Prop or Tach Question

[caddesigner]

I have a question on prop selection or possibly Tach malfunction. I have an 20 ft pontoon boat with a 1993 Yamaha C30ELRR outboard. My tachometer reads 7000 rpm at WOT and my GPS speed is approx 15 mph at WOT. The prop that I have is a 9 7/8 x 13 F. This would seem I need a larger diameter or higher pitch prop. Looking at what is available for this motor I see there is no easy way that I could get there. Now I am now wondering if my Tach is registering correctly. Anybody have any suggestions.

[boats.net]

Sounds like a tach problem, check to make sure tach is set for 6 pole aternator setting. May need to try a know good tach to check reading.

[caddesigner]

Thanks

Thanks for the reply. I will see what I can find.