Raymarine True & Apparent Wind calculation

[robheath]

I noticed on my Raymarine Instrumentation that at anchor, in a current, the true and apparent winds are different: If I am at anchor, in a 5 knot tide, then switching between true and apparent wind should make no difference as I am stationary. However the readings seem to change.

I asked Raymarine about this, and their response was:
With regards to the True Wind Calculations, we use Speed Through the Water (STW) to perform this calculation. This is what we refer to as True Wind relative to the Water. We do also show you Ground Wind which uses SOG from the GPS (True wind relative to the Ground), but this is relative to a compass rose and not the vessel Bow.

Surely this is flawed, they should be using COG and SOG-or am I missing something?

 

[Lucky Duck]

As the boat is (presumably) sailing on the water you are interested in the wind strength and direction relative to that rather than what the situation would be on dry land without any tidal influence

 

[noelex]

Surely this is flawed, they should be using COG and SOG-or am I missing something?

No you are making a very accurate observation. "True wind" is refrenced to the water. This is not what most people understand by the term true wind. True wind refrenced to ground is called "Ground wind".

It is a pity more instrument manufactufacturers do not offer the option of displaying ground wind instead of, or as well as true wind. Ground wind tends to be much more accurate. I think it is the better display for crusing boats.

Raymarine should offer the option. All the raw data is present. It should be a simple software fix.

 

[bbg]

No you are making a very accurate observation. "True wind" is refrenced to the water. This is not what most people understand by the term true wind. True wind refrenced to ground is called "Ground wind".

It is a pity more instrument manufactufacturers do not offer the option of displaying ground wind instead of, or as well as true wind. Ground wind tends to be much more accurate. I think it is the better display for crusing boats.

Raymarine should offer the option. All the raw data is present. It should be a simple software fix.

What is the relevance of ground wind on a sailboat?

 

[ShortarseYoda]

What is the relevance of ground wind on a sailboat?

Everything. In essence to resolve apparent wind you need to know the speed and direction of the boat of the earths surface which for a boat is the surface of the water. Using speed through the water to resolve relative wind will not work. Here's an example to simplify matters. Assume there is 10 kts of wind directly on the bow of the boat. The tide is aligned with the wind and the tide is 5 knots. The boat is motoring at 5 kts through the parcel of water so will be standing still, more or less. Ignoring the wind resistance of the boat and other minor effects, the log will be reading 5kts & SOG zero kts. The relative wind will be (wrongly) calculated by Raymarines's logic as 15 kts when it's actually 10 kts.

What troubles more is a scenario, say in the Channel Islands when you could be in a tidal stream within a parcel of water doing 10 kts, in the direction your going and the apparent wind could be 16 kts, 3 wind forces more (due to your sailing or motoring speed) than the true wind yet would only be calculated at 6 kts more.

All that matters is boat speed & direction (it's velocity) whether acted upon by tide, wind, the boats engine, leeway or any other force you care to mention.

 

[noelex]

What is the relevance of ground wind on a sailboat?

Both ground wind and true wind are useful on a sailboat. Ground wind is actually how most sailors would define true wind as the OP has indicated. They don't realise that their instruments are indicating what would be more accurately called "water wind" when the display is set to read "true wind".

Ground wind is more useful than true when answering questions like:

When I get back to my berth how strong will the wind be and will it be blowing from port or starboard?

Does the wind match the forecast? Is the strength and direction what was predicted?

Is the wind strength going up or down?

Will the anchorage I am heading for be protected?

Etc etc.

The big advantage of ground wind is that it is not affected by paddlewheel fouling/poor placement/poor calibration. So it is always accurate. If the windspeed does not change it will read the same upwind or downwind and on both port and starboard tack. Thus it is easy to see if the wind is picking up or dying down or if it changing direction. This, combined with the important apparent wind display tells most cruising sailors what they need to know.

True wind is also great to have, but your paddlewheel needs to read accurately at all points of sail.

The ideal is to have instruments that can be switched to display true wind or ground wind. In the days before GPS, ground wind was impossible to calculate and some instrument manufacturers like Raymarine have been slow to incorporate this option.

If nothing else, the option of switching to ground wind is useful if your paddlewheel becomes fouled during a voyage.

 

[lw395]

To get ground wind properly, you really need a compass sensor.
The wind direction sensor is relative to heading not COG.

People need to be realistic about what they want from all this stuff.
Ground wind is useful if you are on a long reaching leg, you can compare what you're getting with the forecasts and see long term back/veer.
On a beat, you know the real true wind, short term, is 40 or so degrees above your heading.
That is likely to be more accurate than what the Raymarine box will tell you.

Bear in mind that this magical single vector that is 'True Wind' doesn't really exist. The windspeed will be different at masthead from deck level, and the direction may differ too.
It's also not constant. So there's always a lot of averaging going on and errors creep in.

I've always found on boats with lots of expensive Raymarine (and other) kit, you can be sailing along with a fairly steady true wind indicated, you alter course and the indication changes.
I've never been on a boat where it's been calibrated well enough to stack up.
Only one exception to that. A certain race committee boat. They have the advantage that it only ever motors at 6 knots or anchors and it doesn't heel very much.

It's like any other data.
Only any point gathering it if you are going to use it.
And if you're going to use it, it needs to be good enough for that use.
So wha do we really want from it all?

 

[lpdsn]

Ground wind is actually how most sailors would define true wind as the OP has indicated. They don't realise that their instruments are indicating what would be more accurately called "water wind" when the display is set to read "true wind".

Most sailors would have learnt about true wind when learning to beat to windward efficiently, so would consider the water referenced calculation as true wind. And of course, ground wind calculations have only been possible since accurate GPS came along.

Apart from anything else, true wind and ground wind seem to be the established terms.

 

[lw395]

The tide is generally up to 2, maybe 3, knots. 5 knots is extreme tide.
The wind varies by more than that second to second.
If you move for instance out of the main tide in the western solent, you mght lose 3 knots of tide at full ebb.
But your true ground wind will likely change by more than that.

True wind relative to the boat has its uses.
Ground wind has different uses.

 

[noelex]

Apart from anything else, true wind and ground wind seem to be the established terms.

Yes, I have no illusions that the established terms will change, but I think the term "true wind" is misleading.

For example. Try this quiz:
A boat that was tied up to a marina leaves and enters a channel. The wind in the channel is blowing exactly the same strength and direction as in the marina, but there is some current in the channel.

There will be a change in the apparent wind in the channel caused by the boat's motion through the water (if any) and the current.

Does the true wind change?

The correct answer is:
Yes.

Despite the wind blowing exactly the same speed and direction in the channel the true wind has changed. Even perfectly calibrated correctly reading instruments will indicate a change in true wind. As there is no change in the wind speed and direction ground wind will be unchanged and instruments reading ground wind will reflect this.

I think many would see instruments reading ground wind, showing no change, as more accurately indicating the true state of the constant wind in this example.

 

[Bru]

Raymarine should offer the option. All the raw data is present. It should be a simple software fix.

They do on the plotters

And have done for yonks - my legacy C80 plotter can display apparent, true and/or ground wind as could the A50 on our previous boat

You won't find ground wind on the older instrument heads such as the ST50/ST60. The legacy ST70+ does offer all three but the current range of heads do not (I'm a little surprised the multifunction i70 doesn't) but then they're aimed at sailing where ground wind isn't all that useful anyway

 

[noelex]

And have done for yonks - my legacy C80 plotter can display apparent, true and/or ground wind as could the A50 on our previous boat

You won't find ground wind on the older instrument heads such as the ST50/ST60. The legacy ST70+ does offer all three but the current range of heads do not (I'm a little surprised the multifunction i70 doesn't) but then they're aimed at sailing where ground wind isn't all that useful anyway

Ok thanks.

It is good to know Raymarine are making this available in some instruments at least.

 

[Triassic]

I sometimes wonder if we as sailors are becoming far to preoccupied with instrumentation and as a result forgetting who is actually making the decisions. It is important to understand how our electronics gather and interpret information however ultimately we all have an interface that oversees all of their decisions, it's called our brain. Delegating this function to something electronic will ultimately lead to the brain not being updated with the latest software..... In the example given by the OP if you're sitting on your boat in that position why on earth would you look at true wind speed. You know you're stationery, use apparent!

Pretty much the only time I ever look at the true wind speed on my instruments is when I am sailing fast off the wind and I need to make sure I'm not exceeding the parameters for being able to round up, but having said that you're only really looking for reassurance.

 

[flaming]

There are very good reasons to use True Wind, but in my experience ground wind is something of passing interest only.

Remember, your boat does not derive its motive force from the speed of the wind over the ground. It derives its motive force purely and simply from the difference in velocity between the water and the air. Thus we see it is entirely possible for a sailing dinghy to cross a fast moving river in a flat calm.

For example, a racing boat heads out for the day. Being a good racing boat they have some polars that tell them how fast they should be going upwind and downwind in a given true wind speed. If they are sailing on a lake then ground wind and true wind are the same, so no matter. If however they are sailing on the sea, it's entirely possible that there is a 2 knot tide. So lets say that the ground wind is 8 knots. When the tide is running in the same direction as the wind your boat is also being swept along at 2 knots, so you must subtract the tide from the ground wind, and see that you are only sailing in 6 knots of true wind.
When the wind and tide are opposed you are still being swept along at 2 knots so you must add the tide to the ground wind and see that now you have 10 knots of wind. Quite a difference!
It's easy to see that in either one of those cases your polars would be way off if you were relying on your ground wind instruments, and you would not know if you were up to speed, as speed potential in 6 knots true and 10 knots true is very different.

And lets imagine that a cruising boat is on passage. It's one of those delightful summer days where the skipper of our big heavy cruising boat wants to sail, but the wind is light so he is motoring. Now our skipper knows that when the wind reaches 8 knots he has enough wind to sail at a speed that won't frustrate his semi-willing passengers. Reading his ground wind instrument he sees that he has 8 knots, so he starts disturbing his sunbathing passengers and unrolling sails and generally fussing with things. However, as he is a good skipper and has planned his passage to be both downwind and to take advantage of the 2 knot tide, he discovers that the 8 knot wind his instruments are showing is not moving his boat very fast at all. Cue lots of fiddling etc before he admits defeat and puts the donk back on.
If his instruments had been set to true wind (or if he really understood what his instruments were telling him) he would have known that he only had 6 knots of wind to sail in.

In the example given by the OP if you're sitting on your boat in that position why on earth would you look at true wind speed. You know you're stationery, use apparent!

Ah yes, don't forget to do a bit of mental maths to work out what will actually happen to the wind you're sailing in once you're floating free. I've seen it many times in the Solent when you record 15 knots at anchor in Newtown bay, so decide to snug down a reef before you leave. Only to realise that the 3 knot spring tide has left you underpowered in 12 knots. Or vice versa!

And this is before we even start to think about the effect the tide can have on the wind direction when the tide and wind do not line up.

 

[lw395]

Yes, I have no illusions that the established terms will change, but I think the term "true wind" is misleading.

For example. Try this quiz:
A boat that was tied up to a marina leaves and enters a channel. The wind in the channel is blowing exactly the same strength and direction as in the marina, but there is some current in the channel.

There will be a change in the apparent wind in the channel caused by the boat's motion through the water (if any) and the current.

Does the true wind change?

The correct answer is:
Yes.

Despite the wind blowing exactly the same speed and direction in the channel the true wind has changed. Even perfectly calibrated correctly reading instruments will indicate a change in true wind. As there is no change in the wind speed and direction ground wind will be unchanged and instruments reading ground wind will reflect this.
.....

But that's not a true situation, more of a Dazed Kipper Excerise Fiction.

 

[Triassic]

Ah yes, don't forget to do a bit of mental maths to work out what will actually happen to the wind you're sailing in once you're floating free. I've seen it many times in the Solent when you record 15 knots at anchor in Newtown bay, so decide to snug down a reef before you leave. Only to realise that the 3 knot spring tide has left you underpowered in 12 knots. Or vice versa!

And this is before we even start to think about the effect the tide can have on the wind direction when the tide and wind do not line up.

But that is exactly my point, surely even the most basic of sailor should understand that and be able to carry out those calculations in their head? I suspect I'm starting to sound like a bit of a patronising know it all now but if we continue to replace all the thought processes and knowledge that we should possess as sailors with automated instrumentation that just gives us the end product then we de-skill the individual and eventually things become unsafe.

 

[lw395]

But that is exactly my point, surely even the most basic of sailor should understand that and be able to carry out those calculations in their head? I suspect I'm starting to sound like a bit of a patronising know it all now but if we continue to replace all the thought processes and knowledge that we should possess as sailors with automated instrumentation that just gives us the end product then we de-skill the individual and eventually things become unsafe.

For what they're worth.
For ever occasion they give the right answer, I bet there's two when you leave the bay and find the true wind is rattling up the Solent a fair bit harder once you're clear of the headland.

Where a true wind calculation would be really useful is when you're sailing a fast boat downwind with a big kite.
You accelerate, but is that an increase in wind speed or a change in direction?
I find it's not always easy to tell.
Unfortunately, we don't have instruments on asymmetric dinghies.
True and apparent can be a long way apart sometimes!

Perhaps every sailor should get a ride on a land yacht or ice yacht?

 

[Triassic]

Where a true wind calculation would be really useful is when you're sailing a fast boat downwind with a big kite.
You accelerate, but is that an increase in wind speed or a change in direction?
I find it's not always easy to tell.

You do very rapidly get a feel for that and if you are watching the apparent it's a dead giveaway. I always find it amusing when I get someone on board who isn't used to multihulls and they are trying to work out why the apparent is going forward even though they are bearing away.....

 

[lpdsn]

In the example given by the OP if you're sitting on your boat in that position why on earth would you look at true wind speed. You know you're stationery, use apparent!

Actually, I'd probably just look at the surface of the sea.

 

[bbg]

Everything. In essence to resolve apparent wind you need to know the speed and direction of the boat of the earths surface which for a boat is the surface of the water. Using speed through the water to resolve relative wind will not work. Here's an example to simplify matters. Assume there is 10 kts of wind directly on the bow of the boat. The tide is aligned with the wind and the tide is 5 knots. The boat is motoring at 5 kts through the parcel of water so will be standing still, more or less. Ignoring the wind resistance of the boat and other minor effects, the log will be reading 5kts & SOG zero kts. The relative wind will be (wrongly) calculated by Raymarines's logic as 15 kts when it's actually 10 kts.

Your example is wrong. The boat speed will be 5 knots through the water. The apparent wind speed will be 10 knots. The instruments will calculate that you are motoring at 5 knots into a 5 knot headwind, giving a true wind speed of 5 knots. That is correct.

I take the point that there might be some interest in knowing ground wind speed when you are in one tidal current and you will be sailing in something significantly different, but I can't think of any time when I am actually out sailing that I really care what the wind is doing in relation to the sea bed. I care only what it is doing in relation to the water.

That is what TWS gives you.