To: Tom, Shonda and friends,
I'm a new member and was going through old postings. I've got an `89 23' Weekender with a 5.7 Merc, 260 hp/350 ci. I'm turning a 14 1/4X 19 prop. When I got the boat I'd have to turn at least 3200 RPM to maintain a decent trim. I was getting about 25 MPH, but burning too much gas and working the engine too hard. I use my boat for long distance cruising, so fuel economy and reducing stress on the engine are more important than higher speed.
I replaced the OEM trim tabs that were 12' wide (and 12' cord) with 24' wide tabs. Used same pump and cylinders, just relocated the cylinders to center on the new tabs.
I can now cruise at 2850 RPM at 22 MPH with an average load and full water and fuel. The engine is just loping along and I'm burning a little less than 6 GPH. And, when I have a load of people, those larger tabs pushes the stern right up out of the water.
But, I haven't changed the prop, and can get 35 plus at about 4400 RPM. Increasing the trim tab size effects performance by allowing you to maintain an efficient planeing angle, but at a much lower RPM, rather than changing the top RPM or speed. Depending on your use, maybe looking at increasing the width of the trim tabs rather than changing the prop size, will do more for your performance.
Just some thoughts.