The "lee bow effect"

[lw395]

I feel nervous about even asking, you being a whizz-bang racing skipper and me being a pottering cruiser,but are you sure about that definition of "true wind"? The first ten Google hits agree with what I thought, which is that it is the velocity of the wind relative to the earth.

As distinct from the magnetic wind?

You are not wrong, it is a loosely defined term, people should always make sure how these things are defined.
Meanwhile in the real world, the 'True' wind usually varies by a lot more than the tide vector over any 15 minute period.

And the motion of the water with the friction between air and water will move the true wind and the ground wind close to the surface...

 

[Mark-1]

what I thought, which is that it is the velocity of the wind relative to the earth.

That's always been my usage. True wind is what I see moving smoke away from chimneys, apparent wind is what I feel on my face and what I sail to. If I can't see something that tells me what true wind is doing then I don't need to know.

 

[JumbleDuck]

True wind is relative to the water.

Ground wind is relative to a point on the chart.

Is this a racing thing? I have now, finally, found a web page which uses that definition but it seems a very odd way to describe a crucial quantity.

 

[flaming]

Is this a racing thing? I have now, finally, found a web page which uses that definition but it seems a very odd way to describe a crucial quantity.

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.

 

[grumpy_o_g]

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.

I take it you mean the only thing contributing to any lee-bow effect? I understand why you call windspeed relative to the water "true Wind" but it's a terrible phrase to use as it goes against just about every other form of transport. It's not true at all - it's relative the water the boat is moving through it's a relative wind speed, just not relative to the boat. TWS, AWS and maybe WWS (Water Wind Speed) SWS (Surface Wind Speed) but hey ho, it's too late now.

I'm totally confused by this post as I thought all lee-bowing really was was to ensure one used the current to offset the fact that one couldn't make a waypoint that was upwind. In other words try and stay on the tack that means the tide is carrying you towards your waypoint and not away from it if you can, the classic being going from Poole to Cherbourg with a forecast westerly component in a gentle breeze - head into the current, putting in your tack as the tide changes. I understand all the arguments about the effect on the apparent wind and pinching and boat speed VMG to windward but but I never associated any of that with lee-bowing. I reckon any difference in AWS due to the current would be tiny and quite likely to be offset by the sea-state it would be so small - in fact most of the time I'm sailing we try and stay on the most comfortable tack if we can but then I don't do racing . What have I missed?

Edit: Ignore this (I know you all would anyway) - I hadn't seen the other post about the lee-bow article.

 

[BelleSerene]

Is this a racing thing?

No. Just as relevant cruising at 3 knots in the middle of the channel. You’re sitting in a blob of water. The wind is moving relative to that water. That’s what puts any force in your sails. What the wind is doing relative to a point in space may be ‘truer’, but isn’t affecting you.

Alternatively viewed, let’s all use True Wind to refer to that wind across the chart. But we then need a new term, say, Water Wind, to describe the wind across the water which is what we’re sailing with or against.

 

[flaming]

I take it you mean the only thing contributing to any lee-bow effect? I understand why you call windspeed relative to the water "true Wind" but it's a terrible phrase to use as it goes against just about every other form of transport. It's not true at all - it's relative the water the boat is moving through it's a relative wind speed, just not relative to the boat. TWS, AWS and maybe WWS (Water Wind Speed) SWS (Surface Wind Speed) but hey ho, it's too late now.

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.

And with yachts, we have a readout available that calculates the true wind using the water speed, wind angle and wind wind speed. This gives us a number that we can use for things such as selecting the correct sail to hoist and using my polars to provide a target speed that we should be able to do in that windspeed. If this was derived from GPS rather than STW, then the number would be meaningless, as it is not necessarily representative of the actual power available to my sails. Renaming this "water wind, or tide wind" is a weird idea. It is the true wind, has been called this since we derived the instrumentation to measure it and is the measure of the available power to move your boat.
If you also want to measure the ground wind, then fine, but don't confuse it with true wind.

I reckon any difference in AWS due to the current would be tiny and quite likely to be offset by the sea-state it would be so small - in fact most of the time I'm sailing we try and stay on the most comfortable tack if we can but then I don't do racing
image: http://www.ybw.com/forums/images/smilies/rolleyes.png
. What have I missed?

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....

 

[lpdsn]

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

It's not that. If you use the term true it should mean true. If you get too hung up on the true wind as determined by on board instrumentation being true you lose sight of the tides influence on that wind.

It's not difficult to switch back and forth between the two concepts according to context.

 

[lw395]

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

Generally the course is defined in that frame.
The only exception being gate starts AFAIK.
The object is to get to the windward mark, which is nailed to the ground, not to sail up more tide than the opposition.

 

[bdh198]

I think I'm beginning to understand what is going on (or not) with the lee bow argument/myth. Without going into vectors, is it essentially that those who believe there is a an advantage from better apparent wind by lee bowing are double counting the effect of the tide?

Ground Wind + effect from the tide/current = True Wind (the wind experienced at sea or on the river)

True Wind + the effect of speed through the water = Apparent Wind.

The mistake is then to assume the tide/current will affect your Apparent Wind (e.g. by lee bowing), when the affect of the tide/current has already been taken into account in what you're already experiencing in the True Wind!

So the simple answer (assuming even tide, constant rate etc) is that lee bowing will make no difference if you are simply trying to get a better Apparent Wind speed/angle from the tide. Instead, the way to look at the tidal effect is only as a force pushing you towards/away from a fixed position, and to take that into account separately from your calculation for best angle for boat speed through the water. Only then can you work out the best combination of those two for a course to steer to get the best speed over the ground while travelling the shortest distance.

 

[savageseadog]

True Wind + the effect of speed through the water = Apparent Wind.

The mistake is then to assume the tide/current will affect your Apparent Wind (e.g. by lee bowing), when the affect of the tide/current has already been taken into account in what you're already experiencing in the True Wind!
.

No. The tidal current does affect the apparent wind.

 

[flaming]

No. The tidal current does affect the apparent wind.

No. Apparent wind is only expressed relative to the boat. So tide makes no odds. The only things that impact apparent wind are wind speed, boat speed and boat heading relative to true wind.

Tide affects true wind, which is the measure of the difference in velocity between water and air.

 

[Mark-1]

No. The tidal current does affect the apparent wind.

Agree, but I think bdh198 understands that. It's just he's already factored tide into his 'true' wind so if he adds it again to apparent wind he'd be double counting. (Which, rightly or wrongly, is his point.)

 

[bdh198]

No. The tidal current does affect the apparent wind.

I'm still confused, but it seems it is the stage at which the tidal current influences the apparent wind that is in question.

Obviously tidal current does affect apparent wind because it is the tidal current that contributes to the true wind you experience when drifting on the water. Then, when you start moving through the water the wind you experience is the apparent wind (apparent wind is therefore a combination of ground wind, tidal flow and speed/direction through the water).

When you are on the water you are experiencing true wind, and when you start moving through the water you experience apparent wind. It would therefore be wrong at that stage to assume there is an additional effect that can be felt from the tidal flow (e.g. by lee bowing) because that has already been accounted for in the true wind. To assume you can get a "lift" in the apparent wind by having the lee of the bow in the tidal flow is double counting the effect of the tide.

 

[Mark-1]

When you are on the water you are experiencing true wind, and when you start moving through the water you experience apparent wind. It would therefore be wrong at that stage to assume there is an additional effect that can be felt from the tidal flow (e.g. by lee bowing) because that has already been accounted for in the true wind.

Agree. If you define true wind as wind an object drifting with the tide would experience then without doubt if two boats tack dead upwind into the true wind on opposite tacks neither will get an advantage.

However, if the *ground* wind is on the nose, but there's some tide, then yes one of the boats get lifted, and one gets headed by the wind 'generated' by that tidal movement.

As you say, essentially the question becomes "If I've already factored out the tide, does tide have a further effect?"

We now have 6 or 7 different definitions of lee-bow and 2 different definitions of true wind. That means we can have 49 different opinions on this topic, and every one would be logically defendable by one person and logically attackable by everyone else. What a great topic.

 

[bdh198]

Agree. If you define true wind as wind an object drifting with the tide would experience then without doubt if two boats tack dead upwind into the true wind on opposite tacks neither will get an advantage.

Excellent, it's starting to make sense!

However, if the *ground* wind is on the nose, but there's some tide, then yes one of the boats get lifted, and one gets headed by the wind 'generated' by that tidal movement.

Now, is there ever a situation where you would be sailing in tidal waters or river currents where you would be aware of the "ground wind on the nose"? Even if you had an instrument telling you what the ground wind is you'd still be sailing by the true wind.

 

[savageseadog]

I'm still confused, but it seems it is the stage at which the tidal current influences the apparent wind that is in question.

Obviously tidal current does affect apparent wind because it is the tidal current that contributes to the true wind you experience when drifting on the water. Then, when you start moving through the water the wind you experience is the apparent wind (apparent wind is therefore a combination of ground wind, tidal flow and speed/direction through the water).

When you are on the water you are experiencing true wind, and when you start moving through the water you experience apparent wind. It would therefore be wrong at that stage to assume there is an additional effect that can be felt from the tidal flow (e.g. by lee bowing) because that has already been accounted for in the true wind. To assume you can get a "lift" in the apparent wind by having the lee of the bow in the tidal flow is double counting the effect of the tide.

If there was no wind at all, with the boat sat without moving through the water in say 3 Knots of tide, the boat will experience 3Kn of wind in the direction opposite to the tidal direction. The boat will be moving relative to the ground at 3Kn in the direction of the tide. If you introduce wind add the wind and tidal wind vectorially. After the boat starts to move through the water a wind vector equal in speed to the boat but opposite in direction is added to obtain AWA. There will be things like drag to take away but that's the basics.

 

[Mark-1]

Now, is there ever a situation where you would be sailing in tidal waters or river currents where you would be aware of the "ground wind on the nose"? Even if you had an instrument telling you what the ground wind is you'd still be sailing by the true wind.

Not sure I endorse all the terms you're using but it's clear what you mean and yes, a boat sails to the aggregate of all the different things that 'create' wind relative to it: If you're in the middle of the sea you have no way to directly measure what part 'ground' wind is playing in the wind you're physically experiencing. (If you know all the other factors you can work it out, of course).

PS: I'm using the the term 'ground' wind under protest! 'Ground' wind has always been 'True' wind to me and having to change the meaning of that just so Flaming can use instruments to factor tide in so he can choose the right sails really sticks in my throat!

 

[NealB]

PS: I'm using the the term 'ground' wind under protest! 'Ground' wind has always been 'True' wind to me and having to change the meaning of that just so Flaming can use instruments to factor tide in so he can choose the right sails really sticks in my throat!

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.

 

[johnalison]

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.

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.