Shaft issues - connecting gearbox to shaft

hey guys - simple question here. I’m just trying to connect my gearbox to the primary motor shaft, following the instructions on https://www.instructables.com/Building-an-ELECTRICAL-Hydrofoil-Part-12/

I know i need a shaft collar, but I have two issues:
1 - the gear box is 13 mm, and the propeller shaft calls for 10 mm
2 - what do I do about this protrusion on the gearbox? (ignore it?)

thanks!

Hi, it might be best to get rid of the engine and transmission. There’s a fundamental error in the instructions (https://www.instructables.com/Building-an-ELECTRICAL-Hydrofoil-Part-12/). It states that “The gearbox reduces the RPM of the motor by 5:1 thereby increasing the torque and allowing a larger propeller --> higher efficiency.”

While it’s true that the gearbox increases torque, it’s not true that it increases prop efficiency …or whatever the idea is. Higher torque will only allow the propeller to generate more thrust (at the expense of speed). But the higher thrust is mainly needed by tug type vessels, not sophisticated e-foil type vessels …or when a tug is towing a broken e-foil to port:-).