I have a C&C 33 with an Atomic 4... Prop Shaft 1" OD married to 16 x 12 Martec folding prop.
Cruise all day @ 1800 rpm...for 5 knots, with my engine temperature never exceeding 150 F
Fuel burn 1 litre / hr.
No issues revving my engine up to 2200 rpm to compensate for stronger tidal currents...
If you are slipping out of forward gear it is most likely that your forward pressure plate is in need of adjustment. I would try that before changing out your prop.
Good luck..
Cheers,
Mel