Quoting from David Dellenbaugh (with some paraphrasing): “Locating the best angle is not so difficult when you are sailing upwind. The close-hauled groove that gives you optimum performance to windward is fairly well defined and stays pretty much the same even as the wind increases or decreases. The front of your jib gives great feedback about whether you are sailing too high or too low. …other clues to help you find the best close-hauled angle is the tug of the windward helm, a steady angle of heal and the constant feel of the breeze on your face.
“Almost all of these things are missing when you sail downwind.”
Let’s first look at some “boat speed theory” -
The Polar Curve and VMG