Prop slip is not the same as propeller efficiency. If the prop did not slip at all as it screws through the water, each rev theoretically propels the boat the forward a distance equal to the prop pitch. (a 15 inch pitch prop theoretically propels the boat 15 inches in one revolution)
Usage of dilemma a usually undesirable or unpleasant choice; a situation involving such a choice; broadly : predicament; a problem involving a difficult choice.
the cost of a DVD
the cost of a CD
with 
in the second equation and get
