How to Calculate True and Apparent Wind
Finding the direction and speed of True and Apparent wind is fairly easy and requires a simple understanding of force triangles. For a Mariner, calculating the true wind direction and speed is essential as it is the true wind that is recorded in the logbook.
In this Blog, we will provide everything you need to know to calculate true and apparent wind speed and direction, including easy examples.
1:- What is Apparent Wind
Apparent wind is the wind experienced by a moving object, such as a ship or a sailboat. It is a combination of the true wind (the wind you feel when standing still) and the wind created by the object’s own movement. Understanding apparent wind is crucial for sailors as it affects how the vessel is navigated. In simple terms, apparent wind is the wind that “appears” to be blowing from a certain direction when you’re in motion, and it plays a key role in optimizing sailing performance.
2:- What is True Wind
True wind is the wind that blows across the land or sea when you’re stationary. It’s the natural wind direction and speed, unaffected by your movement.
3:- Difference between True and Apparent Wind
| Aspect | True Wind | Apparent Wind |
|---|---|---|
| Definition | Wind felt by a stationary observer. | Wind experienced by a moving object. |
| Source | Natural wind blowing over the land/sea. | Combination of true wind and wind from movement. |
| Direction | Consistent and based on natural conditions. | Changes based on the object’s movement. |
| Speed | Remains constant in a stationary context. | Varies with the speed and direction of the object. |
| Importance for Sailing | Used to determine initial sail settings. | Critical for adjusting sails during movement. |
4:- Points to Remember Before Solving True & Apparent Wind Problems
The following points will help you understand and solve the problems better:
- A shipboard anemometer measures the speed of apparent wind.
- The direction of wind obtained by observing the line of waves is the direction of true wind.
- The wind is named by the direction from which it comes. “Wind if ‘from’ and set is ‘to’.
5:- How to Calculate True and Apparent Wind Speed and Direction
Let us take one step at a time and solve basic problems. This will form a strong base for understanding harder problems.
Let’s assume the Vessel is on a course of 000 Degrees and the Vessel’s Speed is 15 Knots
Case 1: No wind and Vessel Moving Ahead.
True Wind speed = 0 knots.
Thus, the observer on the vessel would experience apparent wind at a speed of 15 Knots coming from North
Case 2: Wind From North and Vessel Moving Ahead.
Say, the true wind speed from the North is 20 Knots.
The observer would experience an apparent wind of 35 Knots coming from North
Case 3: Wind from South and Vessel Moving Ahead.
Say, True wind from the south is 20 Knots
The observer would experience an apparent wind of 5 Knots coming from the South
These were simple scenarios where the apparent wind was found easily. In other cases, a Vector Triangle needs to be formed to find the direction of true/apparent wind. This Triangle is called the ‘OAT’ Triangle.
6:- Steps to Find True and Apparent Wind
In the OAT Triangle,
AT – Course and Speed of the vessel
OA – Direction and Speed of Apparent Wind
OT – Direction and Speed of True WInd.
Consider the following problem:
Course 045° speed 15 knots. Apparent wind 100° at 20 knots. Find the direction and speed of the true wind.
Step 1: Make a line representing North-South and take any point A on it.
Step 2: At A, draw an angle equal to the course (045° in this case) and cut off AT equal to the ship’s speed (15 knots in this case). Use a Suitable Scale
Step 3: At A, draw an angle equal to the apparent wind (100° in this case) and cut off AO equal to the apparent wind speed (20 knots in this case), using the same scale.
Step 4: Join OT and this represents the true wind. Using the same scale as before, convert distance OT into knots.
Step 5: To obtain the direction of true wind, draw a North-South line through T and read off the angle between it and OT.
In this case, the true wind direction speed and direction are 17 knots and 147 degrees, respectively.
Similarly, using the same triangle, apparent wind direction can be found.
7:- How to find Apparent Wind using the OAT Triangle?
Course 200° speed 14 knots. True wind 300° at 18 knots. Find the direction and speed of the apparent wind.
Step 1: Draw a line representing North-South and take any point A on it.
Step 2: Make AT equal to the course and speed of the vessel (200° at 14 knots), using any appropriate scale
Step 3: At T draw a North-South line and insert the true wind OT (300° at 18 knots), using the same scale.
Step 4: Join OA, which now represents the apparent wind. Using the same scale, convert distance OA into knots.
Step 5: The angle that OA makes with the North-South line at A is the direction of the apparent wind.
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