Hi Chuddster .. yup.. at first thought ...there appears there be something not quite correct with your setup .. I get approx the same speed with my Quicksilver powered by a 25HP outboard.
However that is with me only in it. I recall a guy recently saying his planes with seven folks in it.. but mine would be stuck at fast displacement speeds ( around 10 mph) with that amount of folks. Im mentioning this simply because you have not given any info other than top speed and 40HP...therefore we cant offer you suggestions to try.
That is why Fenlander or anyone else requires more info from you. Your engine may be fine ..but if over propped.. or if the boat is heavy loaded .. it may only be putting out around 3K revs at full throttle ..instead of 5K or so revs.. (if that is what the 40HP should put out) Remember..boat outboards donít have gears like a car.. so if load or prop is too much..it cant reach max revs. That is why you need the rev counter to help identify the issue.
An example is the FRIB with a 6HP goes at 16mph with one person on it. With two folks..it cant get on the plane..it only gets 2.5K revs instead of 5.5K as recommended by manufacturer. However ..with a smaller prop..it gets 5.5K revs again but around 13mph with two on board. Cant keep the smaller prop on when only one on board though..as it will over rev the OB at WOT.
Good luck with getting the boat trimmed for your requirements.