"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?


Probably more practical than a setup that I used on a 50' workboat that had
a 671 in it. I used a Briggs & Stratton 2.5 HP gas engine as an auxiliary
generator to top off the 24 VDC batteries when the main engine wasn't being
used for a few days (bilge pumps were pretty active on that boat!). I
mounted this auxiliary high above the propshaft, on a couple of heavy 2x12
planks. I also rigged an auxiliary bilge pump at one side of the B&S engine,
with the 24VDC generator on the other side. I could run either by slipping
the V-belt over the appropriate pulleys.

I then noticed that the auxiliary engine shaft was roughly parallel to the
prop shaft, so rigged a split pulley around the prop shaft and got another
real long V-belt so that I could drive the prop shaft from the B&S engine.
This wasn't too good, since the pulley ratio was about 3:1. The B&S engine
needed to turn maybe 3000 RPM to yield full HP, but the low pulley ratio
bogged down the engine quite a bit. The B&S engine might have died a quick
death if I asked it to work full-load for a day or two. But still, I could
motor around at a couple of knots in protected waters.

I never had to use this emergency system, as my 671 was very reliable. Just
as well, because, in retrospect, it was a pretty dumb contraption. Your
concept is OK, and it will likely work. How long, in an emergency, is
another question. Cooling, of all segments, is critical. Bearing alignments,
lubrication and vibration control too. I think I'd overhaul the 671 first.

Ed