Roni, with enough growth of the prop, even if the hull is clean you can reach full throttle and have zero forward speed. On pretty much any boat.
If you have growth ("a good beard") on the hull and some on the prop, hitting 6 knots would be quite good actually. It can easily cut 1-2 knots off the speed of a boat.
Its amazing how much just a little growth can slow you down, Just like HS said. Even a small bit of seeweed on your prop will significantly alter the proformance of your boat.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White