Sailing: True Wind Calculation

Why can’t I use SOG and COG for calculating True Wind?

If you substitute SOG (Speed Over Ground) and COG for Boat speed and Heading in the calculation of True Wind Direction, you are negating the fact that when sailing on waters with tides or currents you are, in effect, travelling on a ‘moving carpet’.

Clearly if you were to take the sails down and stop the boat you may still be moving over the ground because of the tide but you would be stationary relative to the water! This is where the distinction between True Wind and Ground Wind comes into play. The wind speed and direction when measured relative to a fixed point on the ground is given the term Ground Wind, while the wind speed and direction measured relative to the water is given the term True Wind.

When sailing, you are more interested in the affect the wind has on the boat rather than measurements from a purely meteorological point of view, so True Wind is the preferred choice. Racing navigators constantly convert between True Wind and ground wind to determine the accuracy of weather forecasts which use Ground Wind.

If you have a large difference between the speed of the boat and the speed of the tide (tide rate) then it is possible to use SOG without many issues (e.g. a maximultihull travelling at 40kt in 1kt tide is not hugely interested in the tide rate effects), however if you are sailing a more normal boat (say 8kt in 1kt tide) then you should stick with boat speed (speed through the water) so that you have a clear understanding of the effect of tide on your boat.

The units of Wind Speed in instrument systems are usually measured in knots (usually abbreviated kt or kn). However sometimes it is useful to display wind speeds in Beaufort Scale numbers – this allows “at a glance” checking, sometimes preferred by those who cruise and aren’t too interested in the difference between 18 and 19 knots.

Racers should use knots exclusively and get a feel for the wind speeds at which their choice of sails work best. It is common to have a simple sail selection chart on the boat, with a look-up table showing each sail against True Wind Speed and True Wind Angle.

And this is our tip of the week, but don’t miss the next chapter where we will deepen into the of “VMG” – Velocity Made Good -. See you soon!

⇐ Sailing: Displaying Data

⇒ Sailing: What is VMG?