Is your engine 2 or 4 stroke? A 2 stroke should rev to 5500rpm and the 4 stroke should rev to 6000. It sounds like the Yamaha prop you have got is slipping, check the bush in the middle of the prop. If it has play in it then that could cause your problem.
The general rule for props is for each pitch of prop you will go up or down about 200 rpm.Maybe with all your weight you should try a 15" pitch. If its a four stroke maybe try a 13" pitch.