Prop diameter vs pitch

I read a post from 5/20/2010 but it doesn't really help because of the type of boat used as an example. I have a 1997 20' Monark pontoon with a 1998 Force 75 HP turning a 12x9 3 blade prop. I bought the boat used so I don't know if this is the original speced prop or not. I am only getting about 15-17 mph at 5100 rpm wot. This is rpm's that the motor specs call for but I feel like the boat should do a little mph better than this. The boat is used in fresh water and trailered between uses so there is no build up on the toons. Is this the right diameter and/or pitch I should be using? Thanks in advance for your comments.

 

If the motor runs at the rated RPMs, it is at its optimum power.

 

Could decreasing the diameter while increasing the pitch push more water and thus increase speed?

 

That will decrease the total blade area and may cause cavitation. You need more power to increase speed.

 

Thanks Gonzo.

 

9" is pretty low pitch, but if it won't pull any more than 5100 rpm, it is not too low for the job. Maybe the motor is tired, as in down on compression. Is that an accurate speed measurement (15-17 mph), it should be going faster at that rpm with that prop, if the gear ratio is the 1.64 to 1, I see quoted elsewhere on the net.