The beauty of conventional polars is that they show both absolute boatspeed and VMG to windward, which is simply the vertical component. The best VMG to windward is at the highest point on the curve. The best VMG to leeward is at the lowest point on the curve.

Keep in mind, however, that only a single polar curve is shown, and it's for a constant wind and sea state. The curves can change dramatically with wind speed, especially the downwind part. See Bethwaite's High Performance Sailing for dramatic examples.

--Glenn