I'm aware of the formula, but I suspect that you are taking the word "maximum" too literally. Earlier, I questioned it when you said that a boat will never exceed its displacement speed. My understanding is that these formulae give you a practical (rather than an absolute) maximum. However the application of more power will give more speed, albeit with increasing inefficiency.
You then went onto say that it's different when under sail. Granted it is different in that the "balance" of the boat will differ, but in general terms surely 30HP of thrust from the sails is much the same as 30hp from the engine?
I'm not trying to score points...honest. Just curious about the truth of this.