prop ft50c

Problem is that the 13 1/2 x 16 prop is not giving very clear water out the stern i am maxing out at 4800rpm with 2 on board at 20knts on a 19ft semi planing hull 1500lbs,the cavitational plate is 1/2 inch below the boat,should i drop a pitch size?,raise the outboard?
Cheers from the outer hebrides

[boats.net]

The rpm range for that engine is 5000-6000 rpm's at wide open throttle. Try a 12 pitch prop, it should be between 5500- 6000.

Cheers for that,i tryed a 14x11" and it took the w.o.t up to 5100 i am wondering what i can take the diameter down too?
I seen a prop for sale but the guy was asking if my outboards propshaft has 15 splines and 4 1/4" gearcase but i am not sure?
Thanks and regards in advance i am sorry for being a pain.

Anybody got any idias?

[boats.net]

I haven't ever counted the splines but he if doesn't know if it is for a T50 Yamaha then probably shouldn't buy it.

thamks for that i will give it a miss.