I gotta agree with RW... Something just ain't right. 220 HP you should be skeedaddleing across the water close to 50mph.
Some things you can do...
First, get a hand held GPS... the one from a car will do fine, like a Tom-
Tom or a Garmin. This way you'll get an accurate measurement of your speed.
Second, look at your prop.. any dings on the edges of the blades with take MPH off your top end. Take the prop off the outdrive and look inside the prop on the hub. Mercury usually puts their prop part number and the prop pitch both on the blade and on the inside hub. The pitch will be a number like 17p or 21p or something like that.
Third, check your tach with another tach if you have one. Make sure you are really turning 5100 RPM.
Forth.. like RW said, Pics are always a good idea. One picture is indeed worth a thousand words.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God)
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.