OK, In summary, I found the prop that came with my engine is either too low a pitch or diameter.
I had been looking for a Merc one at 14", but having had way too many ventilating moments at Easdale where I came to a grinding halt & had to come back to idle & wait a copuple of seconds to get going again, I decided that instead of up -pitching, I would up -diameter instead.
Cutting long stories short, it would appear the Merc props with the "stud hubs" only come in one diameter (well, in the 13 spline version, not sure about the bigger ones) Now, I have what I think might be a suitable Yam prop lying in my garage, (Certainly WRT pitch & diameter), BUT of course it relies on a split pin & castelated nut to stop the nut unwinding, which of course the Merc shaft isn't drilled for as it uses a tab washer.
Option 1 - Drill & tap a couple of holes & put studs on the Yam hub.
Option 2 - Use a "Nord-Lock" washer.
Nord-Lock wedge-locking washers
Anyone used one on a prop? If so how did it fare?