Perceived Wind, True Wind, and Surface Wind: Data Required to Calculate Them

Different Raymarine displays can show a variety of wind data types, which can sometimes cause confusion. Therefore, we will explain a bit about what each data type means and what other data is required to calculate and display them. In the Swedish version of this blog post, we have chosen to stick to the English terms for wind as they are established and minimize the risk of misunderstandings.

Angle or direction?

When we refer to a data item as a wind angle (e.g., Apparent Wind Angle, AWA), we mean relative to the bow (0 to 180 degrees port or starboard). When we refer to something as a direction (e.g., Ground Wind Direction, GWD), this is relative to north.

Apparent Wind

Apparent Wind Angle (AWA) and Apparent Wind Speed (AWS) are the standard displayed data on dedicated wind instruments such as i60 and predecessors. This is measured directly by the masthead sensor and does not depend on any external data. Apparent Wind varies depending on your boat's movement. For example, if you motor at 10kn straight into the wind in a 15kn breeze, your AWS will be 25kn and your AWA will be 0 degrees (wind on the bow). On the other hand, if you motor straight with the wind at the same speed, in the same wind, you will see 5kn AWS from the stern (180 degrees AWA).

True Wind

What Raymarine refers to as True Wind (TWA, TWS) is always wind over water, not wind over land (more on this below). We calculate this from AWA and AWS, plus speed through water (STW, from a paddle wheel or equivalent). It is not possible to correctly calculate True Wind from GPS speed (SOG). If you look at your wind instrument and you have AWA and AWS but TWS is displayed as dashes (-.-kn), you have no speed data coming in. In addition to TWA and TWS, our latest multifunction displays and i70/i70S instruments also offer TWD (True Wind referenced to north rather than the bow). This requires compass heading as well as STW. We cannot use GPS COG for this because COG is the direction you are moving in, not the direction you are facing (which is what heading measures). Lighthouse 2 MFDs also require heading to display TWA/TWS.

Ground Wind

Ground Wind is what you would measure standing on the dock, referenced to north (GWS and GWD). This requires AWA/AWS (direct wind measurement), GPS COG/SOG (how the boat moves over land), and compass heading (where the boat/weather vane points, relative to north).

Why is Ground Wind not particularly useful for sailing?

Simply put, your keel is in the water, not on land, so the wind angles you sail to are wind over water, not wind over land. On a lake or in slack water, the biggest difference will be your drift, but in tidal waters or offshore currents, Ground Wind can be very different from True Wind, which is why our sailing-oriented wind instruments only show True Wind (although on faster boats Apparent Wind is probably more useful still of course).

As an example to illustrate the difference between Ground Wind and True Wind, imagine you are sailing due east from Sydney Harbour, Australia, into the wind in an easterly breeze (Ground Wind 090 degrees True) and in the strong south-going East Australian Current of Finding Nemo fame. The first screenshot I have attached is what the situation would be if we had zero current (which is what we would try to sail if we referred to Ground Wind). As you can see, we have two symmetrical tacks to reach our destination. Simple:

Before we start talking about how the wind would be if we sail in what is actually 2+ knots of current going south, let's imagine we have stopped (zero STW) and are drifting south with the current: the wind we measure will seem more southerly than if we were stationary. If we now resume sailing on starboard tack, we will go upstream: if we try to sail close to the wind (45 degrees from the wind to pick a nice simple number) from GW data, we will go on a course of about 045T (NE) as in the first screenshot, while if we sail to TWA we will go on a much more easterly course due to the offset True and Apparent Wind, and actually we will come at least 10 degrees closer to the wind than if you blindly followed Ground Wind data. On port tack (downstream) if we tried to sail to Ground Wind we would go on about 135T, which will actually be close to straight into the wind, TWA, and we would be in the no-go zone. Instead, you would need to steer more like 150T to be close to the wind on port.