Although I am not familiar with working on anything with a duel prop setup, I disagree on the statement "No way to prop low enough to gain 1800 RPM." but it would have to be an extreme over propping issue in the first place that I feel you would have found engine lugging as a first sign. I agree with checking compression and making sure the motor is in good running condition and confirming you are getting real tach readings. This is a big heavy boat, 7200 lbs dry. even so, I have read that factory should run about 30 mph cruising speed with 40 mph top end. The 5.7 L. motor is the smaller motor but after a further short research, I have found others discussing same issues in other forums with no mentioned resolution, and also stating that an 18P prop is getting to the small end of what factory would agree to be using. I still personally feel that your boat may be over propped with a 22P (I can again admit if I am wrong) but may be experiencing any combination of other issues like being very heavily loaded, false RPM readings at the tach, motor old and tired, low on fuel pressure, dirty or stuck fuel injectors, (if this motor is equipped with a fuel injection system) in need of tune up, and or throttle not opening to WOT position. Start with what you plan on doing with the compression test and also check that the throttle plate is fully open at full throttle, I would plan to replace spark plugs if your compression readings are good as you have to pull them anyway to test compression. Might as well do wires, cap and rotor at the same time if all is well.
