On Wed, 2009-05-27 at 15:38 -0400, iloveamercedes@
> Hey Dave, I am intrigued at the prospect that a rated 8 hp electric motor
> can have the same pulling power as a 'rated' 50 hp Diesel motor!
> Now I may not understand what "Bollard Pull" is...
The trouble is you are not comparing like for like. 'RAW' HP isn't
telling the whole story, especially when the ICE HP is probably it's
peak, and the Electric motor HP generally continuous. Electric motors
then have a different peak rating based on time, where you may be able
to for example run at 150% rated power for an hour. The big railway
diesel electrics I've been on have their ammeter colour coded by time
giving a 5,10,15,30 and 60 minute motor current ratings.
Just look at those electric drag bikes - they probably push their
motors way over the rating as they only have to be able to do it for a
few seconds.
My own boat (a 'launch', not a sale boat) seems to be running happily
at approx 3.2hp when the old petrol was about 5.5hp. (Didn't have GPS 20
years ago when the boat was last running off it's 2 stoke smoker, so I'm
only guessing at the relative speeds based on the bow wave)
The torque characteristics are totally different. I've come to the
conclusion that on my boat, we had to run the engine at a higher
throttle than was strictly necessary to get the required torque out of
it. (Note I'm still using the prop from the 2 stroke engine).
'Bollard pull' would be one sensible rating, but I imagine that could
only be determined by measurement. (I've come across that term before,
that's how tug boats are rated :-)
You have now got me contemplating what I would need to actually measure
this, some sort of load cell, strong ropes and a good solid bollard to
tied up to.
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