Learn the Difference Between True and Apparent Wind Speed
Have you ever thought about how different sailing toward versus sailing away from the wind can be? If you are a new aspiring sailor, you must learn the difference between true and apparent wind speed.
True wind speed is the actual wind velocity measured by a stationary object. Apparent wind speed is the wind velocity perceived by an object moving through the air, such as a boat or yourself. In other words, apparent wind speed combines the actual wind and the effective wind created by your motion.
Wind awareness is critical when you’re out at sea. And when you’re on a sailboat, it’s essential because your safety depends on your understanding of how the boat behaves in different wind conditions.
In this guide, we’ll dive into how the true and apparent wind speed affects us when we sail at different angles to the wind and get familiar with the compass rose. Then we’ll learn some related terms and look at examples.
We’ll wrap it up by summarizing the content in this guide. By then, you should have a good understanding of the difference between true and apparent wind!
Understanding the difference between true and apparent wind
What might feel like a chill and relaxed Sunday sail off the wind can turn into an intimidating and challenging experience if you have to turn around and sail upwind. When you understand how the wind works on a moving object like a sailboat, you can consider it when planning your trip or deciding whether to go or wait for better conditions.
The angles we sail relative to the wind and the wind created by our speed over the ground equals the apparent wind we experience onboard. And on a sailboat, we sail and trim the sails according to the apparent wind.
Our apparent wind speed changes when we adjust the course and angle towards or away from the wind direction. Our boat speed also affects the apparent wind angle. This can be confusing in the beginning. To fully understand how the wind speed and direction affect us on a sailboat, we first have to familiarize ourselves with the compass rose and learn some terms.
The compass rose explained
We often talk about wind direction in terms of the cardinal points in the compass rose.
The most normal to use are the following:
|E – East|
|SE – Southeast|
|S – South|
|SW – Southwest|
|W – West|
|N – Northwest|
Knowing your compass rose is essential when sailing and navigating at sea!
These are commonly known and used by most. However, specifying the angle in degrees is much more accurate. It is also what you will read from your sailing instruments and the weather forecasts.
True wind explained
True wind is the wind that affects you when you are stationary. When we talk about wind in general, we usually refer to true wind speed, direction, and angle. Using nautical terms makes it easier to identify the specific element we are talking about.
These are the terms used about true wind:
- TWS – True Wind Speed
- TWD – True Wind Direction
- TWA – True Wind Angle
The wind speed is usually measured by a wind anemometer in knots, meters per second, kilometers per hour, or miles per hour. For us sailors, knots are the most commonly used measurement as it makes calculations regarding speed in knots easy. Here you have the conversions:
Wind speed conversion:
10 knots = 5.14 m/s = 18.52 km/h = 11.50 mph
If the wind is blowing 10 knots north-easterly, the wind comes from the northeast, which is a 45 degrees direction (TWD), blowing 10 knots (TWS). If you sail on a south-easterly heading, the vessel’s angle to the wind (TWA) is 90 degrees.
However, since we move in direction and speed when we sail, the apparent wind always behaves differently than the true wind.
Apparent wind explained
Apparent wind is the wind that affects you when you are moving. It combines the true wind and the effective wind created by your motion. The apparent wind will vary from the true wind depending on the angle and speed of your movement.
Here are the terms used about apparent wind:
- AWS – Apparent Wind Speed
- AWD – Apparent Wind Direction
- AWA – Apparent Wind Angle
Think about this:
You are standing still, and the wind blows 10 knots. If you start to run against the wind, it feels stronger. If you run away from the wind, it feels weaker. That is because even though the actual breeze is consistent, the apparent wind changes.
Let’s give some examples!
How true and apparent wind work when sailing
Imagine the following scenario:
The TWS is 15 knots, and the TWA is 0 degrees, or straight from the north. You are sailing at 5 knots straight south, away from the wind. Your AWS will, in this case, be 10 knots.
If you turn around 180 degrees and motor straight north into the wind, your AWS will increase by the 5 knots of speed you are making and rise to 20 knots.
So, what about the angles?
Let your brain process this for a bit…
The elements work against us when we sail to some degree. This is where most fresh sailors’ plans do fail. I have been there myself. However, don’t be disappointed in your boat’s ability to sail close angles. I actually wrote an article about how high a sailboat can point.
Just take the facts for what they are and work with them to plan your routes.
The correlation between true and apparent wind speed
Finding the correlation between true wind speed and apparent wind speed in an angle requires us to calculate some vectors, which is unnecessary.
Luckily, Most boats today have wind instruments that do this job for us and show us all the information we need.
However, if you want to squeeze your brain around vector calculations, this is a good source from Spinnaker Sailing.
The most important thing is to know how to use this information. When you sail downwind, it might seem like it is just blowing a breeze. But that is only until you turn around and realize your apparent wind has increased significantly.
You need to know and prepare for this. Maybe you should reef your sails before turning around?
When you sail, you sail to your apparent wind, as that is the wind that affects your boat in motion. But always keep an eye on the actual wind speed, especially when sailing downwind!
The video below shows an example of smooth 20 kt wind on a 90 degrees beam reach while enjoying the Caribbean waters sailing above 8 knots!
Most people are familiar with true wind or actual wind. Understanding apparent wind is a bit more complicated. After reading this guide, you should know the difference between the two and why it matters. You learned that the apparent wind will always vary depending on your movement in relation to the true wind.
Now it is time to continue and learn how to sail at different angles to the wind. That brings us to the five points of sail which you can read about in the next guide.
Delve deeper into the world of sailing! This guide is part of an exclusive series designed to equip you with the knowledge needed for your next sailing adventure.
Check out the others below:
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