What propeller do i need?

size and pitch. i have a 20 ft boat with a mercruiser 3.0 tks. and a mercruiser outdrive thats for a 6 cylinder. currently i have a 14.5 in x 17 degree pitch. i dont know how to tell if the out drive was for a 6 cyl engine there doesnt seem to be a serial etching that i can see. the boat is terribly slow and its highest rpm is 2400. any advise would be helpful