On board, we talk about ‘true wind’ or ‘apparent wind’. We use this to calculate the quarter from which the wind blows and the speed. We express the quarter from which the wind blows, in degrees from the bow viewed to the rear. The wind speed is expressed in knots. The ‘true wind’ is the wind as is really blown observed from standstill on a spot.
The ‘apparent wind’ is the wind observed on board, calculated with our own speed.
Easier said: When you are sailing along with the wind, you feel less wind on deck then when you stand still. On this moment the ‘true wind’ is 14 knots, but the ‘apparent wind’ is 7 knots. We sail on the moment 7,5 knots with a quarter of 150 degrees wind angle.
A square rigged ship that goes faster threw the water then the air parts that hits the sails is still a fantastic phenomenon.

Richard Slootweg
Captain Clipper Stad Amsterdam