In this short sailing courses series you will understand the difference between true wind and apparent wind.One of the difficult concepts to first understand as you learn to sail is the difference between true wind and apparent wind.
True Wind
True wind is the direction the wind is indeed blowing on the planet, as witnessed by flags, smoke, trees bending, etc. If your vessel were standing still, at anchor or securely in the slip, you would measure true wind speed and direction.
The weather report will tell you the true wind speed and direction.
However as we know, true wind is always fluctuating depending upon; weather conditions and turbulence of landmasses, tall buildings, cliffs and mountains, etc. Use true wind speed forecasts as an approximation, but be aware that it can change (shift) dramatically.
Apparent Wind
Here is an explanation that you easily grasp because it relates to something you experience almost every warm summers day.
Put your hand outside the window of your car traveling at 60 miles per hour on a still warm summer day and your hand will feel a 60 mile per hour wind. That’s apparent wind yet the true wind is zero. What if the car was driving into a 20 mile per hour head wind? Your hand would feel 80 mph. Or if the wind was blowing from behind at 20 mph, your hand would feel 40mph.
Now what about a cross wind of 20 miles per hour? Well we need to do a little Pythagorean theorem work on this. What is the square root of the sum of 60 squared plus 20 squared? Your hand would feel 63.24 mph and mostly from a direction in front of the car. If the car accelerated to 100 mph your hand would feel 102 mph, again mostly from the front. If the car decelerates to 10 mph your hand would feel 22 mph mostly from the side of the car and if he car stopped you’d feel the full true wind of 20 miles per hour from the side of the car. What ever your hand feels is the apparent wind. The apparent wind equals the true wind when your car is not moving.
When determining direction of the apparent wind, the faster the car goes the more the apparent wind direction comes from the direction of travel of the car. Again imagine the cross wind. At 1 mph forward speed in your car and a 20 mph cross wind, the apparent wind feels almost like the true wind from across the car. As the car accelerates the wind feels more and more like it is coming from the front.
This is similar to a boat. The faster the boat sails into the wind, the more the apparent wind speed increases and the more it feels like it is coming from the front of the boat.
So now that you understand the difference – let’s put the practical application to work for you on a sailboat.
The minute your vessel is no longer standing still, it creates a wind vector of its own and thus the wind you feel (speed and direction) is altered away from the true wind speed and direction. When sailing, apparent wind information is needed for efficient set of the sails and the information required includes both velocity and direction.
There is fun mathematics associated in determining apparent wind speed, but at this time in your learn to sail quest you are luckily spared this computational exercise – sorry. Or perhaps you’re glad!
One more time for repetition purposes then, the apparent wind speed and direction is the resultant determined by the angle and speed the vessel is sailing and the actual true wind speed and angle. It’s the wind that you feel on your face, or the back of your neck and the one that makes you smile when you’re out in it.
Apparent wind is also the wind that the boat and the sails feel just like your hand outside the car. Thus any telltales on the sails, wind vanes and wind meters are working with the apparent wind direction and speed. Wind meters with an anemometer will also tell you apparent wind velocity. A wind meter can also tell you the true wind speed and direction from a set of computations that the wind meter does inside its electronics. But again – as you’re sailing, you’re mostly likely concerned with the apparent wind. True wind knowledge comes in handy when you begin to do navigation, plotting and course planning.
Imagine 4 sailboats, “A” stopped and pointed into wind. “B” is heading upwind on a close haul at 30 deg off the wind. “C” is headed across the wind on a reach and “D” is headed almost downwind on a broad reach.
As you now know, the true wind condition is independent of boat speed or boat direction and remain unchanged for each boat but the apparent wind varies widely.
“A” is pointed to wind and is therefore stopped. People on “A” feel the wind unaffected by boat speed and so they feel the true wind.
“B” is more like a car headed into wind. Thus on an upwind course, apparent wind is higher and alters in direction to feel like it is coming more from the front of the boat.
Traveling across wind, “C” would feel the largest change in direction but the smallest change in velocity. Again the apparent wind direction feels like it is coming more from the front of the sailboat.
“D” feels a reduction in apparent wind speed. Think about a car driving downwind at 20 mph with a tailing wind at 20mph. You’d feel nothing. However, D also feels the direction of the apparent wind become more from the front of the boat than the true wind.
Thus people on A, B, C and D feel completely different wind speeds and velocities, even though the “true wind” is unchanged. In “D” imagine the boat going a lot faster, the “apparent wind” vector would get much shorter and swing more to the side of the boat.
As a general rule of thumb, when on a boat, if you point towards where you feel the wind is coming from (the apparent wind) then move your finger 15 to 20 degrees towards the stern of the boat, that is where the true wind is coming from. It’s more so going across the wind and less so heading into or away from the wind.
Next time you’re on a boat with an electronic wind meter, toggle between true wind speed indication and apparent wind speed indication. Except standing still, going dead down wind, or headed directly into the wind, in every case, going from apparent to true, the needle will flick more towards 180 degrees – guaranteed. If your headed into wind the true wind speed will be less and if you’re headed down wind the true wind speed will be higher.