Understanding true and apparent wind when sailing
02 May 2018
We are regularly asked what is the most important skill when learning to sail. Given the nature of sailing the answer shouldn’t surprise you; it’s wind awareness.
What’s wind awareness? The answer is it’s simple - but not necessarily easy.
Some of our novice students ‘get it’ right away. Other more experienced sailors join us with a surprisingly poor wind awareness. Given that we all learn at different rates and have different ways of learning, we expect this variance but if there is one thing that will limit your ability to safely and efficiently sail a yacht it’s a sufficient level of wind awareness.
Knowing where the wind is blowing from and at what angle the boat lies to the wind direction is critical if we are to sail the boat efficiently, trim the sails properly and protect the crew from an accidental tack and, more critically, an accidental gybe. A sailor without wind awareness cannot be a safe skipper and is not a competent crew member.
However, in addition to knowing where wind is blowing from, what angle the boat is to that wind direction and how strong that wind is blowing (all of which the boat will have instruments measure) there is also a need to understand the definitions of true wind and apparent wind. If you don’t know the difference or why these definitions matter, you’ve come to the right place! Read on..
What is True Wind?
True wind is the actual speed and direction of the wind blowing as experienced by an observer that is stationary. He might be at sea level or at the top of the mast but as long as the boat is entirely stationary (e.g. perhaps at anchor or alongside) then the wind speed and direction are the TRUE wind speed and direction. These two terms are defined as TWS and TWD respectively. In addition, there is the term TWA (True Wind Angle) which is the angle between the boat’s heading and the true wind direction (TWD).
True wind speed and direction will vary dependent on how close to the surface the observer is located. As you go up true wind speed will increase (due to the lessening effect surface friction at ground or sea level will have on the air mass blowing over it). True wind speed and direction might also be affected by headlands and other obstacles, making the true wind back or veer and decelerate or accelerate.
In all cases, the true wind is the actual wind experienced by a stationary observer at a given location and altitude.
What is Apparent Wind?
Apparent wind is the wind speed and direction as experienced by an observer but in this instance the observer need not be stationary. This can make a big difference. For example, consider you are at the top of a small hill. The wind is blowing hard on your back (say at 20 knots true wind speed).
If you were to turn to face the wind and start pedaling down the hill, directly into the wind, you’d notice two things. Firstly, the it’d be hard work! But secondly, and more relevant to this example, as you accelerated into the wind, the wind speed would seem to increase. If you got up to 10 knots in speed it’d feel a lot windier, in fact, the wind you feel would be a combination of the true wind speed that we know is blowing (20 knots) and the speed at which your bike is travelling into the wind (10 knots).
The wind would seem to be blowing at 30 knots, even though it is still only blowing at 20 knots. The 30 knot wind speed is your apparent wind speed.
Given that you’re not stupid, you decide to turn around and cycle away from the wind. This has two benefits. First of all, it is much easier with the wind on your back (not strictly relevant to this example) and secondly, the wind seems to drop! The reality is that the wind is still blowing at 20 knots but because you are now travelling in exactly the same direction as the wind the apparent wind is now less. In fact, it’s 20 knots (true wind speed) - 10 knots (cycle speed in same direction as wind direction) = 10 knots apparent wind speed (AWS).
Of course, we can’t sail directly into wind, with most sailing boats only ever managing to sail at say 30 degrees off the true wind. Therefore, the AWS and TWS will not vary by just your boat speed but will instead be a vector proportion of the boat speed, the boat’s angle to the wind and the true wind speed. Thankfully, as long as properly calibrated, modern wind instruments, along with an accurate log, will calculate True Wind Speed (TWS), Apparent wind Speed (AWS) and True Wind Angle (TWA) and Apparent Wind Angle (AWA) in the blink of an eye.
Why do I need to know the difference?
Whilst the true wind speed and direction are critical when planning a passage or port entry, in reality we will decide our sail plan and course based on AWA and AWS. For example, we can safely hold more sail when sailing downwind on a broad reach, but if we change course and ‘come up’ to wind the AWS will increase as our TWA reduces. This will probably require us to reef or otherwise reduce sail.
Always sail your boat by Apparent Wind Speed and Apparent Wind Angle but keep regular logs of True Wind Speed, course and speed. That way, you’ll have a clear understanding of the environment around you and sail your vessel safely and efficiently. Of course, because of the way your boat’s instruments work, if your log or anemometer is faulty or poorly calibrated the wind speed information calculated will be inaccurate. This is where experience comes into its own.