"Jim, that does sound a touch
"Jim, that does sound a touch slow on that rig. That particular boat should max out at just a hair over 40 mph (the hull design won't allow it to go any faster no matter what you hang on the back).
Since you are running "light" with that motor you should be able to easily move up to a 21 pitch and squeeze out a couple more mph - however, without the tach there is really no way to easily determine if you are "underpropped" or perhaps the motor is not running at it's potential.
Maybe start by grabbing a can of Merc's "powertune" (sells for about 5 bucks) and just follow the directions on the spray can - gets carbon and other "cooties" out of the powerhead - you may just have a bit of a carbon problem robbing you of some top end performance (if the motor is running well otherwise)."