I have 25ft versatillity with a 2.5 BMC when i first start the boat there is white smoke for 5 to 10 minutes then it clears, and when leaving the yard at halve throttle no smoke but as i increase the speed there is black smoke ?
The white smoke is fairly normal, but the black smoke as you describe it is typical of “overloading”. This is where something prevents the engine reaching the revs set on the throttle.
Once you have checked that the prop is clear, I would suspect that the prop is too “large” and is holding the revs down. I would advise talking to someone like Steel Developments (South London) or Crowthers (Manchester way?) to find out the size they suggest and then check it against yours when the boat is next out of the water.
Operating the boat is in displacement mode and trying to drive it at more than the hull design speed or a very badly fouled hull could also cause overloading. I estimate that the maximum displacement speed of your boat to be about (without any data) and trying to drive it faster that this without a much larger engine may well cause black smoke.