Apparent Wind vs. True Wind

K3 reaching
If we place the anemometer ashore, then the data on wind speed and direction will give the so-called true wind. However, if we use the same device on a moving ship, the data will be different. The device will then be measuring the apparent wind. And it is precisely this wind that is so crucial for the correct sail trim and why it is so important to us.

How does apparent wind come about? 

It is a combination of wind direction and speed, but also the direction and speed of a moving object. For a better idea, let’s use the example with a steam locomotive. If a locomotive is stationary and no wind is blowing, the steam will rise straight up. However, if the wind is blowing, for example from the right, then the steam will go to the left. But what happens if the locomotive starts moving? In addition to the real wind, the wind created by the movement of the locomotive itself also begins to act on the steam. This is the wind from front to back. The effect of both winds, the real wind and the wind created by the movement of the train, will add up and the steam will be affected by what we call the apparent wind. The steam will not go directly to the left but obliquely. The faster the train travels, the more there will be an apparent wind from the front and the steam will eventually go straight back instead of to the left.  

What does the difference between apparent and true wind depend on? 

The above example shows that the difference between true wind and apparent wind will always depend on our course, and its strength will be different with a tailwind or crosswind. Plus, it changes depending on the speed at which we sail. During the voyage, we don’t change power depending on the direction of the real wind (according to the weather and other influences) but also the apparent wind, which is related to our course and speed, which is constantly changing.

Imagine this 

1) When the wind blows at 10 knots and you are piloting the boat at a speed of 5 knots directly against the wind, the wind will feel 15 knots (10 + 5), i.e. the apparent wind will be 15 knots. It will seem like the wind is blowing strongly.

2) But if the wind is blowing at the same 10 knots but you have a tailwind of 5 knots, the apparent wind only be 5 knots (10-5), because you are travelling with the wind. It will seem that the wind isn’t blowing so much. 

Trim trim trim

So when it comes to really using the boat effectively, we can’t adjust the sails to the current conditions and just take a siesta. Basically, it is necessary to trim constantly, because the conditions that affect the trim are constantly changing. So if you didn’t know it before, you must always trim the sails according to the apparent wind, not the true wind.