I have an older dying style inboard mudboat.
Boat originally had an 18hp kohler M18 from the 70's,its a horizontally opposed twin cylinder.Motor mount was at an extreme angle for the carb float.
I currently have a 10hp diesel in its place with a very worn prop.

I bought the orignal 18hp and I want to but the correct propeller for the inboard set up.
I have a 9x6 but I think that is far a 20hp and up.
Should I be looking for a 8" propeller instead.