Hey Big dipper,
I have ran those yanmar 315's a lot hotter than that, in similar size boats.
You are right tho, something sounds pretty wrong, the new prop's obviously are the first thing to try, but sounds like either a monday morning engine, or badly set up.
just a bit confused, did you re-engine the 7.4 Redbay ?
Even a 9m Redbay cabin will run hotter than that.
If you want to PM me with some more info on the problem, happy to see if i come up with any suggestions. I may have some guys i can ask some advice on it for you if you want. What kinda speed are you getting when you are WOT ?