Nice to see so many mad scientists jumping into this thread :-)
Ok so I totally understand that the alternator
would consume HP, and that that HP would no longer be available for pushing the boat. This is why I proposed a clutch
or disengagement of the alternator
in the case you wanted all your revs to push the boat.
On my boat I rarely run the 8HP motor
past about 1/3 throttle. She hits hull speed
at just over that and the motor
is more fuel
efficient at lower revs for pushing the boat anyway. If I wind
it to 90% throttle I might get an extra half knot
above hull speed
for double the fuel. In theory, for my specific application I've got about 4 of my 8 horses unused most if not all fo the time (seeing as I hate motoring and would rather sail)
As to the specifics. My 2500W 8HP generator example was just from my experience in using generators. Yes it was 120V not 12V. I was guessing at the amps. But I digress.
While I have seen some people pack both a generator and an outboard
(and, as some have noted, a battery
charger) this is loads of extra equipment
and with every interconnection there is not only loss but another possible point of failure. Of course an outboard isnt designed to run a large alternator either so its not going to be the most efficient design, but you will get considerable loss in running a generator, converting that through a charger, and then charging
a battery with all those steps in the cycle. Most consumer generators also run a 120V block, so transforming that back to 12V through the charger is also a considerable amount of loss and quite wasteful when you could just generate the 12V off the movement of a motor in the first place.
It is interesting that someone found an alternator option on outboards. I'd love to see a link.
In my particular case my 8HP Mariner is about 20 years old. So if I butchered the cowling and made it look a bit like a mad scientist's creation it would probably make the motor look better rather than worse (LOL). I wouldn't propose anyone who is not a serious professional to hack away at a nice newish Yamaha 4-stroke.
In my own situation I would intend to use such a creation in one of the two following uses:
a) When putting in and out of port or on trips too short to hang all the rags (perhaps a 4HP example), or;
b) When realizing that the battery is low and wanting a charge when shore power
is unavailable or at anchor
(perhaps using the full 8HP).
I've already got lines run with a 2A fuse from the motor to the battery; but as I said before the 1.5A trickle that comes out of the motor is insufficient to do much charging
of a 1000A battery. I'd have to replace the lines (previous owner did some awful work in there) of course if I was putting out any serious amps but this must be doable.