Lew Hodgett wrote in
ink.net:

You can't get there from here.
To get any decent output, you need at least 2,000 alternator RPM.


Shaft alternators, like the Motorola spinning from Lionheart's shaft, don't
have to spin so fast as engine-driven alternators. At 6 knots, under a 20A
load charging the monsters and running the electronic suite and 12V fridge,
it turns about 140-160 RPM, which is just a guess watching it run nowhere
near as fast as an alternator on an idling engine.

The prop is a 3-bladed spun by a Perkins 4-108 about 50 hp pushing the
heavy 41' ketch. I'm not sure what the pitch on it is, sorry.

The shaft pulley is about 10" in diameter and grooved for one of those
serpentine belts like a car uses, the one with all the little 'v's in it.
A rib up both sides holds it in place on the big pulley. The pulley on the
alternator is probably 2" in diameter, something like that.

The voltage regulator is built into the alternator. All you do to energize
it is switch on the 12V power to the field windings (regulator) and it
pumps its output through a 30A simple ammeter back on the switchpanel.

To keep you from running it while the engine is turning the shaft, which it
says in the manual will burn it out from overspin, the engine key is on the
same welded keychain as the alternator switch key. You have to switch it
off to extract the keys so you can start the Perkins.

The only noise you hear is the shaft rattling around in its cutlass
bearing. The transmission is a "hydraulic", made for this operation to
allow the shaft to freewheel without wearing out he transmission. It's
different from the transmission delivered on the Perkins for other boats.

Once running, it tends itself and if the load drops you can hear the shaft
spin at higher speeds. As the prop stalls from the load, the alternator
output drops unloading it so it can continue to put out whatever it is
capable of.

Now, what I'd, personally, like to see is for someone to homebrew a
DIRECTLY-powered FLAT alternator like he homebrew wind power boys are
using. Read about them and their HIGH OUTPUT, VERY SLOW TURNING
alternators made out of old disk brake parts on:
http://www.otherpower.com/
Look at the sheer simplicity and amazing output from a wind machine made
from a Volvo 140 steering arm and 11" disk brake with permanent magnets
glued to it and a home-made set of 3-phase coils those magnets only need to
be "near", but not too near.....
http://www.otherpower.com/windfarm1.html

There has gotta be a way to slip that disk brake full of magnets onto a
sailboat shaft with its stator coils just closed enough to maximize output
current without stalling the prop....same as they do in the wind
machine....

Or....build a sailboat wind machine out of their designs for $ALMOST
NOTHING!