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

 If, when calculating True Wind Direction, you substitute SOG (Ground Speed) and COG for boat speed and heading, you will be denying the fact that, while sailing in waters with currents, you are effectively moving. on a “moving carpet”.

Even with the boat stopped and stationary relative to the water, you may still be moving relative to the bottom due to the current! That is where the difference between the Real Wind and the Compass Wind is found. When we calculate the speed and direction of the wind in relation to a point on the background we obtain the Compass Wind. However, when calculating the speed and direction of the wind relative to the water, the result is the True Wind.

While sailing we are more interested in the effect of the wind on the boat than in certain measurements from a purely meteorological point of view, so True Wind is our best option. Sailors constantly convert True Wind to Compass Wind 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 many problems (example: a maxi multihull sailing at 40 knots in a 1 knot current will not be as interested in the effects of the current). But if you are sailing in a more “normal” boat and you are going 8 knots with 1 knot of current, then you should use the boat speed (speed through the water) so that you can better understand the effect of the current on the boat.

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

Sailors should use knots exclusively and have a good understanding of the wind speeds at which a given sail selection works best. It is common to have a simple chart for selecting sails to use, with a table that shows each sail in relation to the True Wind Speed ​​and True Wind Angle.

And that's the end of our advice for this week, but don't miss the next chapter where we will delve a little deeper into the topic of “VMG”, see you soon.