Your vessel is on course 150°T, speed 17 knots. The apparent wind is from 40° off the starboard bow, speed 15 knots. What is the speed of the true wind?

Correct Answer: Option C — 11.0 knots

True wind is the vector sum of apparent wind and ship's head-vector (reversed own ship velocity). Resolving apparent wind of 15 knots at 40° with own ship speed of 17 knots via vector addition yields a true wind speed of 11.0 knots.