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Tom,
I have the 21' version of your boat and recently repowered it with a 4-stroke Yamaha. (I went Yammy in part because its a lighter 4-stroke.) Anyway, I had similar issues with not staying on plane under 3500rpm. I have trim tabs, but I didn't get results I was looking for. Going from a 3 blade to a 4 blade with lift characteristics got me up on plane in the 2900-3000 RPM range. Same pitch. It seems to handle better generally at modest speeds.