Shaft Material Selection.

Tobin Bronze has become an unfashionable material for propshafts lately, and preference given to "stainless" steels. This is unfortunate, since these steels are far more brittle and prone to shear, though these properties are useful in long shafts driven by powerful motors. However a stainless shaft carrying a bronze prop is a source of galvanic corrosion. NEVER under-specify propshaft or thrust bearing equipment. At best you may shear the shaft and lose all power, at worst you have a hole below waterline of propshaft diameter.

As you can see, by selecting Tobin Bronze for shaft material I have similar yield to stainless and 50% greater shaft weight, but vastly greater elasticity than any other material. While this does mean the shaft will bend easier (it will still not bend under normal service conditions) if the prop strikes bottom or wraps in some very strong cord, it is far less likely to shear. Even a badly bent shaft will give useful emergency thrust. A sheared shaft will give none, and may also foul the rudder and leave a hole below waterline. A bronze shaft can also be repaired.

If you refuse to use a Bronze shaft, run away from Stainless no matter what the price or convenience and go for an Aquamet shaft. They are no more brittle than stainless and 3 to 4 times stronger for not much extra money.