Thanks you have given lots of useful information here.
If it is a copy of Dan's 10ft then cut in at 135rpm seems normal enough. Now at 350rpm you have an emf of 350/135 x 12 = 31v. Now that is 19v above battery volts. For a current of 10A that implies a total circuit resistance of 1.9 ohms which is crazy for a 10ft 12v machine. Have you connected the thing with string or something, the thing needs to be got down to a fraction of an ohm to do any good. No wonder it is going up to 400 rpm there is no load on it. The furling as intended by Dan isn't going to work as the prop is racing away and will be seeking the wind and this changes everything.
I don't have figures for that alternator but running something similar from somewhere else I think you will have a winding resistance of about 0.2 ohms, about the most you can stand in the line connection is about the same again and ideally you ought to be keeping it down to .1 ohm.
What is that ammeter thing you are using, I am wondering how much resistance is in there, you ought to be looking at something to measure 60 amps and that should be using a shunt for such a current.If it is some moving iron current indicator from a cheap battery charger or something then it may have a lot of resistance.
Flux