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True And Apparent Wind Speed Explained

True and Apparent Wind

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.

So why is this so important in sailing?

Let me explain.

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.

Compass rose

The most normal to use are the following:

N- North
NE- Northeast
E – East
SE – Southeast
S – South
SW – Southwest
W – West
N – Northwest
Cardinal points in the compass rose.

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

So, precisely what is true wind?

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-degree 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

So, precisely what is apparent wind?

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.

Apparent wind vs. true wind

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.

True And Apparent Wind Speed Explained

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.

True And Apparent Wind Speed Explained

So, what about the angles?

  • When you sail in any direction below 90 degrees toward the wind, your apparent wind speed will be higher than your true wind speed. Your apparent wind angle will be less than your true wind angle. If the wind speed increase, the apparent wind angle will also increase.
  • When you sail in any direction above 90 degrees from the wind, your apparent wind speed will be lower than your true wind speed. Your apparent wind angle will be less than your true wind angle. If the wind speed increase, so will the apparent wind angle.

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 fail. I have been there myself. However, don’t be disappointed in your boat’s ability to sail close angles. You can read more about how high a sailboat can point here.

Just take the facts for what they are and work with them to plan your routes.

The correlation between actual and apparent wind speed

Finding the correlation between actual 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-degree beam reach while enjoying the Caribbean waters sailing above 8 knots!

I am sailing Ellidah in the Caribbean in sweet conditions.

Final words

Most people are familiar with true wind or actual wind. Understanding apparent wind is a bit more complicated. I hope you now understand the difference between the two and why it matters. 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 this guide.

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