To review the issues involved, we’ll start with a basic example. Our actual use of compass directions in navigation does indeed complicate things a bit, but that can be sorted out later as it is not really related to the subject at hand. Since wind directions are almost always discussed in terms of true directions, not magnetic, we’ll forget the compass for now and consider all directions as being true. TWD = True Wind Direction (relative to true north) TWS = True Wind Speed (relative to the fixed earth) ![]() SOG = Speed Over Ground (relative to the fixed earth)ĬOG = Course Over Ground (relative to the fixed earth) SET = Current Set (direction it flows toward, relative to fixed earth) S = Knotmeter speed (relative to the water)ĭFT = Current Drift (speed, relative to fixed earth) To help practically illustrate these points, let’s bring in some shorthand:ĪWS = Apparent Wind Speed (relative to the boat)ĪWA = Apparent Wind Angle (relative to the bow, 0 to 180, starboard plus, port minus)ĪWD = Apparent Wind Direction (relative to true north) Slight changes in true wind speed affect boat speed, and in turn, the apparent wind speed and direction, which can easily mask a small but important shift in the true wind direction. This is not a simple observation though, which is the point at hand. If we do not figure this properly, we can miss an important shift. Shifts in the true wind are usually the first sign of changing patterns. But to know about the weather patterns that are causing the wind and how the wind is changing, we need to know the direction of the true wind as accurately as possible. Many sailors claim that the apparent wind is all they care about, and that can be well argued when it comes to setting sails and judging performance. Once we get underway, however, our own motion changes the wind we feel, and then it is called the apparent wind. Tied up at the dock, we feel the true wind. This true wind is the speed and the direction of the wind relative to the fixed earth under the ocean. ![]() For weather work at sea we care only about the true wind. There are several ways to define the wind. Here’s a great article by David Burch from Starpath Navigation on how to calculate the true wind and why it matters to sailors…
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