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Apparent Wind, True Wind and Ground Wind, and data required to calculate - Chuck - Raymarine - Moderator - 06-01-16 10:59 AM
Various Raymarine displays are capable of displaying a variety of Wind data types, which sometimes cause confusion, so we'll explain a little about what each data type is and what other data is required in order to calculate and display.
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 default displayed data on dedicated wind instruments such as i60 and predecessors. This is directly measured by the masthead sensor and doesn't depend on any external data. Apparent Wind will vary depending on your boat's motion. As examples, if you're motoring at 10kn dead upwind into 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're motoring dead downwind at the same speed, in the same wind, you'll see 5kn AWS from astern (180deg AWA.)
True Wind
What Raymarine refers to as True Wind (TWA, TWS) is always wind-over-water, not wind-over-ground (more on this below.) We calculate this from AWA and AWS, plus speed-through-water (STW, from a paddle-wheel or equivalent.) It's not possible to calculate proper True Wind from GPS speed (SOG.) If you're looking at your wind instrument and you have AWA and AWS but TWS is showing as dashes (-.-kn) then you don't have Speed data coming in. In addition to TWA and TWS, our recent 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 can't use GPS COG for this because COG is the direction you're moving, not the direction you're facing (which is what Heading measures.) Lighthouse 2 MFDs also require Heading in order to display TWA/TWS.
Ground Wind
Ground Wind is what you'd measure standing on the dock, referenced to north (GWS and GWD.) This requires AWA/AWS (direct wind measurement), GPS COG/SOG (how the boat is moving over the ground) and compass Heading (where the boat/wind-vane is pointing, relative to north.)
Why is Ground Wind not very useful for sailing?
Put simply, your keel's in the water, not on the ground, so the wind angles you sail to are wind-over-water, not wind-over-ground. On a lake or at slack water the main difference will be your leeway, but in tidal waters or in 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 and true wind, imagine you're sailing straight out east from Sydney Harbour, Australia, upwind into an easterly breeze (Ground wind 090 degrees True) and in the strong south-setting East Australian Current of Finding Nemo fame.
The first screenshot I've attached is the no-current situation (what we'd be trying to sail if we were referring to Ground wind.) As you can see, we have two symmetrical tacks to reach our destination. Simple.
Before we start talking about what the wind would be like if we're sailing in what is actually 2+ knots of current heading south, let's imagine we've stopped (zero STW) and are drifting south with that current: the wind we measure will appear more southerly than if we were stationary.
If we now resume sailing on the starboard tack we’ll be heading up-current: if we try to sail close-hauled (say 45deg off the wind) from GW data we’ll go on a heading of about 045T (NE) as in the first screenshot, whereas if we sail to TWA then we'll head in a much more easterly course because of that offset True and Apparent wind, and in fact we'll get at least 10 degrees closer to the wind than if you were blindly following Ground wind data. On the port tack (down-current) if we tried to sail to Ground wind we'd head off on about 135T, which in fact will be close to dead upwind, TWA, and we'd be in the no-go zone. Instead you’d need to steer more like 150T in order to be close-hauled on port.
For a viewpoint from another electronics manufacturer, try here.
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