I used one on a 14'SK flatbottom with a 1500XS Merc.
It added some top end, but wanted to fall off a plane at slower speeds real bad.
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I pulled the plate off and mounted the motor flat on the transom (15" with plate even with bottom) and had a 2-3 MPH slower boat, but it was more usable for most boating.
I was told by some local hot boaters that I should have run the motor higher on the transom, but didn't really have the money to experiment with props etc to get things right
There also is a jack in racing(or was years ago) that extended through the transom and supported the lower unit....some of these were manual adjusted for trim, and some were hydraulic depending on the size and complexity of the boat....I don't think you mean this though and am assuming you mean a jack plate to raise straight up & down