[MHml] HP to Watts

[Paul Nudd]

Horsepower is the traditional measure of mechanical power. Power (hp) = torque x speed (rpm in the case of engines) or force x linear speed. Recently it has become fashionable to express mechanical power as watts or more often kW.
Power (watts) = torque x speed (rpm), hence the formula 1 hp = 746 watts.

Watts is the tradional measure of electrical power. Power (watts) = EMF (volts) x Current (amperes, amps).
This does not necessarily relate to mechanical power, for example at zero rpm an electric motor can be supplied with electricity at say 12 volts and be drawing say 1000 amps. This means 12,000 watts or 12 kW, but zero hp because zero rpm. It might be producing huge amounts of torque - no, it can't because torque = force x distance (radius) and force = mass x acceleration. No change in rpm means no acceleration, thus no force, thus no torque.

It is possible to increase the power (hp or watts) of an internal combustion engine simply by raising the rpm at a higher rate than the torque is dropping but is this extra power of any use? In a racing car yes, in a cruising yacht no.

Where does all this get us? Not far.

However we can say:
1. Horsepower is a pretty useless way of 'rating' an engine or an electric motor.
2. Watts is also pretty useless.
3. Torque is much more usefull and it is also essential to know the speed (rpm) at which the torque is produced. That is we need to see the torque curve.

Now the torque curves of internal combustion engines and electric motors will be radically different.
Electric motors produce their max torque at zero rpm (OK technically when they are just turning,>1 rpm).
It's all downhill from there. A modern diesel as we might use in our cruising catamaran (oblig' multi content) might produce max' torque at 1000 rpm. So we would want to run the engine at 1000 rpm and would select a propellor to suit and/or gear the shaft to give best prop speed at an engine speed of 1000 rpm.

Electric motors are completely diferent. We can't run an electric motor at close to zero rpm because it will overheat. So we need to know the ideal running speed of the electric motor and the torque produced at that spped.

There is no simple way to 'convert'. We may have a '20 kW' Diesel, that doesn't mean we can replace it with a '20 kW' electric motor.

To Dan's original questions:
"A simple formula of 1 hp required = 746 Watts looks simple but do we really need that many watts to get the same effective power from an electric motor vs an outboard?"

No, we don't, but we can't do any sort of simple conversion.
Say we just wanted to replace the outboard's power head with an electric motor, assuming we are happy with the current propellor and performance, we need to find an electric motor which will run all day at the rpm we have been using on the current power head and will produce the same torque at that speed. This will almost certainly be a lower 'rated' motor. Perhaps a '10 kW' electric motor replacing a '20 kW' power head.

But - we can probably do better by using the superior characteristics of the electric motor to turn a bigger propellor at lower speed for more thrust. An appropriate setup may let us replace a '20 kW' powerhead and propellor 'A' with a '5kW' electric motor and propellor 'B'. (just guessing with the numbers).

"I know Johnson/Evenrude claim's they are measuring HP at the prop but what does that mean in terms of what we would need from an eletric motor to do the same thing?"

Absolutely nothing!

"Should I be looking at units of thrust from the prop or ???  I don't know if the thrust is commonly quoted???"

No, I don't think it is ever quoted, basically it all comes down to trial and error.

"An eletric motor has better torque characteristics if I understand what's going on under water."

Absolutely. More torque at lower speed. It should certainly be possible to replace an internal combustion engine with a significantly 'lower rated' electric motor, even with the same prop and gearing. furthermore it should be possible to significantly further reduce the 'rating' by using a better suited propellor size and speed (bigger and slower). It is all down to trialand error so you would probably end up with an electric motor more powerful than you really need but much less powerful than you might think you need by taking it simplistically.

I have heard of an electric motor conversion for the Yamaha 9.9. Perhaps I heard of it on this list?

Paul Nudd
----------------------------------------
> Date: Fri, 14 Dec 2007 16:00:38 -0800
> From: Dan.Frenette at Sun.COM
> To: multihulls at steamradio.com
> Subject: [MHml] HP to Watts
> 
> A simple formula of 1 hp required = 746 Watts looks simple but do we 
> really need that many watts to get the same effective power from an 
> electric motor vs an outboard?
> 
> I know Johnson/Evenrude claim's they are measuring HP at the prop but 
> what does that mean in terms of what we would need from an eletric motor 
> to do the same thing?
> 
> Should I be looking at units of thrust from the prop or ???  I don't 
> know if the thrust is commonly quoted??? An eletric motor has better 
> torque characteristics if I understand what's going on under water.
> 
> Dan Frenette
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