1,000 more rpm's why?

We have a 2006 21' sweetwater with a 60hp Yamaha 4 stroke and it swings a 14x11 prop. We have had the boat about 7 weeks and it has sat for awhile before we bought it, I don't know for how long.
It runs at 5600 rpm's and 21mph per my gps and handles great. It had a half tank of gas in it when we bought it and I have filled it twice.
Today coming back in from an hour ride the tach is now hitting 6600 rpm's and my speed has increased, I didn't bring my gps but it's obvious faster.
Does anyone know what could cause this increase in 1000 rpm's?
If it stays there I will have to change the prop in order to bring down the rpm's which may not be a bad idea as I might get more speed than before.
What are yout thoughts.

Re: 1,000 more rpm's why?

Well this is a switch . . . Good problem to have. Same weight? Passengers? Stuff? Beer? I am kinda thinking this belongs in the Yamaha OB section as it does not seem to be 'toon specific. Thoughts?

Re: 1,000 more rpm's why?

Maybe Yamaha has the rpm's limited during a break-in period of so many hours? and better check the rpm range for that critter from the manual!

Re: 1,000 more rpm's why?

book says 5600

Re: 1,000 more rpm's why?

I have an older Sweetwater and sometimes the tach will read high. I tap on the front of the tach and it corrects its self. I am not sure what the cause is, loose wire or something?? If it is false reading, speed would not change. I also might guess cavitation/ventilation causing increase in rpm, but speed should drop, I would think. You might want to double check the speed. I changed my motor mount position, lost rpms but gained speed, reduced cavitation?? I think??
Just throwing out a few ideas, as I am no expert. Let us know what you find as that is how we learn.