|Choosing a prop based solely on the distance between the end of the shaft and the surface of the hull won't give you enough data to make a good choice. There are too many other variables to consider - that's why we have the props page, so people can list their choices and their degree of satisfaction with it. If you're buying new, let me say I've had good luck with manufacturer's suggestions.
I will try to get a keyway dimension next week.
C&C 27 Mk IV