Originally Posted by sailingdog
Different materials have different tensile strengths, and as such, may need to be made in different diameters, based on the strength of the material and the strength required for the given purpose.
I'm sure this is true in regards to standing rigging, but prop shaft sizes tend to be based on engine and transmission size and are quite standard, regardless of material used. My experience is that, for instance, a Catalina 27 is always going to have a 1" shaft. It is a rare production boat where shaft sizes vary from one vessel to another. I have it from two separate sources now that the Newport 41 uses a 1 1/8" shaft and I would be very surprised if the Newport 41 I'm going to put a zinc on later this week were different.