With the currents we are talking about in an IGBT, you have to avoid latch up. All the things that I don't know do make this a scary proposition. How well the water will conduct, we need a current limiting device for safety. Might want to do a LOT of research before getting on the water with one of these. Isolate the device entirely from your boat. You dont want current traveling back through your motor into your boats electronics. An SCR also came to mind for this, but you would have to have pulsating DC into its anode in order that it turn off. The main thing is building something that will provide 500 to 600 volts at 4 amps.
Just a lot of random thoughts.
