YSx8 propeller size
 

Hi,
I would like to ear from whom have the YSx8 engine with 2:1
gear ratio,what size of prop are currently using.
I have 2 blade 12 1/4 x 8,sailboat,6200 lb,LGW 24',it seem to me that
I am overpitched...(black water after approx 2400 RPM) but it could be
caused by a very tired engine too.
In very calm condition I reach 5 Kn at "black water point" that occour
at 2/3 of trottle.(prop and bottom are clean)
How can I distinguish between the two things?
How much should be approx the prop slippage in calm for a sailboat?
This a new "used" boat and I cannot ask at the previous owner anymore
and the YSx8 powered sailboats are too rare here in Italy. :-))
Thanks and sorry for the not perfect English
Marco,Italy