Why can't I use SOG and COG to calculate True Wind?

 If you substitute boat speed and heading for SOG (Speed ​​Over Ground) and COG when calculating True Wind Direction, you are denying the fact that while sailing in current-ridden waters, you are effectively traveling on a “moving carpet.”

Even if your boat is stationary and moving relative to the water, you may still be moving relative to the bottom due to the current! That's where the difference between True Wind and Compass Wind lies. When we calculate the wind speed and direction relative to a point on the bottom, we get Compass Wind. However, when we calculate the wind speed and direction relative to the water, the result is True Wind.

While sailing we are more interested in the effect of the wind on the boat than in measurements from a purely meteorological point of view, so True Wind is our best choice. Sailors convert True Wind to Compass Wind constantly to determine the accuracy of weather forecasts, which use Compass Wind.

If there is a big difference between boat speed and current speed it is possible to use SOG speed without too much trouble (example: a maxi multihull sailing at 40 knots in a 1 knot current will not be that interested in the effects of the current). But if you are sailing a more “normal” boat and are doing 8 knots in 1 knot of current then you should use boat speed (speed through the water) so that you can better understand the effect of the current on the boat.

Wind speed is usually measured in knots (abbreviated as kt or kn) on instrumentation systems. It is sometimes useful to display wind speed using the Beaufort scale, especially for those who are cruising and are not interested in the difference between 18 and 19 knots.

Sailors should use knots exclusively and be familiar with the wind speeds at which a particular selection of sails works best. It is common to have a simple sail selection chart, with a table showing each sail against True Wind Speed ​​and True Wind Angle.

And that’s all for our tips for this week, but don’t miss the next chapter where we’ll go a little deeper into the topic of “VMG.” See you soon.