The "lee bow effect"

[lw395]

I've sailed for over 55 years: a bit of racing, a fair bit of cruising, and loads of reading.

I think 'true wind' always used to mean the wind I'd feel when sitting on a deck chair on a beach.

Then along came those bright young things, inventing all those magical, electronic, wind measuring devices, and ..... they decided to invent their own definitions, riding roughshod over centuries of common usage.

That is what the American Meteorological would say:
The true wind is defined herein as a vector wind with a speed referenced to the fixed earth and a direction referenced to true north.
That's a useful definition if you want to plot a weather chart.
Less handy for racing a boat.
Best to just accept that there are different definitions and check which one is being used, if and when it matters.

 

[grumpy_o_g]

I think it disappoints me to see so many sailors using the land as their frame of reference, not the water!

Think of it this way. If I go sailing on a lake with no current, then when the clubhouse wind instrument reads 10 knots then I know that the amount of power available to my sails will be 10 knots. But if I then go sailing on an estuary, when the clubhouse wind instrument reads 10 knots then I might have anywhere between 7 and 13 knots of wind. And there is a big difference. So to my boat out on the estuary, is that 10 knots as measured by the clubhouse true? Of course not, it's basically meaningless...

I fully understand that but you can't simply ignore the current when it suits you. True wind speed and direction is referenced to the Earth's surface, wind velocity relative to the water is not True wind, it's a relative wind to the normal frame of reference which is the Earth.

If you think the effect is small I suggest you do the vector maths for a 2 knot East-West tide that becomes a 2 knot West-East tide and an 8 knot Southerly wind....

I'll do that but my intuition (which is likely wrong) is that that vector will become comparatively small when including the vectors for the boat speed and direction in an 8 knot Southerly. It's a very fair point though and you've made me go and check my intuition. The irony of course is that those very instruments you talk about mean that I simply try things out and see what works best. Lazy...

 

[grumpy_o_g]

I've been sailing seriously for about 62 years and have no difficulty in distinguishing ground wind and 'true wind' as it has been defined for the last forty-odd years. While you are still pottering around in your dug-out, things have moved on. As for lee-bowing, I'll let the children argue it among themselves.

They haven't moved on at all, they've simply been confused by a few idiots. True wind, for the majority of people, does not mean wind speed and direction relative to a body water someone happens to be sailing in and will continue not to mean that. A small group of people have decided to use an existing term for something different that could easily be confused for the existing definition - that's lazy and dangerous. No problem with the concept of wind velocity relative to the water but why use the name for wind velocity relative to the Earth's surface to describe it? Why not simply give it a new name to reflect the fact that it's definition? It's like developing a fuel for internal combustion engines that isn't quite the same as diesel or petrol and isn't compatible with either but just deciding to call it diesel and saying "things have moved on".

 

[johnalison]

They haven't moved on at all, they've simply been confused by a few idiots. True wind, for the majority of people, does not mean wind speed and direction relative to a body water someone happens to be sailing in and will continue not to mean that. A small group of people have decided to use an existing term for something different that could easily be confused for the existing definition - that's lazy and dangerous. No problem with the concept of wind velocity relative to the water but why use the name for wind velocity relative to the Earth's surface to describe it? Why not simply give it a new name to reflect the fact that it's definition? It's like developing a fuel for internal combustion engines that isn't quite the same as diesel or petrol and isn't compatible with either but just deciding to call it diesel and saying "things have moved on".

I don't think it was a small body of people. As far as I remember, it was B&G who introduced instrumentation that would give this figure, perhaps in the early '70s, but it may be someone else, or I may be confusing it with VMG, though they are related and may have come together. The people using the term would have been rich folk who could afford B&G in those days, and those of us who aspired to.
If you really want to confuse us, perhaps you can start using the term 'water wind', but it sounds awkward to me and I think I will continue with true and ground winds, unsatisfactory though it may be.

 

[grumpy_o_g]

I don't think it was a small body of people. As far as I remember, it was B&G who introduced instrumentation that would give this figure, perhaps in the early '70s, but it may be someone else, or I may be confusing it with VMG, though they are related and may have come together. The people using the term would have been rich folk who could afford B&G in those days, and those of us who aspired to.
If you really want to confuse us, perhaps you can start using the term 'water wind', but it sounds awkward to me and I think I will continue with true and ground winds, unsatisfactory though it may be.

Compared to the number of people who work with windspeed the entire leisure sailing community is tiny...

 

[Deleted member 36384]

Compared to the number of people who work with windspeed the entire leisure sailing community is tiny...

But they are not relevant to the sailing community, the sailing community has a name for a feature that they use that happens to coincide with another definition. I am sure no one is spinning in their grave over this, and even less people who care that there is a term with two definitions. That's how I see it, but maybe I am missing a big kerfuffle amongst the scientific community and leisure sailors; anemometers at dawn anyone!

 

[Kerenza]

When I started to learn to sail, a process still continuing every day I sail, I understood that apparent wind was the most important wind factor. To complete the picture there was another term which removed our sailing influences , which was called by all the sailing people I knew, the true wind. It never occurred to me that there might be an appreciable difference between true and what was happening ashore.
As my understanding, available information and instruments improved, I discovered the term ground wind, as shown on my raymarine instruments. I also realised this is fairly irrelevant to most cruisers, sailing on local waters for short periods.
Ground wind (at 10m) is what we get from the forecast and our grib files.
When I look to the next waypoint, either racing or cruising, I try to estimate the wind at that point. if we're racing ground wind is probably what the course maker has considered.
My starting point is the overlay grib , or just the forecast, which I can make some sort of correlation with using my real time ground wind readout.
I can try to factor in the anticipated conditions of tide, etc, and thus deduce the true wind we might expect at the waypoint and perhaps some way before it. Using this over an appreciable distance may make a big difference to how we react to the conditions we are experiencing.
So apparent wind is very important for sailing, true wind is essential for safe short trem planning (including sail wardrobe), but ground wind is also essential for planning and safety.
Now most of my calculations are usually wrong, to the mirth or otherwise of the crew. But I can't attempt the exercise without the three different wind definitions, and they seem to me to be very suitable names, using as we do use the terms over the ground and through the water for speed.

 

[Kerenza]

To return to the OP. I always simplistically thought that lee bowing meant putting the bows into the tide to help negate course and leeway errors, if possible.
There are situations where better angles may be obtained, particularly downwind, indeed I had the same discussion last week racing, failing to convince the crew, and indeed my tide calculations were wrong once again when we got to the gybe point, but I don't think there can be any hard and fast rule.

 

[lw395]

I don't think it was a small body of people. As far as I remember, it was B&G who introduced instrumentation that would give this figure, perhaps in the early '70s, but it may be someone else, or I may be confusing it with VMG, though they are related and may have come together. The people using the term would have been rich folk who could afford B&G in those days, and those of us who aspired to.
If you really want to confuse us, perhaps you can start using the term 'water wind', but it sounds awkward to me and I think I will continue with true and ground winds, unsatisfactory though it may be.

It goes back well before B'n'G.
In dinghies a basic sailing textbook or RYA syllabus from the 1960 will tell you that the burgee indicates apparent wind and not the true wind. You don't need any electronics to 'measure' the true wind direction in this basic context, just your compass. It's half way between your close hauled heading on port and your close hauled heading on starboard.
That is still generally more accurate than your bullsheet and guesswork calculations involving wind instrument and speed log. Provided it doesn't shift too much while you're tacking of course.

I dare say it was like that before sailing vessels ever considered measuring the apparent wind in knots while they were underway.

In dinghies, we don't seem to talk about 'true wind'. Just the wind and the apparent wind.
The wind where I'm sailing will be different to the wind 100 yards away or two minutes ago. Which of those is the true wind?
These variations generally blow the current vector into insignificance.

 

[flaming]

They haven't moved on at all, they've simply been confused by a few idiots.

Well at least you're living up to your name eh?

True wind, for the majority of people, does not mean wind speed and direction relative to a body water someone happens to be sailing in and will continue not to mean that.

Actually, anyone who references TWS or TWD as produced by their wind instruments uses that definition, they just might not be aware of it!

A small group of people have decided to use an existing term for something different that could easily be confused for the existing definition - that's lazy and dangerous. No problem with the concept of wind velocity relative to the water but why use the name for wind velocity relative to the Earth's surface to describe it? Why not simply give it a new name to reflect the fact that it's definition? It's like developing a fuel for internal combustion engines that isn't quite the same as diesel or petrol and isn't compatible with either but just deciding to call it diesel and saying "things have moved on".

Simply because in sailing, True wind has always been relative to the water for as long as it has been measured and displayed by instruments. It's just far too late to change it now.

 

[grumpy_o_g]

Well at least you're living up to your name eh?


Actually, anyone who references TWS or TWD as produced by their wind instruments uses that definition, they just might not be aware of it!



Simply because in sailing, True wind has always been relative to the water for as long as it has been measured and displayed by instruments. It's just far too late to change it now.

Mind blowing - spoken like a true racing yachtie....

 

[flaming]

Mind blowing - spoken like a true racing yachtie....

Yes... And...?

Thing is... I’m not wrong. Think about it this way.

If you don’t have any instruments at all you know to determine the true wind speed and direction with reference to the ripples on the water and the size of the waves. Sailors have been doing that since the first one noticed that it seemed windier when going towards the wind than away from it.
And what is the ripples and the wave size indicating? Simple - it’s the difference in velocity between the air and the water.

So if you’re going to argue prior useage, you have to argue in favour of my definition of true wind, not yours.

 

[grumpy_o_g]

Yes... And...?

Thing is... I’m not wrong. Think about it this way.

If you don’t have any instruments at all you know to determine the true wind speed and direction with reference to the ripples on the water and the size of the waves. Sailors have been doing that since the first one noticed that it seemed windier when going towards the wind than away from it.
And what is the ripples and the wave size indicating? Simple - it’s the difference in velocity between the air and the water.

So if you’re going to argue prior useage, you have to argue in favour of my definition of true wind, not yours.

Apologies it wasn't meant to sound like quite so much of an insult! Just that I thought it indicative of the single-mindedess of racing and other competitive sailing folk (which is probably mildly hypocritical of me in view of my past history in competitive sports).

Sailors start their journey on land - the Shipping Forecast uses true wind, the fisherman checking the weather before putting to sea uses true wind, the master of Clipper ship uses true wind before he sets sail, even Beaufort scale indicators commonly quoted today are true wind. I would say sailors instinctively think in terms of true (i.e. ground-referenced) wind and have ignored the difference as, until GPS, they haven't been able to do much about it.

Ref your argument about using the sea state and conditions to estimate true wind on my one long passage (to the Azores many years ago) I have a vague memory of it being done in reverse - the skipper suspected the current had changed based on slower than expected progress (only astro available) and the sea state.

Garmin have a large market share in the private aviation sector and the idea of quoting true wind as referenced to the a/c would horrify them - they have to deal with wind shear, jet streams, vertical and well as horizontal winds, etc. and ALL wind speeds are ground referenced and always have been. That's why they make the unequivocal statement "[FONT=&quot]If an instrument for measuring the speed and direction of the wind is mounted on shore, then the readings obtained are those of the speed and direction of the true wind." [/FONT]They close with the statement "[FONT=&quot]It is the apparent wind that acts on the sail, not the true wind. and that is why it is so important to understand how it behaves."[/FONT] admittedly ignoring that fact that it may be the water moving the boat, rather than the boat moving through the water. I can't get that to mean that "an apparent wind generated by the movement of a boat caused by a current is called the true wind" though. The idea of windspeed being anything other than ground-referenced just doesn't enter into anyone's thinking in the aviation world.

I think we'll just have to agree to differ...

 

[JumbleDuck]

But it isn’t crucial. It’s almost irrelevant. The only thing powering your boat is the difference in velocity between the water and the air.

The "crucial quantity" I was referring to was the difference in velocity between the water and the air, for which I think still "true wind" is a rotten term - particularly because it has a long established and significantly different definition.

 

[JumbleDuck]

No. Just as relevant cruising at 3 knots in the middle of the channel.

I meant the term, not the concept. Most of the sailing world seems to think that "true wind" is wind over ground.

 

[JumbleDuck]

I think it disappoints me to see so many sailors using the land as their frame of reference, not the water!

You race round free-floating marks then do you? You get weather information from a service which knows the tide where you are?

 

[lw395]

You race round free-floating marks then do you? You get weather information from a service which knows the tide where you are?

Where are you getting weather information accurate enough to tell the difference?
Is your Scottish sea so smooth that the tide doesn't affect the wind at all?

True wind means 'not the apparent wind'. The variation in definitions is generally moot due to the variation in value from minute to minute and place to place being a lot greater.

 

[Buck Turgidson]

I’ve had this discussion more than once on this forum!
Ground wind = wind over ground or anchored boat
True wind = wind as seen by boat drifting with tide
Apparent wind = wind as measured by boat that is Sailing.

This is only correct in sailing parlance. In the rest of the world True wind is wind over the ground. There are rather large industries that rely on true wind for planning. It's what meteorological agencies spend billions predicting.

Sadly the small manufacturers of niche sailing instruments chose to make their own stuff up hence the confusion.

 

[JumbleDuck]

Where are you getting weather information accurate enough to tell the difference?

As Flaming himself has said, +/- 3kts of tide can have quite an effect on the effective wind speed

 

[JumbleDuck]

This is only correct in sailing parlance.

I suspect that it is only in racing sailing parlance. Which is fine, just as it is fine for the rest of us to use the more conventional definition.