Hull speed is not the max. speed of the boat but rather the speed of the WAVES which are produced by the bow and stern and which AT calculated 'hull speed' are still fully supporting both the bow and more importantly the stern. Beyond 'hull speed' the produced stern wave will be 'behind' the boats stern and no longer fully supporting the stern ... and the boat begins to 'squat' because the stern is no longer supported by the stern wave.
Other considerations: Running a boat in relatively shallow water will cause the bow and stern waves to become greater in amplitude and with the bow and stern waves 'slower' ... wave 'reflection' off the bottom. It takes more power to attain the same speed in relatively shallow water than when in deeper water. ;-)
To determine speed vs. engine output (rpm) either assay this in FLAT (deep) water with no to little wind blowing ... or less preferably, run several 180° courses and then average the results. Just plot several speeds vs. engine rpm ... and where the line
you plotted starts to rapidly change direction ... is your 'functional' hull speed.
Prop 'efficiency' has a lot to do with how much 'speed' you get out of a boat and no matter how fast the engine is turning, even if the engine rpm is developing maximum HP from the engine.
A .5 knot current against you, plus a 9 kt headwind can easily drop your speed vs. 'hull speed'. Do several 'assays' in various light wind and zero current conditions ... and then and only then if the expected isnt resulted, .... then search for the cause.
Also be aware that most 'boat instruments' arent 'calibrated' and only give 'approximate' and sometimes 'variable' results. ;-)