Originally Posted by nimbusaviation
Thanks Zippy, appreciate you going to the trouble to reply, V useful!
There are only two reasons why a 250 would be preferable to a 300 if the hull is rated to 300hp, weight and cost. If they weigh the same, that only leaves cost. As for performance/economy, the 300 will obviously out perform the 250 in terms of acceleration and grunt, top end would be determined by propping. The 300 may possibly be more economical, as it wont be working as hard most of the time, or the difference between the two wouldn't be worth worrying about.
225hp/7m = 48knts is pretty meaningless. Which 225, which 7m? Every combination will give a different result.