Definition
VMG is the component of boat speed resolved along the direction of interest, so it equals boat speed multiplied by the cosine of the angle between heading and that direction. Sailing at six knots at 45 degrees to the true wind yields about 4.2 knots of upwind VMG. Pinching raises the angle but kills speed, so the best upwind and downwind angles are those which maximise VMG, taken from the boat's polar.
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