A submersible hydrogenerator

[Joseph Antoine]

I am currently under a projet to fabricate a hydrogenerator. I had iniatially thought to use an alternator from a old use car but I heard that it needs a high rpm to generate any useful output. I also heard that the starting torque for a car alternator is very high though I don't really know what's the difference between these two.
So I thought that maybe I can use a wind alternator instead. I did a bit of research and I found these two alternator:
http://www.windbluepower.com/Permanent_Magnet_Alternator_Wind_Blue_Low_Wind_p/dc-540.htm
http://www.windbluepower.com/Permanent_Magnet_Alternator_Wind_Blue_High_Wind_p/dc-520.htm
The submersible hydrogenerator that I plan to fabricate is to be mounted under a boat that sails at 5-7 knots. Does it matter whether I use the one for high wind or low wind?

[Dale Sheler]

Wow, that would be quite a project, the wind blue alternators are basically modified automotive alternators.
At 5-7 knots you would need some kind of fast prop or gear train set up to get close to cut in (charging speed) I've never paid much attention to them but I think the wind blue alternators need several hundred rpm to get to cut in.
Why not use the alternator top side in the wind?

[Joseph Antoine]

Do you know where can I buy a small alternator? On my boat I have a 100Ah battery. It will be completely after empty about 25 hours. Which means that I use about 4A continuosly. And I need an alternator that can produce around 4amps. Am I correct in my calculation?

[Dave Turpin]

4A at 12V? We're only talking 50 Watt. You could do that with a single solar panel.

[Peter Mckinlay]

The smaller the turbine diameter the greater the revolutions needed. The larger the turbine diameter the less revolutions needed. :)