This is the situation: My 40' LWL boat (15 ton displacement) has a 150
PS engine. From the formula for speed I calculated a hull speed of

sqrt(40)*1.34 = 8.47 knots

*but*: when i did trials last week (absolutely calm water, almost no
wind) those are the results:

800 rpm 5 knots no noticeable waves generated
1100 rpm 5.5 knots small waves
1800 rpm 6.5 knots (flat out) - huge waves generated, stern deep in
the water, boat "running uphill".

1100 rpm is around 50/60 PS (from the engine rpm/PS table).

Question: what could be the cause of the "slowness" of the boat ? I do
not pretend to reach 8.4 knots cruising but at least 7 knots should be
in.

I'm thinging of dirty hull (green slime), incorrect weight
distribution (bow tends to "point" upwards even when crossing small
waves).

Any experience ?

Thanks
Matteo