First, all power taken from the boat's forward motion subtracts from the power creating this forward motion and potentially reduces that forward motion. However, with a fairly powerful rig or in strong winds, you are very likely overdriving the
boat at times. In other words, at or exceeding the true
hull speed of the boat. In such case, some of the power provided by
wind and sail is being wasted, and you may as well harvest it and put it to good use. When the boat is significantly below
hull speed, it is a zero sum
game and creating more drag on the prop by loading it with an alternator that is placed under an
electrical load will reduce your speed by some small amount.
As others have pointed out, a common alternator does not operate well or even at all below a threshhold speed in
RPM. A large pulley on the prop shaft and a small one on the alternator will give you an
RPM increase over same size pulleys. As Stormalong suggests, a lay shaft might be necessary. Not all transmissions are good to go, with long periods of freewheeling, so check with your
mechanic.
You can also use a PMAC or BLDC (essentially the same thing)
motor and controller such as is commonly used for
electric propulsion, in regen mode. The required pulley size ratio would be very small, probably around 2:1 or at most 3:1 and there is a LOT of wiggle room. By the way, if you switch to a 48V DC system you can also use that
motor for
propulsion, especially at very low speed when you want to just ghost into a harbor or slip without bumping in and out of
gear, or having to deal with a minimum idle speed. Also, power is instantly available with no startup, no warmup, etc and with a decent size bank, you always have enough power for
docking even if your
diesel craps the
bed. 12V calls for too much
current to be practical, and even 24V is pushing it, unless you have a very
small boat. 48V is the standard for EP
boats under about 36'. The PMAC motor and controller will cost multiples what an ordinary alternator will cost but will be more efficient and as I said, also can provide
emergency power to the prop shaft.
A freewheeled prop produces power more efficiently with a high
pitch. A low speed (such as a sailboat) vessel is usually more efficient with a lower
pitch prop when using the prop for propulsion. And so, you have a compromise to make that will certainly affect efficiency either
charging, pushing, or both. You could of course go with a variable pitch prop. $$$$$$$$$.
You can of course reduce the drag of the prop by allowing it to spin more freely, and you do that by reducing the
electrical load on the alternator or brushless motor. For an alternator with a fixed
regulator, simply increase the sense voltage so the
regulator switches field
current "on", less, and "off", more. Brushless motor controllers have a regen power adjust input usually controlled with a potentiometer.
A towed hydrogenerator would definitely be a better choice than coupling to the shaft, unless you want it to push your boat as needed, and likely, you don't. With a towed rig, you deploy it when you have more wind power than your
hull can use, and keep it aboard when you want to squeeze every itsy bitsy fraction of a
knot out of her. You can
DIY your own
drogue generator easily enough if you can't find one that you can live with. Unless you are quite the shade tree
engineer though, I would look for a turnkey system that you just connect, splash, and go.