60hp 4 stroke won't rev pst 3700

Hello, I just purchased a used 16' slyvan aluminum fishing boat with a 60hp 4 stroke out board yamaha with a 10.25 x 14 propeller. With the throttle wide open it only gets up to 3700 rpm. The engine starts good, sounds good & gets up to 3700 pretty quick but won't really plane out & can't get past 3700. Any thoughts?

[boats.net]

Are you sure tach is reading correctly, should be set for 12 pole alternator.

update

Yup the tach is set & working fine. We took the boat out yesterday with only 3 of us (usually two adults & three kids). Same thing couldn't get over 3700rpm but I had the other two go in the front of the boat & after 2 minutes it started revving higher & actually got on a plane and top rpm was 4400 (motor specs say wot should be between 5000 & 6000). I did this a couple of times to verify but it seems by putting more weight up front made it plane. I am guessing that I need to get a lower pitch propeller to help it plane faster? Or could it be the motor is mounted at the wrong height (it currently is mounted as low as possible). Again any help would be appreciated as I am new to owning a boat but already loving it!

Thanks in advance.

[boats.net]

Sounds like you need to try a 10 pitch prop.