We're sort of doing this on Fintry.

We have a Perkins 6-354 that was driving a 30 kw 220VDC genset that we're
going to use for hydraulics, primarily for a sixty horse bow thruster, but
also for docking capstans, crane, windlass, etc. The main reason for not
driving the hydraulics off the Cat 3406 main is that while docking, you
probably want the main to be idling most of the time, just when you want to
be taking sixty horse for the thruster.

In the conversation with John Champion at American Bow Thruster, he asked
about get-home.

We've ended up specifying an extra hydraulic motor driving a roller chain
sprocket on the propeller shaft. We've calculated the sprocket sizes so
that the shaft will turn the right rpm with the available horsepower (sixty
less losses). We'll put the chain over the two sprockets only when needed.
For short term use, lubrication is not a real issue -- hitting it with an
oil can every few hours will get us home from anywhere. This may get messy,
but it's an emergency.

So, while your idea should work, and my instinct tells me that 6hp may be
enough, I'm not sure I would go electric. For one thing, a 6hp diesel will
not drive a 5hp electric motor. For another, you'll turn on the motor with
the shaft stopped, so you'll need enough torque to start it turning. That
will require careful design.

If the main is soft mounted, you need to think about the fact that the shaft
will be moving around -- either mount the get home motor on the main engine
or look at how much relative motion the chain drive will accept.

One thing to do carefully. Ask your propeller manufacturer or naval
architect what shaft speed will absorb the horsepower you have available at
the shaft. This has to be done in steps, because then you have figure out
how fast the boat will move at that shaft speed and adjust the number for
the slip at that speed. This is one place where having hydraulic or
variable speed electric drive would help, as you can accelerate the shaft
slowly up to speed as the boat gets moving, taking only as much HP at each
speed as you have.
--


Jim Woodward
www.mvFintry.com


..
"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?