I have a 2000 115 FICHT on a 1984 24' Harris Flote Bote. It currently has a 13.25" x 17" 3 blade cupped Aluminum propeller. It goes about 22 MPH at WOT of 5200 to 5400 RPM.
I think the motor is running too deep, I believe the anti cavitation plate is about 5" - 8" below the water line. I am planning to rework the transom to raise the motor so it is 1" to 2" below the water while the boat is running.
I am also thinking about replacing the propeller. I am considering a Hydrus 13" or 11" pitch.
Should I try the motor at the proper depth before changing the prop?
Will going to a 11" prop be too much of a change? (i.e. too little pitch?)
I hate to say this but after running your performance numbers through a propeller slip calculator program, your data is not correct.
The numbers show that the propeller is experiencing just under 50% slippage and that your actual top speed should be 34mph at 5200 rpm. The 22 pph speed is probably close but your other numbers are suspect. The tachometer could be inaccurate or on the "5" setting and not the "6" setting. A dealer with the Evinrude diagnostic software can check the accuracy of your tach. A 17" prop is way too big for a pontoon boat. An 11" is close to what you need. The motor has to turn up near 6000 rpm with a light load to avoid being overloaded and lugging the engine when a number of people come aboard. See your dealer for assistance. Start with getting reliable engine data with the motor at the proper height, then experiment with propellers.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.