RPM's & Prop

[Joker2]

I am only getting 4-4200 RPM's with my prop 15- 1/2 x 15 Alum. get 32 mph on gps. One person, no gear, 20 gal. gas, trim adjusted. The boat is light, recently rebuilt the whole transom and rear deck, totolly dry now.Checked the linkage at motor, it is maxed out. Engine height is OK the fin on lower unit is even with keel. Engine just repaired, runs great. Could a prop change make up 1500 RPM's ? I don't know what else to try.