Garmin wind inst needs speed through the water input to give true wind direction. Why?

[bedouin]

Yes, I did use that example, because it shows why you're wrong in a very clear example. You didn't give any examples, just more waffle why the "accepted wisdom" is what you believe, none of which explains anything at all. Yes, I do sail, and real true wind tells me a lot more than the almost random concoction you're making with forward speed in the water and apparent wind. Unfortunately sailors don't often think about what they've been told so it's sometimes hard to break habbits.

So why not give us a real example of where GWS influences how you sail your boat?

It is a genuine question. I find TWS very important and haven't yet come across a situation where I would use GWS - am I missing something?

 

[Wing Mark]

Ground wind speed, when it's significantly different from TWS would only be of interest if I expected the wind to stay the same and the tide to change.
Maybe if I'm sailing a dinghy in the deep channel in Chichester Harbour, I'll be aware that wind against tide is increasing the wind speed.
So I might think that the ground wind id what I'll have when I leave the deep water and hit the calm shallows.
But the reality is a couple of knots of tide varies the wind by up to 2 knots, but the ground wind (and every other sort of wind) varies by more than that every few minutes and every few boat lengths.

As a dinghy sailor, I'm used to having one instrument, a compass. That tells me about changes in the true wind direction, when I'm sailing upwind.
In one sense that's 99% of what you need to know about the wind from instruments, you can tell the strength from how much you lean over or your arm aches.
In another point of view, you need to have a feel for how the tide is affecting your boat and how that might change as you move around the course.
So you need a grip on all the vectors.

I think sometimes different sailors add the vectors up in a different order, the end result is they make the same decision using the same information when you boil it down?

Nobody's mentioned the 'lee bow effect' yet!

 

[lustyd]

So why not give us a real example of where GWS influences how you sail your boat?

It is a genuine question. I find TWS very important and haven't yet come across a situation where I would use GWS - am I missing something?

We're still waiting for you to explain your own position rather than waffling and deflecting from the fact that you don't have a real explanation. GWS is useful because it's actually the speed and direction of the wind, everything else can be deduced from that. Wind speed derived from speed through the water is genuinely a meeningless number which was only ever used due to limited technology, it's not actually useful in any way whatsoever.

 

[lustyd]

will both be 4kn from the north and most definately not from the south as you state.

I said the wind direction would be due South since the boat is travelling due north. Nothing at all wrong with that statement.

as an experienced sailor I will be using the tide to assist my passage so will be happy to see this number and use the sails to tack into the true 4 kn northerly wind

As an experienced sailor you'd be aware of the tide so you'd know to add that to the real wind speed. If you weren't aware of the tide speed then your version of true wind would be useless as I described.

 

[Wing Mark]

How often do you believe the windspeed your instrument gives to +/- 2 knots?

Personally I will concentrate on the direction, as indicated by where my boat heads close hauled.
That's 'True Wind'.
I will make decisions on when to tack, based on how that changes.


For longer passages, IMHO, it's less about measuring the wind and more about having a picture of how the direction is expected to change over time.

You can get 'ground wind ' data from the likes of Bramblemet, and the airfields on XC weather, but how often does what you see on there tell you anything which would change with a tide vector added?

What are people actually doing with the ground wind or true wind information they have?


When you 'measure' ground wind direction on your yacht, what are the possible errors? It's a very 'processed' number, whereas True Wind is 42 degrees up from the compass reading.

 

[lustyd]

I will make decisions on when to tack, based on how that changes.

This makes some sense if the current/tide definitely won't change so I can see why it might be useful in a short race. For cruising though I like to just know what the wind speed and direction is, and now we have the tech that's possible. Real wind tells you what the weather is doing, not just when to tack. When to tack can easily be derived from real wind if you're aware of the current.

 

[johnalison]

OK take the example of your boat drifting with the tide on a completely calm day at 4kt due North. Apparent wind will be 4kt due South, and using STW true wind will also be 4kt due South. How is that useful? You can't turn South and sail against the current, you'd still be going North at 4kt. Using SOG the true wind will show 0kt which tells you a lot of useful stuff, while the apparent wind indicates what the tide is up to.

Please give an example of where the other number is genuinely useful.

It is not quite true that you couldn't sail against the current. In your example a boat drifting would be going north at 4kn against what showed on its instruments as a 4kn northerly wind. This wind would be enough for a boat to beat northwards at another knot or two, to some advantage. There are also craft that can sail downwind at a speed greater than the wind. This might not be practical for those of us who don't own an Anderson 22, but it is possible, meaning that an indication of such a wind would be useful to the sailor.

As another example, I might be sailing with a following wind in such a 4kn tide, both with me. My wife will be nagging me to take in a reef, and if she sees your ground wind speed of 30 kn her fears will be confirmed. On the other hand my TWS meter merely shows a balmy 26 kn, well within the capability of my boat downwind, with an apparent wind of only 19 kn or so. In practice though, I seldom use TWS when actually sailing since I need apparent for sail setting and the mental arithmetic to find true wind approximately is not hard.

 

[srm]

As an experienced sailor you'd be aware of the tide so you'd know to add that to the real wind speed. If you weren't aware of the tide speed then your version of true wind would be useless as I described.

I do not understand the point you are trying to make. You do not understand my use of standard navigation terminology.
So nothing more to say.

 

[John the kiwi]

Its all about frames of reference:
A boat that is just sailing in an environment where for the set of sails and so on the only relevant factors are wind speed and direction with respect to the water. If the tidal current was a steady 10 knots SOG and the wind was 30 knots SOG in the same direction then the boat would be sailing on all points as if there was a 20 knot breeze. You could set a racing course of untethered buoys and have a good race around them.
The problems arise as soon as the buoys are anchored. Now we are sailing to try and achieve a direction and distance comparative to fixed ground positions. For setting the sails the situation is unchanged - its still a 20 knot breeze, and your apparent wind will still be 20 knots plus or minus your speed towards or away from it. Problem is that unless you have the proverbial A22 or a foiling catamaran, you may struggle to tack upwind with a positive Velocity Made Good over ground.
So knowing the wind speed and direction with respect to the body of water the boat is what is needed for the setting of sails.
Knowing what the boat speed over ground is important from a navigation point of view.
Knowing that the wind speed over ground is 30 knots is really only of academic interest.

Lee bowing is also a thing related to sailing in a body of water that is moving with respect to your desired destination. What you are doing when lee bowing is to possibly sail less than optimally with respect to the moving body of water to minimise your distance over ground to your target.

Fuse lit - now running away......

 

[lustyd]

This wind would be enough for a boat to beat northwards at another knot or two, to some advantage. There are also craft that can sail downwind at a speed greater than the wind.

My point wasn’t that the boat couldn’t sail, but that with one setup the skipper is falsely told there’s 4kt wind and the other correctly told there is zero wind. You can’t sail up tide with zero wind.

Its all about frames of reference:
A boat that is just sailing in an environment where for the set of sails and so on the only relevant factors are wind speed and direction with respect to the water.

I think you’re right. When racing it doesn’t matter because you’re not going anywhere. When cruising you’re travelling somewhere on the ground so it does. Cruising you’re also more likely to see changes in current and tide which would show as unstable winds if using the STW, and that’s assuming the paddle wheel even gives useful info. In all situations apparent wind is what the sails set to.
Ultimately I think some of us are making skipper decisions and some helm based on the readout and I can see why the helmers like a more apparent wind number.

 

[Wing Mark]

I'm not really sure why a cruising sailor needs any complicated wind calculations at all TBH.

 

[Moodysailor]

AWS and TWS are both handy at different times is my observation. AWS has more benefit for me when the wind is in front of the beam, and TWS I find more useful when wind is abaft. Both are good to know. As others have said - not too difficult to work out, but I don't see the need when our instruments do it more accurately and faster than I can.

But what has really impressed me is the automatic sail steer vectors you get when it all works together on a course. Takes what I used to do by feel, experience and calculation a millisecond - and constantly adjusts. FWIW, I was not as good as our Axiom+ at calculating the best tacking vectors on our previous boat

 

[Wing Mark]

My point wasn’t that the boat couldn’t sail, but that with one setup the skipper is falsely told there’s 4kt wind and the other correctly told there is zero wind. You can’t sail up tide with zero wind.
..

But in your example, the boat has 'apparent wind' and can sail across the moving body of water.

If true wind is zero, your boat is dead in the water.
But I think you chose a bad example to support your POV.
Such extreme cases don't help, everyone's normal modes of sailing are failing in these cases.

A more useful example.
Wind and tide in same direction
You're doing a cross channel race and about to round a mark, from a broad reach to close hauled.

Which wind tells you what genoa to set at the mark?

 

[Moodysailor]

Which wind tells you what genoa to set at the mark?

It depends if you are sailing a skiff, a cruising mono, or a fast cat ?

For us we just had a roller furler....

 

[johnalison]

My point wasn’t that the boat couldn’t sail, but that with one setup the skipper is falsely told there’s 4kt wind and the other correctly told there is zero wind. You can’t sail up tide with zero wind.

Your peculiarity is in supposing that the information is false. If the boat is experiencing 4kn of air pressure, then it is in reality subject to 4 kn of wind, and that is the wind it will be sailing to. Sailing on your boat must be a strange, even hallucinatory experience. While the rest of us are happily sailing to and fro, you are engaged in the exercise of trying to work out what the wind might be doing if the sea had evaporated.

 

[srm]

The bottom line is the wind / air pressure on the sails - the strength and direction of the apparent wind. The essential is a burgee or other wind direction indicator plus the feel of the wind on your skin. The boat will tell you if there is too little or too much wind for the sails set.
All the rest may be nice to know or an academic distraction. Either way the information is not essential for a cruising sailor but possibly useful when racing for tactical reasons.
I have sailed quite a few thousand miles without wind instruments, and only use wind direction relative to the boat when instruments are available. The boat tells me the rest.

 

[lustyd]

But in your example, the boat has 'apparent wind' and can sail across the moving body of water.

But only in one direction, which is why it was a great example because it actually does apply when sailing too but I took that out of the equation to make the point that it actually is useful to know what the wind is actually doing.

 

[Buck Turgidson]

the bottom line is the difference in the dynamic pressure on the sail vs the dynamic pressure on the keel. That gives the boat performance and is what the polars for your boat are measured from. However, as someone who uses wind in my professional career. True wind for me is over the ground with reference to true north as apposed to magnetic which we only use for take off and landing. For sailing I like to know where I am with respect to the weather patterns when on passage therefore I set my instrument to show that using SOG rather than speed through the water. I use apparent wind and speed through water to judge if I'm sailing well.

"Ground wind" is a label used by some manufacturers who already used True wind for Sailing wind so needed another name.

 

[lustyd]

Your peculiarity is in supposing that the information is false. If the boat is experiencing 4kn of air pressure, then it is in reality subject to 4 kn of wind, and that is the wind it will be sailing to. Sailing on your boat must be a strange, even hallucinatory experience. While the rest of us are happily sailing to and fro, you are engaged in the exercise of trying to work out what the wind might be doing if the sea had evaporated.

The thing is that most of us don't merrily sail to and fro, we're going somewhere. Knowing what wind there actually is helps with that because it informs the direction in which one can sail and therefore where one can go. The information is false in the example because there isn't any wind, yet both AWS and traditional TWS say there is 4kt wind because there isn't any flow over the paddle wheel. As such you can't do anything with that wind if you turn around, and with real wind speed you'd know that without first turning.

 

[johnalison]

The thing is that most of us don't merrily sail to and fro, we're going somewhere. Knowing what wind there actually is helps with that because it informs the direction in which one can sail and therefore where one can go. The information is false in the example because there isn't any wind, yet both AWS and traditional TWS say there is 4kt wind because there isn't any flow over the paddle wheel. As such you can't do anything with that wind if you turn around, and with real wind speed you'd know that without first turning.

I take it that we are still on your own electronically-equipped raft drifting north in the Alderney race at 4kn. The information you gave in #13 is confusing because the ‘wind’ experienced is actually a northerly wind, but your boat’s heading has no effect on whether or not you are going to be able to sail. If when drifting the boat feels a 4kn wind pressure, it will be able to sail in any direction, just as it would with a 4kn wind in still water with no tide. If perfectly accurate, the instruments would continue to record a TWS of 4kn from the north, whatever the boat speed. With this information, the sailor will know how and in which directions he or she can sail.

Armed with only ground wind speed (zero), it will be impossible to know that sailing is possible, if slowly, without information about the tide at that point, which, being only from a prediction usually, will be only approximate.