Re: right prop?
The first thing to check is that the tach is set properly. Look at the back of it for a small dial with numbers like 5P, 6P. That dial should be set to 6P. Gently turn that dial back and forth a time or two and then set it to 6P. Sometimes the internal contacts get a bit of corrosion and turning the dial cleans them up. A 13x11P prop is about right for that pontoon. If the tach is accurate, then I suggest your engine is not making full power so make sure it's running up to par before spending money on props. If the engine is performing at full potential, then you would need to drop pitch by 6 inches to pick up 1200 rpm which is not a practical swap so something else is wrong. That engine is lugging at 4500 rpm so it should be closer to 5500. It is either the tach or the engine. A simple check at the engine is to make sure the throttle is opening fully. At full throttle the butterflies in the carburetor throat should be perfectly parallel with the throat.
