[MHml] HP to Watts

[Martti Halminen]

Graeme Sutherland wrote:
>     The trouble with going from an electric to a diesel motor is that power 
> is measured at different places on a power curve.
>
>     A diesel engine is rated on its PEAK POWER, which is obvious if you look 
> at the power curve.
>
>     An electric motor will be rated on its DUTY CYCLE, which is the amount 
> of output it can sustain. Sometimes this is based on a one hour run, other 
> times it's more open ended. Electric motors don't have a peak power rating 
> as such, rather it's the point where they burn out due to being fed too much 
> electricity. However, it's peak output is likely to be two or three times 
> its rated output.
>   
Well you'd better use a motor rated for continuous use at full power.
Whatever you can get out of the motor for short term use is meaningless 
in this context.

The propeller doesn't know what is turning it, at the same rpm and speed 
it will need the same power, regardless of the motor type.


>
>     The power output of an electric motor matches propeller curves better 
> than a diesel, and having full torque from 0 RPM also means that larger 
> propellers can be used. So there are some advantages.
>
>   
Way back when I was studying naval architecture, we went through plenty 
of various propulsion system torque characteristics, and the previous 
sentence doesn't really fit with that.

In case you haven't noticed, most of the cargo ships in the world use 
slow-speed diesels directly connected to the propeller shaft. This 
system works quite well, just because the propeller curves are a 
reasonably nice match for a diesel engine. The torque curve for a 
fixed-pitch propeller in normal open-water use is close enough to a 
parabola, so even if you get more torque out of an electrical motor at 
low rpm, that is useless as the propeller doesn't need that.

If you are designing an icebreaker, then a higher low-speed torque is 
quite desirable for situations when your propeller is carving ice, and 
so plenty of those are diesel-electric. For open-water use it is 
insignificant.

Electrical propulsion is no free lunch, you still need the same power to 
achieve the same performance (plus a little extra for losses, you lose 
about 10% efficiency).

- In ship usage the system obviously can't use batteries or stored power 
at any reasonable scale (unless you are building a submarine...), so the 
electrical system is just a transmission system, usually done as a 
frequency-controlled alternating-current system nowadays.

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