I've read through this whole thread, just to see how it came out. I don't know why. I'm in a different world: fresh water with a bass boat.
For curiosity's sake, I did a little on-line research on your boat. I think you're expecting more top speed than it can give you. I read through a couple dozen reports and ads on your model, and the top speeds listed are ALL 38-40 mph.
Think about it.... it weighs over 4000 pounds and it's a V hull design. That's a lot of weight and friction, pushed by only about 300 horses, and a single 19" prop. The guys can talk about prop slip all they want, but I think 38-40 mph is all you can expect. It's unfortunate that Larson saw fit to put a 60 mph speedo in it. That just raises expectations.
You were right to be concerned about WOT RPM, and that "dragging" feeling at cruise. You definitely needed a different prop. A 4-blade prop will probably make it feel a little better at cruise, and save some on fuel. But it could knock 1 or 2 mph off the top speed in the process. But if you think about it, the only people who run WOT for more than a mile or two are racers and professional bass fishermen. "Cruise" performance is way more important for most everybody.
If you want to run 60 mph, then you have to shave a LOT of weight, increase power (a bunch), and/or get a different hull design. My bass boat is 17 feet, weighs less than 1400 pounds, has 150 horses, and runs over 60. It cruises sweetly at 45 mph. But then, I wouldn't dare take it off-shore, or expect to sleep inside it, or expect my wife ever to be fully comfy on it. (She thinks we need a boat like yours.
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