Lots of valuable insight here, and all appreciated. My thoughts were if I'm going to have a generator anyway, why not have it large enough to drive the propulsion system too. Especially since I had heard of others (though definately not in the majority) doing something similiar.
Concerns about wet environments for electrical systems, and the need to change the AC oupout of the Genny over to DC hadn't even crossed my mind. That in itself is probably enough to eliminate my changeover.
Even if I could overcome those issues, I have a feeling there are a dozen more that haven't been discussed yet that would in turn lead to a dozen more...
Thanks to all for the input. I haven't necessarily crossed it off my "must have" list - but is is moving further and further down it.
More comments are most welcome!
I know this thread is two years old but I just came across it. As far as I can see it is a very good idea if you are someone who already owns a good generator, such as a diesel 1800 rpm 3-phase one.
The first thing is I hope your generator is 3-phase. Big electric motors especially the cheap used ones found on eBay are 3-phase. 3-phase motors are smaller since they do not need the second coil and capacitor for starting.
You can get a big used motor in 1800 or 3600 RPM and hook it up to where the old engine used to be. Reversing the motor is done by reversing two of the 3 input wires with a switch.
Lowering the speed of the motor will be the hardest thing. You will have to lower the engine RPM on your generator, and you might have to modify the voltage regulator on the alternator. To run at half speed you run the generator at 900 RPM instead of 1800 RPM. Instead of 240V @ 60 cycles you'll have 30 cycles at hopefully around 140V (the optimal amount that the voltage should be lowered when the frequency is lowered is determined by the coil resistance over reactance in the motor). Hopefully the regulator in the generator won't regulate the voltage back up near 240V when you're running at half speed. Doing that will cause the motor to overheat, even though it will still be running at half speed.
You can lower the speed of your generator when it is under some load and measure the voltage with a volt meter. If the voltage lowers proportionally to the rpm then this is a piece of cake.
All of the other motors in your boat will slow down too. This won't hurt them though. Lights will get dim. Everything will slow down. The air conditioner won't be as cold.
Or you can get some kind of motor speed controller which runs an AC or DC or a fancy brushless DC motor. That'll be expensive.
I assume you're not interested in using batteries. But if you do go the DC route you could throw in a battery bank of maybe 4 batteries which would let you creep a long for a while in relaxing silence.