have fitted merc 150hp seems like prop is slipping but have tried three different props with same result .When revs get to 1500rpm prop seems to loose grip and revs rise with no increase in speed . Engine install is correct and dont think can be cavitation as revs not high enough .Is there a clutch or anything in gear box that could slip reverse is fine and there dosnt seem to be any grateing or grinding noises .Also drive feels smooth with no jumping or clunking when selecting gears .