What no-one has considered here is that powering calcs are usually done from
the other point of view:

My boat has X lbs of drag at 5 knots, how many horsepower do I need?

Factor in windage (the lee shore situation), foul hulls, appendage drag, prop
efficiency (an oxymoron if ever I saw one) and other such truck like Taylor
Wake Fraction, and you have a real problem on your hands.

Doing it in reverse? Priceless....

Steve