My engine sounds like it should go faster?

1972 Glastron 17.5 ft swinger v-177 tri hull with volvo penta AQ130C and 270 outdrive. first time working on boat engines, first time we had it on the water yesterday. To me, it sounded like we should have been going faster than what we were going by the way the engine sounded, it was going right up to 6000 rpm on the quage(max rpm it shows) and it didnt sound like it was that high rpm but you could tell it was high, but we would only be going about 25-30 mph. and when the rpms said 4000 we would go about 15. Does that sound right? i figured i could water ski behind this bu ti need to go atleast 36 mph and not be redlining the engine the whole way. Could the timing be off do that?