large one-design boats are sailing upwind on opposite tacks in a
beautiful 10-knot breeze. Both boats are identical and are going
exactly the same height and speed through the water. The 'sailing wind'
is exactly the same all over the course area and remains steady. There
also happens to be a constant two knots of current running from right
to left, so the port tacker (P) is sailing into the current while the
starboard tacker (S) is sailing more with the current. Both boats are
outfitted with very high-tech and sensitive wind instruments at the top
of their mast.
QUESTION: If you look at the apparent wind speed readings from each boat, how will they compare?
Correct Answer: Both boats will have the same apparent wind speeds
Both boats are being affected equally by the current and therefore they will have exactly the same apparent wind speeds. The sailing wind in which they sail is a vector combination of the true wind (felt by an observer on an anchored boat) and the current wind (a result of the boats moving with the current). Since the effects of the current are already included in their sailing wind, the apparent wind generated by sailing in that wind will be exactly the same on either tack.