Hi Dan. What you are looking for is I think a flex coupling. You also need a thrust bearing mounted on a thrust plate, to take the forward push of the prop shaft and apply it to the hull instead of to the motor. You do not want to apply any axial force to the motor itself unless it is rated for axial load. Additionally you need a motor mounting plate to stabilize the motor case against its torque and maintain motor/shaft alignment. This is for a direct drive setup.
It is more common to use a reduction gear, and this is typically a belt and pulley setup. A thrust bearing is still needed for the shaft but it is not as critical, and a tiny bit of axial play is not a deal breaker. The reduction ratio can be changed by changing pulley sizes. The motor alignment is not as critical and there is no chance of axial loading the motor. You can also use an enclosed gearbox such as the one I use right now, made by Baldor and sold by http://www.electricmotorsport.com/ and probably other vendors in the market. DISCLAIMER... my recent experiments with direct drive raised questions about the desireability of a reduction gear in my own application, contrary to what is predicted by modeling. YMMV. However, this unit has standard C face mounting input and output, and bolts directly to the motor, and of course has an integral thrust bearing, simplifying mounting greatly.
Here in the US we have a vendor called Glen-L that sells a lot of coupling, bearing, and shafting solutions for small homebuilt boats. There are industrial suppliers like McMaster Carr and Grainger, too. There are prefab reduction gears available from electric boat suppliers that are sensible solutions for low power applications. You can use an ordinary marine transmission if the ratio is in the ballpark for your prop/hull/motor/voltage combination.
The bad news is that just having a boat with a shaft and prop, and a motor, leaves you with a lot of component sourcing and engineering still to do. The good news is your task will become a hobby for the next year or two and keep you occupied.
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