Steve, I think the definitive answer FOR YOU about whether a prop will slow a boat down, is going to depend on the prop (2 or 3 blade
? etc.) and the aperature.
In rough rough numbers, you can only pull so many horsepower from the prop shaft before you stall it out and the generator becomes a friction brake. Since I'm not an engineer
I"m guessing here...but let's say you have a nominal 18hp engine and a maximum speed under power of 6 knots.
To me that indicates you can pull about 18hp off the shaft before you lock it up, probably well less. let's say, maybe 1/3 of that (6hp) would take 2 knots off your speed. One hp is about 750 watts, so at 1500 watts (14.4v @ 100 amps) you'd lose two knots.
I think in reality you might turn a 40-50A genset while losing those two watts although I wouldn't be surprised to be off by 2x either way.<G>
Ideally you would want a 2- or 3- pulley wheel
on the shaft, s the alternator
could sit to one side, and an idler wheel
could sit on the other side to counteract the side load on the prop shaft. Having that pulley machined up in a machine shop, so you could clamp it down over the shaft (and maybe have two lock screws going into the shaft from opposite sides as well) is probably a $150-250 machine shop job.
Size of the pulley...dunno, you'll have a limited space to work in, but you'll need to check the shaft speed when you are sailing, and then ratio that to the shaft speed your alternator
needs, and compromise on the size of both pulleys to get the alternator turning at the speeds IT needs. Most of them put out next to nothing under 1000 RPM
, so typically you'd want a really small alternator pulley and a large prop shaft pulley.
If you run those numbers--for the pulley ratios--and check the space you have available around your prop shaft, you may find a conventional alternator just can't be "geared up" enough to work in this application.
Anyone selling kits on the internet
Or, how about just towing one of the purpose-built water generators that are sold for sailboats? I'm afraid to ask if you could ust stick a larger prop on a MinnKota (electric trolling motor) and clamp it over the transom. AFAIK those are 12VDC motors that shoudl work as gensets, but I don't think they'd live long or prosper in this application.<G>