You''re still confused. If the wind is from the West, and you''re on a close starboard tack reach, the masthead vane will be pointing more Southerly than Northerly. It might be easier to visualize if you imagine a windvane on your car in the same West wind. Parked, you figure the wind is blowing about 15 knots, and you see the vane is pointing due West. You head off South (on starboard tack), and as your car moves forward, you notice your windvane isn''t pointing due West any more -- it''s shifted a bit to the South too. When you speed up onto the interstate, still headed South, the windvane will be pointing almost due South because the forward speed of the car (65mph?) pretty much overcomes the West wind, When you stop for gas, the vane will point West again. On the way back, headed North,(on port tack, now) the vane will again shift forwards, to the North, despite the steady 15 knot West wind, because of the car''s movement.
The same thing happens on your boat, though perhaps not as dramatically. As you go faster on ANY point of sail, the apparent wind goes forward. It is important to know where the apparent wind is coming from, because it is the apparent wind that your sails are using to move the boat. That''s why iceboats go off on 50 mph "reaches" with their sails sheeted in tight. They start out parked in the true wind, and head off on a reach, same as you & me, with the boom off their quarter. They then get going faster, so the wind moves forward and blows a little harder, and they sheet in a little bit. This gets them going faster, which increases their apparent wind still more, so they sheet in some more, and go still faster. You & I have hull speed keeping us from doing this, but planing dinghies
work on the same principle.
In any case, a windvane would help give you an idea of how to trim your sails, depending upon
your heading. It''s important to realize that it''s the relative directions of the wind and the boat -- the apparent wind -- which is crucial. Knowing where the true wind is coming from -- heading into it to see where it is -- is nice, but it isn''t going to get you anywhere.