Without a working propeller an outboard motor can't move a boat at all. Important characteristics of a propeller are it's pitch and diameter, the number of blades it has, the number of splines in the hub and the material it is made out of.

The video belows shows how to measure the pitch and diameter of a propeller.

How to measure a propeller's pitch and diameter

Measuring a propeller's pitch and diameter.

Spun propeller bushing

This topic covers the situation where the propeller is not spinning at the same speed as propeller shaft, therefore reducing forward thrust.