Thanks once more for the new replies.
This is getting closer. It is always good to get replies from people who have prop shaft generators or alternators installed . A bit more info on my setup....
My propshaft turns at about 300rpm at 5 kts. I have geared up the ratio to 5.5:1 which gives me 1500rpm. Minus the slowing down effect of my alternator,belt pulley,amps etc = 1000rpm. With this setup and speed my 24v propshaft alternator takes out 3 amps and puts nothing in. It starts to work at 6 kts. Taking out 3 amps (to run the field coils) is a major problem as anyone with a propshaft alternator will know, it means that it costs you amps if you switch it on in marginal conditions.
This is why I would like to try a PMM.
As a person with absolutely no experience of permanent magnet generators on sailing boat propshafts, I would say the following permanent magnet motor specification is required to charge a 24v battery
Produces at least 27 volts at 1000 rpm
Produces around 5 amps at 1000 rpm (then it would work on the calm days)
Produces at least 20 amps at 2000 rpm (then it would be delivering useful amps on the windier days)
Has a maximum rev limit of around 8000 rpm (not sure about this but if it was then it could probably remain connected when the motor was on, otherwise it might need some sort of magnetic clutch
disengagement system) This is actually an advantage of my setup, that I can run both alternators at the same time if the house batteries are down and I am motoring.
Could anyone comment on whether my specification is in the ball part and if so, would anyone know of a permanent magnet motor that would be within the required parameters?
It would also be interesting to know how many amps could realistically be expected at different sailing speeds.