You’ve heard about “true wind” and “apparent wind”, but you’re unsure how this relates to sailing. 

As you are aware, wind is a significant factor when sailing. The wind direction and our destination determine our point of sail, and consequently, we trim the sails accordingly.

Firstly, true wind is measured, whereas apparent wind is calculated. Also, apparent wind is the wind you “feel” when you’re moving, unlike “true wind.” 

For example, the forecast is for 20 km/h (that is 20 km/h true). When you step on your bicycle and cycle at 20 km/h directly into the 20 km/h headwind, you effectively battle a 40 km/h headwind (40 km/h apparent). Ie, 20 + 20 = 40.

Conversely, when you turn around and are now cycling at 20 km/h with the 20 km/h wind in your back, it appears there is no wind at all. I.e., 20 – 20 = 0

This calculation only holds if you are 100% with or against the true wind. But as we know, true wind can hit our boat at an infinite number of angles. Our speed further complicates this, as it also affects our apparent wind direction.

Generally, sailors use “true wind” to “select” the right sails, whereas “apparent wind” is used to “trim” them.

To calculate true wind speed and apparent wind speed, we can use the following definitions and formulas:

• True Wind Speed (TWS) is the actual speed of the wind blowing over the surface of the water.
• Apparent Wind Speed (AWS) is the wind you feel on a moving boat, which combines the true wind and the wind created by the boat’s movement.

The formula for Apparent Wind Speed (AWS):

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Where:

• $AWS$ = Apparent Wind Speed
• $TWS$ = True Wind Speed
• $BS$ = Boat Speed
• θ = Angle between true wind and the boat’s heading (in degrees)

The formula for True Wind Speed (TWS):

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Wind Direction:

• Apparent Wind Angle (AWA) is the angle between the direction the boat is heading and the direction from which the apparent wind is coming.
• True Wind Angle (TWA) is the angle between the direction the boat is heading and the direction from which the true wind is coming.

Sailors use apparent wind because it directly affects how their sailboat moves and performs. The apparent wind is the wind that sailors actually feel while the boat is moving, which is a combination of the true wind (the wind blowing over the water) and the wind created by the boat’s movement through the air. This apparent wind determines the angle and force acting on the sails, making it a critical factor in optimising sailing performance.

Here are the key reasons why sailors focus on apparent wind:

1. Sailing Efficiency

• Apparent wind angle (AWA) affects the shape and trim of the sails. Sailors adjust their sails based on this angle to maximise the wind’s force on the sails and move the boat faster.
• A well-trimmed sail set at the correct angle to the apparent wind helps generate lift, much like an aeroplane wing operates. This allows the boat to move efficiently, even when sailing upwind or at an angle to the wind (known as “beating” or “close-hauled” sailing).( See points of sail)

2. Boat Speed

• The apparent wind speed (AWS) is often much higher than the true wind speed, especially when the boat moves fast. The faster the boat goes, the more the apparent wind “bends” forward, increasing its effect on the sails. This allows the boat to catch more wind and move even faster.
• For instance, when sailing on a broad reach (wind coming from behind at an angle), the boat can gain extra speed due to the increased apparent wind acting on the sails.

3. Upwind Sailing (Close-Hauled)

• When a sailboat is sailing close to the wind (upwind or close-hauled), the apparent wind is significantly stronger and comes from a more forward direction than the true wind. Sailors rely on this stronger apparent wind to generate enough lift from the sails to progress upwind.
• Understanding apparent wind allows sailors to sail efficiently even when heading into the wind at sharp angles, where the true wind alone might seem insufficient.

4. Downwind Sailing

• When sailing downwind (with the wind at your back), the apparent wind is less than the true wind because the boat is moving in the same direction as the wind. In this situation, sailors must adjust their sails to capture as much of the true wind as possible since the apparent wind decreases as boat speed increases.

5. Tactical Decisions

• Racing sailors use apparent wind to make tactical decisions, especially when planning tacks and jibes. By calculating how changes in boat speed affect the apparent wind, they can adjust their course and sail trim to maintain optimal speed and maximise efficiency.
• Understanding how apparent wind changes with manoeuvres allows sailors to plan the best route, especially when navigating around buoys or racing marks.

6. Safety

• Apparent wind can provide early warning signs of increased wind speed, allowing sailors to adjust sails or reduce sail area before the boat becomes overpowered. This is particularly important in strong wind conditions or gusty weather.

Summary

Sailors use apparent wind because it is the effective wind that powers the boat’s sails. By monitoring and adjusting to the apparent wind, sailors can optimise their speed, control, and performance on the water, whether sailing upwind, downwind, or across the wind. It’s the critical factor in sail trim and efficient sailing.