True Wind and Apparent Wind
There are two kinds of wind we need to be aware of:
True wind is the actual wind you’d feel if you were standing still, not moving. It’s the wind that weather forecasters talk about.
Apparent wind is the wind you actually feel on your face when you’re moving. It’s a combination of the true wind and the wind created by your own movement through the air.
Why Apparent Wind Matters
For sailors, apparent wind is the most important type of wind because it’s the wind that the sails actually “feel.” We always adjust your sails based on the direction and speed of the apparent wind. The apparent wind’s speed and direction change constantly as a boat speeds up, slows down, or changes direction.
Click on the image to see an animation of the relationship between the winds and boat speed and direction.
Sailing into the wind
When you’re sailing upwind, your boat’s forward motion adds to the true wind, so the apparent wind is faster and seems to be coming from a more forward direction than the true wind.
Sailing with the wind
When you’re sailing downwind, your boat’s speed subtracts from the true wind speed, so the apparent wind is slower.
Sailing with the wind on your side
A broad or beam reach, and it’s often the fastest point of sail. The apparent wind here is a combination of the true wind and your boat’s forward motion, resulting in a higher speed that helps generate more power in the sails.
Where’s It Coming From?
You can often tell the apparent wind direction by observing indicators on the boat like telltales (small pieces of yarn or ribbon on the shrouds and sails) or a wind vane (windex) at the top of the mast.
What’s This Mean to Me?
When sailing a small boat in a racing regatta, without sophisticated instruments, the helmsman and crew typically rely on their own senses and basic indicators (telltales, windex) to detect and adjust sail trim and steering to the apparent wind without understanding what follows.
Wind Triangle
For the mathematically inclined
The Wind Triangle (OAT) is a vector diagram used in marine and air navigation to calculate the relationship between different types of wind. It’s a key tool for professional sailors and pilots to understand how their own movement affects the wind they experience.
The letters in the OAT triangle stand for:
- O: Origin (the point representing the vessel or aircraft’s starting point).
- A: Apparent wind. The vector OA represents the direction and speed of the apparent wind, which is the wind you feel while moving.
- T: True wind. The vector OT represents the direction and speed of the true wind, which is the actual wind in the atmosphere.
- The third side, AT, represents the boat’s own course and speed.
For example, sailing on course of 200° speed 14 knots. True wind 300° (WNW) at 18 knots. Find the direction and speed of the apparent wind.
- Draw a line representing North-South and take any point A on it.
- Draw AT equal to the course and speed of the boat, using any appropriate scale. This now represents the induced wind, perceived at point A.
- At T draw a line to represent the true wind OT, using chosen scale.
- Join OA, which now represents the apparent wind. Using chosen scale, measure distance OA as knots.
- The angle that OA makes with the North-South line at A is the direction of the apparent wind.
By arranging these three vectors into a triangle, navigators can solve for an unknown value if the other two are known. For example, if you know the true wind (from a weather report) and your vessel’s course and speed, you can use the OAT triangle to determine the apparent wind, which is what you’ll need to set your sails by.
It is useful to consider what happens when any of the values of these vectors change. For example, if close hauled, how would the angle and velocity of the apparent wind change if the true wind suddenly doubled in strength (a big gust)?