RPM irregardless of torque.

Power at 0 RPM = Torque at 0 RPM x 0 RPM = 0

 

Note that the forumulas I gave are for power at the output shaft, not input power.

 

Okay. Power is usable work done. No movement, no use.
If you beat on a hammer wrench on a large nut to break it loose, And after several blows, it finally turns a fraction and then a wrench will turn it off, was no work done till the last blow?
A sail vessel underway with a sailing brake locking the prop from turning, is there no power in the water flowing over the prop?

 

ah! imput power vs output power.

 

And torque=power divided by RPM yet you can have positive torque with zero power and zero RPM? With steam and electric and a combustion engine driving a torque converter.

 

Thankyou Mr Cockey
Actually I was just looking for a "rule of thumb" as to how big an electric motor should be, to replace a propulsion diesel of any given size.
Everybody seems to agree that for less than planing speeds, a smaller hp rated electric motor will suffice, producing the required torque.
Also seems agreed, no usable rule of thumb exists. Calculations are in order per each application.
The use of a smaller electric motor to accomplish the same work of a higher hp rated diesel doesn't translate into electric hp is greater than diesel hp.

Thanks everyone

 

The obvious question without an obvious answer.

At zero speed power is zero by definition. The equation for torque then gives
Torque = 0/0
So what is the value of 0/0? A mathematician's answer would be "indeterminate" which is another way of saying "more information is needed". In this case it would be what ever the torque actually is.

Another way to look at is to apply a load so that the output shaft moves very slowly. The formula holds with both speed and power non-zero. Increase the load so the speed drops. The forumula still holds. Plot the results for various slow speed and then extrapolate to zero speed. The result will be very close to what the measured torque would be at zero speed (assuming the non-zero speeds are sufficient slow).

 

Thankyou sir

 

I agree that that's the conventional wisdom. But if the propeller is kept the same then the shaft torque and speed for a given boat speed stays the same. Which means the power/torque/speed from the motor or engine is the same. Now if the shaft speed can be reduced when an electric motor is used then it might be possible to replace the prop with one with higher pitch which might improve propeller efficiency and require less power. And if the propeller diameter can be also be increased then that might improve propeller efficiency and allow a smaller motor. The same can be accomplished with a diesel motor if a higher ratio reduction gear can be used.

Or perhaps folks are more willing to run an electric motor at a higher fraction of it's rated power than a diesel engine. If so that would allow a smaller electric motor.

 

I think the immediate torque of the electric motor allows it to swing a big prop that a diesel of the same power would stall on. Thus requiring a larger diesel to handle start up. Usually not a concern, because everybody wants plenty of power top end anyway to get on plane.
Turning this statement around, a smaller electric can swing the prop of a larger diesel. But can't match top end speeds.

 

Let me recount what I have done.
I have an Albin 25 motorsailer I lengthened to 30 ft. 8.5 ft beam. Weighs 4500 lbs
Originall 27 hp diesel powered throwing 16X14 prop.
When I bought her, she had 13 inch prop, pitch? and no diesel, but there had been a 15hp yanmar. Which gave her 5.5 knots the previous owners claim.
British Seagull Silver Century was rated at 5 hp +/-.
I have 4 11 inch dia, compound pitch (average 10 inch pitch), 5 bladed Hydrofan props from Silver Century Seagulls.
I purchased a Minn Kota EM (engine mount) 160 Riptide trolling motor.
That is two 80 lb thrust motors side x side. They came with 2 bladed platic prop, 10.5 inch diameter. Pitch unknown, and they're "weedless" so hard to determine pitch with the blade angled here and there.
Not important, because I installed the 11 inch Hydrofans on the Minn Kota.
I also installed 13 inch dia Kort nozzle style prop guards.

I don't know the HP rating of these 80 lb thrust motors, but the 101 lb thrust Minn Kota I found here:

Minn Kota makes the RipTide with 101 lbs thrust, on 36 volts, that draws 46 amps at full power. The RipTide is a salt water version. Minn Kota rates the motor at 1.79HP

Mine are 24 volt, and accept 5 to 117 amps.
The 13 inch prop that was driven by 15 hp Yanmar was removed.
I have two 11 inch 5 blade props from 5 hp gas outboards, on electric motors that are less than 1.75 hp rating each.

I can get 6.5 knots under power alone with this rig at 60 amps 24 volts.

WRONG!
I got 5 kts on power alone,
I got the 6.5 knots motorsailing at 30 amps.
Checked my recorded data and discovered I posted an error.
Sorry

 

Sounds like a nice setup. What are you using for batteries and how do you charge the batteries? Any pictures of your boat and motors, etc.?

 

Check out my thread Alternative Motorsailer Rig.
I only had a few photos postable. Most photos I took on my cell phone. Can't upload to computer. There are a couple concept drawings there and some photos

I have 10 old 12 volt car batteries. I charged them using car battery chargers. They were for testing the system. I'll get better batteries when I get more money