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

[fredrussell]

I may have this all wrong, but I’ve been looking at the Garmin GNX wind instrument and their website says “for true wind info, speed through the water and heading inputs are required”.

I understand why heading input is required, but why is speed through the water required, rather than speed over ground?

One kit comes with a thru-hull transducer. The wind instrument has no knowledge of tidal factors affecting boat speed, surely? I just can’t get my head around it.

 

[paulajayne]

If the boat is moving then it will not give true wind speed.

5 knot wind on bow with boat moving at 5 knots will indicate 10 knot wind.

 

[bedouin]

I may have this all wrong, but I’ve been looking at the Garmin GNX wind instrument and their website says “for true wind info, speed through the water and heading inputs are required”.

I understand why heading input is required, but why is speed through the water required, rather than speed over ground?

One kit comes with a thru-hull transducer. The wind instrument has no knowledge of tidal factors affecting boat speed, surely? I just can’t get my head around it.

That is the usual definition of true wind speed - the wind speed relative to the water. May seem counter-intuitive but consider most people use it to understand how well they are sailing and in those circumstances it is speed of wind relative to the water that matters.

 

[srm]

If you use ground speed/GPS speed you are adding the tide or current vector in to the equation. As said above for sailing it is wind speed relative to the water surface that effects the sails, both for wind angle and strength.

If you sail around some of the Scottish or Channel islands you can get tide vectors up to around 8 or 10 knots. On a day of zero wind speed over the ground you could be comfortably sailing to windward while in the tide stream with a true wind equal to the tidal speed. A meteoroligist will record a true wind of zero at a ground station but the sailor experiences a 'true' wind over the water of say 5 knots. I have done this many times, using 'the tide wind'.

Its just a case of which surface we use as reference for the wind speed.

 

[RobF]

Interestingly, both factors (heading and speed over the water) mean that the True wind is based on the water having no lateral movement .
If we were to measure its direction and strength based on ground, we would have to use both COG and SOG rather than Heading and SOW.
I presume that, unless you're in the Alderney race, the tidal streams won't make a material impact to the overall true wind and direction.

(and whilst we're on the topic, has anyone else sailed with someone who insisted on recording the wind speed in precise degress and knots (e.g. 229deg, 14.5 knots)

 

[john_morris_uk]

Interestingly, both factors (heading and speed over the water) mean that the True wind is based on the water having no lateral movement .
If we were to measure its direction and strength based on ground, we would have to use both COG and SOG rather than Heading and SOW.
I presume that, unless you're in the Alderney race, the tidal streams won't make a material impact to the overall true wind and direction.

(and whilst we're on the topic, has anyone else sailed with someone who insisted on recording the wind speed in precise degress and knots (e.g. 229deg, 14.5 knots)

Even 2 or 3 kn of tide can make a significant difference in light airs. 2kn of tide is very common around the English Channel.

 

[davidaprice]

I sail in waters (the gulf of Finland) where tide is not a factor. Towards the end of the season when fouling is making the log paddle wheel tend to stick, I have a second page on my instrument panel which shows GPS speed instead of log speed. It's annoying that there aren't alternatives to True Wind Speed and True Wind Angle calculated from GPS speed, it would be trivial to do. (Mostly this affects us when we're motoring because the wind dropped, and we can't see if the wind has risen again enough to stop the engine and sail again).

 

[fredrussell]

Ah ha! Yep, got it now, thanks. I’ve just had a massive revelation too - I now know why when I go sailing expecting, say, a nice close-hauled sail up the Orwell, I may well end up tacking instead. It’s because the weather forecast gives true wind direction whereas my boat sails to the apparent wind direction. Oh well, only took me ten years to work that one out.

 

[johnalison]

I sail in waters (the gulf of Finland) where tide is not a factor. Towards the end of the season when fouling is making the log paddle wheel tend to stick, I have a second page on my instrument panel which shows GPS speed instead of log speed. It's annoying that there aren't alternatives to True Wind Speed and True Wind Angle calculated from GPS speed, it would be trivial to do. (Mostly this affects us when we're motoring because the wind dropped, and we can't see if the wind has risen again enough to stop the engine and sail again).

Clean the paddle wheel. My experience in the Baltic, though not Finnish waters, was that very little fouling occurred, but retracting the paddle occasionally shouldn’t be a hassle.

You are right though that TWS is most useful when motoring, or motor-sailing perhaps, when it would be my usual choice, for the sort of reason you gave. I can’t think of a good reason to use it when sailing. With an increasing following wind, it is not hard to add your boat speed to the apparent wind speed. I find that changing to TWS is undesirable as it tends to frighten the crew if they see 30 knots on the dial.

 

[bedouin]

I sail in waters (the gulf of Finland) where tide is not a factor. Towards the end of the season when fouling is making the log paddle wheel tend to stick, I have a second page on my instrument panel which shows GPS speed instead of log speed. It's annoying that there aren't alternatives to True Wind Speed and True Wind Angle calculated from GPS speed, it would be trivial to do. (Mostly this affects us when we're motoring because the wind dropped, and we can't see if the wind has risen again enough to stop the engine and sail again).

There are alternatives - called Ground Wind Speed / Angle that I know some instruments support. Not sure how common it is

 

[Wing Mark]

Ah ha! Yep, got it now, thanks. I’ve just had a massive revelation too - I now know why when I go sailing expecting, say, a nice close-hauled sail up the Orwell, I may well end up tacking instead. It’s because the weather forecast gives true wind direction whereas my boat sails to the apparent wind direction. Oh well, only took me ten years to work that one out.

No, the weather forecast gives ground wind.

It's all about naming conventions.
Apparent wind is wind relative to the boat, what you measure at the masthead, no other inputs required.
True wind is generally agreed to mean wind relative to the water.

Ground wind is a term for wind relative to the earth's surface.

So ground wind and true wind will be different due to any current.

Bear in mind that outside the sailing community, other people may use names differently.


However, I suspect the real reason you end up tacking up the Orwell is often that the wind direction is affected by the landscape, it tends to funnel up or down the river.
'True Wind' is not the same everywhere. It shifts, bends and shears. The true wind is different at deck level compared to the masthead etc etc.

 

[Bodach na mara]

I worry about people who believe the weather forecast! They can give a guide to expected weather, including wind, but you must expect some uncertainty, in both direction and speed of the wind.

Several years ago I was persuaded to sail to Campbeltown to try going round the Mull of Kintyre by someone who showed me how perfect the conditions would be according to the app on his iPad. Well the north west wind that should have taken us down to Campbeltown turned out to be a south easterly so I was not surprised to wake the next morning to find that instead of a sunny day with light south west winds it was p1ssing down in a strong northerly. When it calmed down in the evening the skipper's of all boats in the fleet opted to move to Portavadie and go through the canal instead.

 

[lustyd]

I have mine set to SOG on my B&G. I want to know the actual speed and direction of the wind rather than some arbitrary number people used to use due to limited technology. I've yet to see a good justification for the continued use of STW here, especially given how ropey the STW data is on most boats. Having two values for apparent wind is no more useful just because one of them knows the speed of the vessel. It doesn't tell me what would happen if I turn 180, whereas the actual real wind speed and direction does, and that only comes through SOG.

 

[johnalison]

I have mine set to SOG on my B&G. I want to know the actual speed and direction of the wind rather than some arbitrary number people used to use due to limited technology. I've yet to see a good justification for the continued use of STW here, especially given how ropey the STW data is on most boats. Having two values for apparent wind is no more useful just because one of them knows the speed of the vessel. It doesn't tell me what would happen if I turn 180, whereas the actual real wind speed and direction does, and that only comes through SOG.

Horses for courses, or is it the other way round? I want to know what forces are acting on my boat. My frame of reference is the water I am sailing or motoring on, which is why I need the TWS related to my boat speed. I don't care what the ground is doing; that belongs to the navigation department.

 

[bedouin]

I have mine set to SOG on my B&G. I want to know the actual speed and direction of the wind rather than some arbitrary number people used to use due to limited technology. I've yet to see a good justification for the continued use of STW here, especially given how ropey the STW data is on most boats. Having two values for apparent wind is no more useful just because one of them knows the speed of the vessel. It doesn't tell me what would happen if I turn 180, whereas the actual real wind speed and direction does, and that only comes through SOG.

If you had a sailing boat you would know why True wind is so useful and Ground wind almost useless

 

[lustyd]

Horses for courses, or is it the other way round? I want to know what forces are acting on my boat. My frame of reference is the water I am sailing or motoring on, which is why I need the TWS related to my boat speed. I don't care what the ground is doing; that belongs to the navigation department.

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.

 

[lustyd]

If you had a sailing boat you would know why True wind is so useful and Ground wind almost useless

If you sailed you'd see why my post #16 proves this to be nonsense.

 

[bedouin]

If you sailed you'd see why my post #16 proves this to be nonsense.

So the example you use is when the boat is drifting with the tide - that is not sailing in most people's definition. Yes of course if you are stationary in the water TWS = AWS and there is no need to distinguish.

For sailors who sail on the water then it is the speed of the wind relative to the water that matters. Close hauled TWA tells me my tacking angle and therefore what course I will be tacking onto. TWS and TWA are used to assess how well you are sailing the boat - most sailors will know what boat speed they should be getting for a given TWS/TWA. When about to make a course change TWS is very useful to allow you to anticipate the AWS on the new course

I agree none of these are any use if you aren't sailing

 

[lustyd]

So the example you use is when the boat is drifting with the tide - that is not sailing in most people's definition. Yes of course if you are stationary in the water TWS = AWS and there is no need to distinguish.

For sailors who sail on the water then it is the speed of the wind relative to the water that matters. Close hauled TWA tells me my tacking angle and therefore what course I will be tacking onto. TWS and TWA are used to assess how well you are sailing the boat - most sailors will know what boat speed they should be getting for a given TWS/TWA. When about to make a course change TWS is very useful to allow you to anticipate the AWS on the new course

I agree none of these are any use if you aren't sailing

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.

 

[srm]

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.

If my boat is drifting on the tide on a completely calm day at 4 kn due north (assume this is the ground track you refer to) then the apparent and true wind, (using the convention that wind is named by the direction it blows from) will both be 4kn from the north and most definately not from the south as you state. 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, keeping in the strongest tide stream, and in so doing increase the apparent wind thus shortening my passage time (see my post # 4).

I think this answers your question with an example.

You are, of course, free to use whatever naming convention suits you, and whatever instrument settings and other ideas you prefer. The nice thing about sailing in this increasingly regulated world is that each of us is free to set up and use our boats in whatever way we prefer.