The term 'alternator' is a little misleading when it comes to the charging system of small outboard motors. "Alternator" conjures an image of the devices one finds under the hood of automobiles and that's exactly what they fit under the cowl of the larger outboards. On the smaller < 10Hp motors its just a magneto type charge coil that provides the electical power. This does NOT have a field exitation like a true alternator, the magnet in the flywheel is all that's needed to generate the limited power available. It is AC current and has to be rectified, the rectifier is usually bolted under the cowling somewhere.
You need a rectifier if you want to charge your batteries, if you just want to power the nav lights as in a small runabout, there is no battery and hence, no rectifier required ( the light's don't care if they're fed AC or DC).
These magneto type charging coils are VERY depending on RPM for a reasonable charge current. Do not expect to get anything useful by way of charging at below half throttle setting. Borrow an Ampmeter and see just how much you get - its enlightening.
You *may* be able to overcharge the battery if you have a 10 Amp rated output and you motor for a long time. In such a situation it would require a regulator fitted ( a simple solar regulator would do). Most trailer sailors only motor for a short time anyway, if you rely entirely on a small outboard for charging your sailboats battery you'll ought to consider alternative battery charging as well.
If your motor has an electric start then it's usually fitted with regulated high output charging.
The diodes in the magneto type are hopefully rated for the full AC voltage and should be not harmed if the battery is disconnected (they are not on my Mercury 9.9).
NOT SO if you have a true alternator fitted. Disconnecting the alternator output ( while it's running) from battery could produce an excessive voltage and destroy the alternator's internal diodes if the field remains connected to the battery. That is why there are extra field terminals on the better battery bank selector switches.
I hope this adds to the confusion 
Klaus
How to connect outboard alternator to charge battery... -- Doug C. -- 2/11/02 8:26a.m.
I don't think you will need to add a regulator -- Roger, Potter 19, Seattle -- 2/11/02 9:05a.m.
-- Doug C. -- 2/11/02 9:49a.m.
pic) -- Roger, Potter 19, Seattle -- 2/12/02 9:24a.m.
