Boat 1996, 2000 Keywest WA, length 20', engine Johnson 225 (99), current has Viper SS, 14 3/4 x 17, hits 5500-5800 depending on load in boat, @44-49mph
While I can't be too specific; the prop size seems right, the numbers work with decent speed and about 8% slip.
Is the speed by gps? It does seem that a 225 might get you over 50 with a 1200lb boat.Is it a really deep V?
Are you hauling a lot of gear?Is the bottom clean and smooth?