To get 60hz (or 50hz for that matter) AC
power, you need some means to control the frequency. I'm aware of three ways this can be done on a
boat:
1) Control frequency by running the driving engine at a fixed, controlled speed. This fixes the rotational speed of the generator and in turn fixes the AC frequency. This is the method used by your typical engine driven stand alone generator. Doing this in conjunction with a
propulsion engine is problematic because, with few exceptions, the engine's speed is varied to control the boat's movement.
2) Generate DC
power, then invert it to create stable 60hz AC. Any inverter-equipped
boat already does exactly this. The engine's
alternator charges the
battery and the
inverter draws off DC to make AC. If all you want to do is power an
inverter, these can be made more efficient by raising the DC voltage, but that's a whole other topic.
3) Make a hydraulic AC generator. The engine drives a hydraulic
pump at varying speeds, then a hydraulic
motor drives the generator at a steady speed/frequency. At lower RPMs the engine will
pump less hydraulic fluid, and might require lighter loading on the generator, but systems like this are commonly used on larger boats.