One thought.... After 20 years of diesel boating I have had to limp home
3 times...(I have almost always had twins) Two times from prop issues
and once due to a broken shaft... If you are looking at a "get-home"
solution I would lean towards something that is 100% redundant. Even
with Spare shaft and a low pitch wheel, you still have single point of
failure with the strut, etc. If you do decide to go forward with
this I would suggest getting a very low pitch wheel and dumping the
gearbox.



Jim Woodward wrote:
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.