Mark I am only quoting from your article in ADB. summer 2020:
In case of a propeller with slip of 14% (three bladed as in Stephen Pigott proposed notebook) this means it is working at 86 per cent of it maximum efficiency whereas a propeller with a slip pf 11.7 per cent ( four bladed as in Stephen Pigott proposed notebook) is working at 88.3 of its maximum efficiency.
Mike, I did not dispute the figures given.
I can only repeat myself:
It must be remembered that they sought to maximise efficiency and minimise vibration.