5.6m Ramco with 90hp merc That would put your boat if motor correct height and at best trim around 1350/1400kg gross weight, which I think is a little high. maybe indicating motor not right yet.
I guesstimate actual weight, say 3 guys up, fishing gear, 100L fuel more like 1200/1300kg (???)
If my guestimate gross weight is right.. say 1250kg you should have a W0T around 37mph, and would relate to engine height not right yet, or not adjusting to best trim at each speed.
At 5000 I was doing 35MPH
prop 13x 18 pitch
Motor yr unknown
on older model gearbox 2.3:1
Thats a WoT slip around 5.5 % which is pretty good for a general use boat, with a cruise around 25mph @ 4000 rpms
Newer model merc 2.33:1 WoT 4% which is bit on low side ok
Either , cavitation(ventilation) is not an issue yet
I have all but eliminated most of the steering torque.
Generally takes a couple adjustments to get neutral at best cruise.
Revs have increased as has top speed, not that top speed was necessarily something I was aiming for. At 5000
Engine spec is 5000 to 5500
you are already on the low end, drop motor you will drop below the min 5000 rpms which means will be over loading the powerhead thru all the speed range...
which may mean going to a 17" pitch prop
If the 35mph/ 5000 rpms was not on a normal full loaded boat then as soon as you go fully loaded (as described above) the powerhead falls out of recommended rpm range and becomes overloaded.
Which means wrong prop selection.. drop motor makes even more wrong.
Generally a approx 5.5/5.6m boat well powered for best hull performance in chop etc will have a WoT laround mid 40 mph,. min best performance around the high 30s low 40 mph