You say you have about 3 kW available. By the time you go through the
controller, motor, perhaps a gearbox, and chain drive how much power are you
going to be able to deliver to the prop?
And this assumes that you have the prop, it does not have a rope wrapped
around it, the stern gland hasn't seized, the shaft hasn't broken ... etc
etc. If what you are after is an alternative drive, I'd make it totally
independent of the existing system. What about borrowing the largest
trolling type motor you can possible power, and seeing how that goes.

David

"BOEING377" wrote in message
...
Is this practical?

I have a 35 ft boat with a very old 671 diesel with a 1.5 to 1 reduction
gear.
I am installing a 6 HP Petter diesel aux which will probably give me about
3 KW
of electric power when coupled to a generator. I have a really nice 5 HP
24 VDC
PM continuous duty motor and a DC motor controller that can handle 375
AMPS.
When my main engine gearbox is in neutral, it is fairly easy to turn the
shaft,
not a lot of friction. If I put the boat in gear and crank my starter
motor
with the fuel shut off to the main engine, I move some water with the
prop, not
a lot, but it is turning slow and doesnt seem to bog down the starter much
motre than if the gearbox were in neutral.

I'd like to just put a flat (no protruding center coupling) chain sprocket
between my propshaft coupling faces and have the coupling bolts go through
holes in the sprocket to affix the sprocket firmly between the flanges
when
they are recoupled. That should only move my prop back an distance
equivalent
to the thickness of the sprocket. I propose to have the DC motor set up to
drive the propshaft through roller chain which would only be connected in
case
of a main engine failure. I'll figure out the right ratios between the
motor
sprocket and propshaft sprocket to avoid overloading the DC motor. If my
main
engine is diabled, do you think this set up would get me home using the
Petter
driven gen as a power source? I'll probably use a 110 AC gen and convert
it to
24 DC with a power supply. Even one or two knots might help in a jam.

Your thoughts? Any good web info on this type of setup?