Lee bowing

[TLouth7]

You see. That’s your definition.
It’s not the correct definition of true wind.
https://www.wmo.int/pages/prog/amp/...AR-99/Presentations/session_08/08.2_Smith.pdf

Relevant xkcd? https://xkcd.com/927/

I see nothing in that report claiming that its definition is anything other than internally consistent: "True wind is defined herein as
a vector wind with a speed referenced to the fixed earth and a direction referenced to true north." (my emphasis).

Hopefully we all agree that differences in tidal stream (either in time or space) result in changes to the direction and speed of wind relative to the water surface, regardless of what we choose to call that wind. It is these changes that can be used by a sailor to gain distance to weather by knowing which tack will be favoured at a particular time and place.

Edit to make the quote clearer

 

[Buck Turgidson]

Relevant xkcd? https://xkcd.com/927/

I see nothing in that report claiming that its definition is anything other than internally consistent: "True wind is defined herein as
a vector wind with a speed referenced to the fixed earth and a direction referenced to true north." (my emphasis).

Hopefully we all agree that differences in tidal stream (either in time or space) result in changes to the direction and speed of wind relative to the water surface, regardless of what we choose to call that wind. It is these changes that can be used by a sailor to gain distance to weather by knowing which tack will be favoured at a particular time and place.

Edit to make the quote clearer

It also quotes the reference used my met, oceanography and merchant shipping. Everyone except a few sailors use the standard definition for true wind. When you get wind predictions from all providers they give true wind by standard definition.
Don't be disappointed if the reported True wind doesn't match your boats instrument when the instrument is mislabelled.

Oh and on my Triton I can switch it to use CoG SoG in the wind calc which makes the value correct for true wind, but less useful for sailing.

 

[lw395]

Personally, I don't really care very often whether somebody else's 'True Wind' is ground or sea true wind.
The difference is usually only a knot or two.
That's usually going to be lost in the minute-to-minute variation.
And the variation from point A to point B within 100metres.
And the inaccuracy of their instruments.
And the variation due to height.

Places where the tide is fast because it's flowing around headlands etc, guess what, the wind is often doing the same, maybe from the opposite direction...

By the time you've worked out the 'true' true wind, it's probably changed anyway, even if you have a 'puter to do it for you.

Have a look at some real-life data:
https://weatherfile.com/location?loc_id=GBR00001
Then can you seriously say you are worried about a knot of tide in some other fool's report of true wind?

 

[TLouth7]

It also quotes the reference used my met, oceanography and merchant shipping. Everyone except a few sailors use the standard definition for true wind.

It specifically includes a table (page 100, 3rd page in the segment linked) which shows that there are four definitions of True Wind habitually in use. I admit that none of these is relative to the water, but it rather refutes the idea of a single standard definition.

Oh and on my Triton I can switch it to use CoG SoG in the wind calc which makes the value correct for true wind, but less useful for sailing.

And surely this is the crux of the debate. Leisure sailors have no requirement to be consistent with anyone else and in general will use the most useful system, which for many means considering the wind that one actually experiences when out on the water. In the absence of another term we use the perfectly acceptable True Wind.

 

[Buck Turgidson]

They all use fixed earth reference!
You think i didn't read the whole document before posting?

the point is not what is more useful for sailing. The point is the use of standard terms in language. The whole point of which is the transfer of information. If i call your cat a dog is that useful?

 

[dom]

the point is not what is more useful for sailing. The point is the use of standard terms in language.

On a sailing forum ...Oh Dear

 

[ghostlymoron]

From observation true and apparent wind are equal when you are tied up or anchored so true is relative to the ground. Once your free from the ground you feel the apparent wind. Or is that too simple?

 

[flaming]

From observation true and apparent wind are equal when you are tied up or anchored so true is relative to the ground. Once your free from the ground you feel the apparent wind. Or is that too simple?

Yes, too simple I'm afraid.

The physics of the situation is simply that the movement of your boat due to the tide creates a wind in the opposite direction. So with no wind over the land but a 3 knot tide a drifting boat would feel a 3 knot wind. And that would be reported by its wind instruments (be they raymarine, Garmin or anything else) as 3 knots True. If drifting then the instruments would also register 3 knots apparent. However the instruments that also display ground wind using the GPS SOG and COG would be saying zero knots ground wind wind.
Now suppose that our boat starts motoring with the tide at 3 knots boat speed, so making 6 knots SOG. His apparent wind then increases to 6 knots. But the true wind stays at 3 knots and the ground wind stays at zero knots.

Now imagine that the tide turns and runs at 3 knots in the other direction. Ground wind is still zero, true wind is still 3, but has changed direction 180 degrees and apparent wind is now zero, as the boat is doing zero SOG.
All a bit academic when you consider what happens with zero ground wind, but much more relevant when considering what happens in typical Solent conditions. For example.

You're anchored up in Newtown creek for lunch. There's a nice Solent sea Breeze blowing from the SW. Say 12 knots GWS. You finish lunch and decide to head to Yarmouth for the night. The tide is ebbing to the SW, running at about 3 knots in the channel.
You hoist full sail just after you exit Newtown in a lovely 12 knot TWS breeze and very little current. Then you head out into the Solent close hauled on Port tack. As you get into the full force of the tide the wind picks up to 15 knots TWS. GWS is still 12. If you're using your GWS readout you might wonder why you're suddenly rather overpowered given there is apparently only 12 knots of wind.
Of course if the tide was going the other way you'd only have 9 knots TWS, but the GWS would still show 12...

So you can see that knowing about the difference between GWS and TWS is actually very useful even in a cruising context.

What those who advocate calling Ground wind True wind never seem to be able to answer is what to call what I term True Wind....?

 

[dom]

From observation true and apparent wind are equal when you are tied up or anchored so true is relative to the ground. Once your free from the ground you feel the apparent wind. Or is that too simple?

Flaming provides some good examples and ends up asking how those who conflate True and Ground define the wind experienced by a floating boat - i.e. the wind that can power the boat, define where it can go, and not to forget build the seas the boat will experience.

This is vital, though once defined the calcs are way easier than lee-bowing and other voodoo nav. A boat floating in a constant tide, and a boat moored may be defined by two inertial frames in a constant state of linear motion (rectilinear) with respect to each another. A person sitting in either frame will experience no acceleration or other feeling of movement. Windspeed, motion and other measurements taken in one inertial frame can easily be converted to the other, with much useful data falling out as a byproduct - lee bowing very much included.

In fact, we can now deal freely with tides, wind shifts, AWS, TWS and to the extent our data allows, we'll never get caught out. Flaming speaks of Newtown Creek.Now imagine floating off Cap de la Hague. The lighthouse data tells you that the wind is a pleasant southerly 15kts. If the tide is running south at 7kts, the boat will experience a punchy 22kts of wind. When the tide changes direction the same boat will experience a balmy 8kts. Lighthouse keeper will experience no change.

And don't forget: the sea will very much be a 22kt sea in one tide and an 8kt sea in the other, which explains a good part of wind over tide effects!

An accelerometer on a boat fitted with sophisticated scientific equipment would detect the change in direction of the boat's frame of reference. As no boat can do this, one simply uses tide tables, buoy measurements, or whatever, after which so nothing fancy is necessary.

And perhaps best of all, this is exactly how Raymarine, NKE, B&G, Garmin, etc approach the problem, do so one can fully attune oneself to one's instruments

 

[ghostlymoron]

As someone who doesn't use wind instruments, I have a rather different perception of true and apparent wind.

Yes, too simple I'm afraid.

The physics of the situation is simply that the movement of your boat due to the tide creates a wind in the opposite direction. So with no wind over the land but a 3 knot tide a drifting boat would feel a 3 knot wind. And that would be reported by its wind instruments (be they raymarine, Garmin or anything else) as 3 knots True. If drifting then the instruments would also register 3 knots apparent. However the instruments that also display ground wind using the GPS SOG and COG would be saying zero knots ground wind wind.
Now suppose that our boat starts motoring with the tide at 3 knots boat speed, so making 6 knots SOG. His apparent wind then increases to 6 knots. But the true wind stays at 3 knots and the ground wind stays at zero knots.

Now imagine that the tide turns and runs at 3 knots in the other direction. Ground wind is still zero, true wind is still 3, but has changed direction 180 degrees and apparent wind is now zero, as the boat is doing zero SOG.
All a bit academic when you consider what happens with zero ground wind, but much more relevant when considering what happens in typical Solent conditions. For example.

You're anchored up in Newtown creek for lunch. There's a nice Solent sea Breeze blowing from the SW. Say 12 knots GWS. You finish lunch and decide to head to Yarmouth for the night. The tide is ebbing to the SW, running at about 3 knots in the channel.
You hoist full sail just after you exit Newtown in a lovely 12 knot TWS breeze and very little current. Then you head out into the Solent close hauled on Port tack. As you get into the full force of the tide the wind picks up to 15 knots TWS. GWS is still 12. If you're using your GWS readout you might wonder why you're suddenly rather overpowered given there is apparently only 12 knots of wind.
Of course if the tide was going the other way you'd only have 9 knots TWS, but the GWS would still show 12...

So you can see that knowing about the difference between GWS and TWS is actually very useful even in a cruising context.

What those who advocate calling Ground wind True wind never seem to be able to answer is what to call what I term True Wind....?

 

[flaming]

As someone who doesn't use wind instruments, I have a rather different perception of true and apparent wind.

That's great.

It doesn't change the physics though.

 

[dom]

As someone who doesn't use wind instruments, I have a rather different perception of true and apparent wind.

Not trying to be argumentative here, but without wind instruments all you could possibly experience is true wind as so defined in a freely floating stationary boat, and apparent wind as so defined when you're sailing

Unless you're looking at a riverbank, transits, or something.

 

[Buck Turgidson]

That's great.

It doesn't change the physics though.

Problem is the physics used in this case completely disregards drag/windage, log vector error. Hull form and sail plan.

The true wind described as vector difference between airflow and water flow will be completely different on each vessel that calculates it and unique to that vessel at that moment.

Very useful if you’re sailing that vessel at that moment. Not exactly a universal “truth” though. Hence my dislike of the term.

 

[dom]

Problem is the physics used in this case completely disregards drag/windage, log vector error. Hull form and sail plan.

The true wind described as vector difference between airflow and water flow will be completely different on each vessel that calculates it and unique to that vessel at that moment.

Very useful if you’re sailing that vessel at that moment. Not exactly a universal “truth” though. Hence my dislike of the term.

No, in fact a set of boats sailing randomly around in a uniform tidal flow will - subject to their instruments being perfectly calibrated - calculate exactly the same True Wind. And that is the True Wind as defined by all the major instrument OEMs

 

[flaming]

Problem is the physics used in this case completely disregards drag/windage, log vector error. Hull form and sail plan.

Actually, it doesn't... If you put your boat in moving water it must go at the speed of the water if drifting. Unless it is acted on by the drag of being moved through the air. Which is in effect a wind, and will change the velocity of the boat relative to the water by exactly the same amount as an equal wind blowing over a still lake and hitting the boat. All you've done is change the frame of reference from the land to the surface of the water.

The true wind described as vector difference between airflow and water flow will be completely different on each vessel that calculates it and unique to that vessel at that moment.

Very useful if you’re sailing that vessel at that moment. Not exactly a universal “truth” though. Hence my dislike of the term.

If you're talking about the inerrant inaccuracies in measuring water speed and wind speed on a sailing boat, then yes... Though the top of the line instruments are getting very good... Still there will always be inaccuracies. We have to consider though what we are using the data for, and what inaccuracies render it useless. For me, the data is most often used for 2 reasons. First, with reference to target speeds, am I fast enough in this windspeed? As you can see with my examples above, using GWS for this is a nonsense as it could be considerably out for a relatively modest tide and targets will be very different in 9 and 14 knots for example. Secondly it's "what will be the wind angle and wind speed on the next leg, and therefore which sail shall we hoist? Again, with big differences in angle and wind speed between TWS and GWS arising from relatively small tides, it's clear that using TWS is the way forward. And then we see that whether my boat decides that this is 14 or 15 knots is not especially important as long as it's repeatable, as I will know from experience that when my dial reads 14 I can carry that kite on a reach. But if that 14 is GWS, then the actual power in my sails could be as much as 17 or as little as 9 with a 3 knot tide. Very different sailing conditions.

But I note you didn't answer my little question.... What do you call the measure of wind that is the difference in velocity between the air and water?

 

[Buck Turgidson]

No, in fact a set of boats sailing randomly around in a uniform tidal flow will - subject to their instruments being perfectly calibrated - calculate exactly the same True Wind. And that is the True Wind as defined by all the major instrument OEMs

If that were true they would all have to have exactly the same drag. Which would require them to have exactly the same crossectional area and Cd.

They don’t.

You are describing spherical boats of zero mass in a vacuum.

 

[Medway Matt]

The only problem with that argument is that it then doesn't have a term for the wind that is the measure in velocity difference between the water and air. Which is the measure of the power we have available to our boats, and the direction in which we are able to point them.

If you call the wind relative to the ground "true wind" then what do you call this wind?

As we seem to be inventing new terms in this thread, I shall call it Wind Apparent Floating, to be abbreviated to WAFt.

Actually I'm in total agreement with you, especially since the true wind experienced afloat and the boat's apparent wind are what we generally care about when sailing.

I expect you can tell that I'm not taking all of this too seriously, I hope that doesn't make me a troll!

 

[johnalison]

If that were true they would all have to have exactly the same drag. Which would require them to have exactly the same crossectional area and Cd.

They don’t.

You are describing spherical boats of zero mass in a vacuum.

Your post also reminds me of spherical objects.

 

[Buck Turgidson]

Actually, it doesn't... If you put your boat in moving water it must go at the speed of the water if drifting. Unless it is acted on by the drag of being moved through the air. Which is in effect a wind, and will change the velocity of the boat relative to the water by exactly the same amount as an equal wind blowing over a still lake and hitting the boat. All you've done is change the frame of reference from the land to the surface of the water.



If you're talking about the inerrant inaccuracies in measuring water speed and wind speed on a sailing boat, then yes... Though the top of the line instruments are getting very good... Still there will always be inaccuracies. We have to consider though what we are using the data for, and what inaccuracies render it useless. For me, the data is most often used for 2 reasons. First, with reference to target speeds, am I fast enough in this windspeed? As you can see with my examples above, using GWS for this is a nonsense as it could be considerably out for a relatively modest tide and targets will be very different in 9 and 14 knots for example. Secondly it's "what will be the wind angle and wind speed on the next leg, and therefore which sail shall we hoist? Again, with big differences in angle and wind speed between TWS and GWS arising from relatively small tides, it's clear that using TWS is the way forward. And then we see that whether my boat decides that this is 14 or 15 knots is not especially important as long as it's repeatable, as I will know from experience that when my dial reads 14 I can carry that kite on a reach. But if that 14 is GWS, then the actual power in my sails could be as much as 17 or as little as 9 with a 3 knot tide. Very different sailing conditions.

But I note you didn't answer my little question.... What do you call the measure of wind that is the difference in velocity between the air and water?

I would call it calculated or relative or sailing.

 

[Airscrew]

Flaming - Thank goodness for clarity at last....

Your skills would be good as a yachtmaster instructor?

But probably too slow for you..?