Sorry, its QD bushing that goes on to the drive shaft. and bolts to the big pulley.
Check out EVDrives - Golf Cart Parts & EV Conversion Parts
for ideas and the Cruisers Forum thread, Electric Propulsion.
Some people us AC motors but then you need an inverter. So I want to keep it simple.
I think the 7.2 kilowatt motor is 10 hp. Knowing the Palmer power curve, at 10 hp, that will give you the Knots your boat will do, as well as that rpm to be sought for, from the
electric motor down gearing.
In other words you may be trying to gear down to 3 to1 ending up with 1200 rpm on the prop.