I'm intrigued by this change-over switch. Is that a safety requirement in the marine world?
Well, my responses have been wide, in scope, because we still have not been informed about the boat, etc. As for being a requirement, it depends on the boat... But, it is always a safety concern. It may also be required by the boat insurance underwriter; it usually is, in the fine print...
Depending on the parts of the system (engine type, battery type, charger type, etc.,) the switch may be an operational requirement. For example, having the charging system tied to the starter
of some types of outboards engines, at the moment you start the engine, can fry the outboard engine electrical components. We still need information about the boat, the engine, the intended sailing region, etc.
Have we been told, exactly, what the generator powering method will be? Wind, or hydro?
JuculianD
A major consideration, in choosing the generator, is the amount of torque that is needed, to turn the generator, throughout all the expected RPM ranges, of use, because, the torque needed changes. For example, I looked at building a floating generator, to ride in a small stream. It was going to be difficult, to turn the generator (of the size needed for the target voltage/amperage) with the water current that was available.
Take any spare motor, of the same type, as you intend to use, and try spinning it, by hand. It will be much more difficult, than you would think. If it is making any significant usable power, it is not going to freewheel.
Then, chuck the shaft, of the motor, into a drill motor. Secure the motor's body. Attach your multimeter leads. Spin the motor, varying the RPM. Observe the power, that you get. You will gain much insight, by this simple experiment.
You may not want to buy anything, until you have a complete design, for the entire system. I think that you have a long way to go... learning the factors... designing what will work, for you... I don't mean to dampen your enthusiasm, just somethings to think about.
Check YouTube and Instructables for project ideas.