"I agree with W D...you should
"I agree with W D...you should take the boat back to the dealer and tell them what is going on.
If it is a fuel injected 50, they are able to check the tachometer accuracy with the HDS. They do not even have to run the motor. Just have to turn the key switch on and run the tachometer test. It is part of something that should be done during each motor set up. Many of the boat manufactures just stick the tach in and do not set it for the engine. Maybe they missed it.
The prop you have is a 9 pitch and it sounds like it is about right for that boat and motor combination. So, I think your tach is just set incorrectly.
If you do not have the time to go back to the dealer or if you are curious...you can check the setting yourself...if you can get to the back of the tachometer. Most likely it is either a Teleflex or Faria Tachometer.
If it is a Teleflex you will see a little rotatable switch on the back that has markings like 2P, 3P,4P,5P,6P...make sure the arrow on the switch is pointed to 2P. You may need a small screwdriver to turn it and probably a flashlight to see it.
If it is a Faria gauge, the markings on the switch will be 1,2,3,4,5,6...set the switch to 1. You will need a very small screwdriver on this on."