Sailing: Wind

What’s so important about wind information?

Every sailor needs to know which way the wind is blowing from in order to make even the simplest manoeuvres and route calculations. The wind can make a difference between a great journey and a terrible one.

The Wind Triangle

The derivation of wind direction is best described by the “Wind Triangle” which shows the relationship between boat speed, the Apparent Wind (the wind you feel while sailing along), the True Wind (the wind relative to the water surface) and the compass heading of the boat.

True Wind Direction

True Wind Direction is the compass direction of the wind, relative to the water. It is measured from the boat, which may be pointing in any direction and, hopefully, sailing along at reasonable speed. So, in order to get an accurate reading, the wind direction must be calculated from multiple sensor inputs, rather than just from an individual sensor.

Angles vs. Directions

An important distinction when considering instruments, and sailing generally, is that a value that is relative to the boat is normally referred to as an Angle (e.g., Apparent Wind Angle is 35 degrees starboard of the bow). A value that is relative to an external point such as the water or a point on the Earth is a Direction (e.g., True Wind Direction is 315 degrees magnetic).

It is, unfortunately, common to see True Wind Angle referred to as True Wind Direction – even by some instrument manufacturers who should know better. Some have even been known to refer to “Apparent Wind Direction”, you should always be sure of the difference between angles and direction when using your instruments.

Advanced wind calculations

Using Heading data and True Wind Angle to calculate True Wind Direction is normally an accurate enough assumption to be able to monitor wind shifts, but this doesn’t take into account how the boat is travelling through the water.

When heeled over and sailing upwind, the effect of leeway (the amount the boat is slipping sideways) will come into play to some degree. Adding the leeway angle that you are achieving to the vessel Heading gives you the Course. Advanced instrument systems will calculate wind with Course rather than Heading.

Course is the direction that you are travelling through the water. This should not be confused with COG, which is Course Over the Ground.

And this is our tip of the week, but don’t miss the next chapter “Displaying Data” . See you soon

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