Let's Get to the Bottom of the Lee-Bow Effect Once and For All

[Dockhead]

Originally Posted by europaflyer

In my mind it works for any angle of current, other than two points (roughly, to windward and to leeward) where it makes no difference to the direction of the apparent wind vector. At these points, it switches from helping to on one tack (coming from your leeward side) to hindering you (coming from windward).

It is a very, very subtle point, that I can picture but not quite explain properly (yet), that these 'switchover points' are not exactly to windward and leeward. Picturing it using my 'sum of vectors' approach for the wind as above, it can be seen that these points where the 'current induced wind vector' does not affect the 'apparent wind vector' are when the tide is just slightly to port of windward and leeward when on starboard tack, and just slightly to starboard when on port tack. As I say, this is a VERY subtle (ie. totally unimportant!) point, that I may try to support with a diagram if I have the time later. No, I'm not sure if I'm right either.

Excellent stuff.

You are the first person I have ever talked to who actually seems to understand all this. You have an advantage over Seaworthy and me in that we started this discussion in a state of utter cluelessness.

The subtle points are not unimportant, because they illustrate the limits of the phenomenon and therefore give clues about its nature. Very good stuff.

Why don't you have a look at these posts:

http://www.cruisersforum.com/forums/...ml#post1388619

http://www.cruisersforum.com/forums/...ml#post1388677

and tell us if you think we're on the right track?


And next question -- do you have any opinion on whether it is ever possible to gain miles by actually pinching, in order to stay on the best tack for a while longer? If so, how can it work if you're giving up VMG to windward?

One of the puzzles is that your polars give you max VMG to windward, but that is to the True Wind, not to the Ground Wind. Maybe that makes no difference, but I haven't figured it out yet.

[Hoofsmit]

Originally Posted by Paul Elliott

You are using "True Wind" for what we have been calling "Ground Wind". We have more or less agreed to call True Wind the wind that is referenced to the water. Some people call this the "Sailing Wind". In any case, it is critical that we differentiate between the ground-referenced and the water-referenced winds. Otherwise we will be forever arguing in circles. The True Wind (water) is the vector sum of Ground Wind and the current-induced wind. The Apparent Wind is the vector sum of True Wind (water) and the speed/course of your boat through the water. If we are going to discuss the Lee-Bow Effect, we should not be discussing changing currents, or changing Ground Wind, or waves. Ockham Instruments describes the mythical lee-bow effect on their website: (Lee Bow Effect « Ockam Sailing Instruments Ockam Sailing Instruments) They then proceed to show why this a myth, using the same logic (and math) that some of us here have been using. We can discuss changing currents, winds, waves, Course To Steer, etc, but I hope we can all agree that there is no lee-bow effect as that term has been traditionally defined.

Ooooops.........

( staying quiet)

[europaflyer]

Originally Posted by Paul Elliott

You are using "True Wind" for what we have been calling "Ground Wind". We have more or less agreed to call True Wind the wind that is referenced to the water. Some people call this the "Sailing Wind".

Perhaps this is UK vs US terminology. I have always heard, and used, the phrase 'true wind' to describe the path and velocity the wind has over the ground. Calling it 'sailing wind' is just confusing because it isn't what you 'sail to' - that is apparent wind!

Originally Posted by Paul Elliott

If we are going to discuss the Lee-Bow Effect, we should not be discussing changing currents, or changing Ground Wind, or waves. Ockham Instruments describes the mythical lee-bow effect on their website:
(Lee Bow Effect « Ockam Sailing Instruments Ockam Sailing Instruments) They then proceed to show why this a myth, using the same logic (and math) that some of us here have been using.

We can discuss changing currents, winds, waves, Course To Steer, etc, but I hope we can all agree that there is no lee-bow effect as that term has been traditionally defined.

I don't agree at all! Perhaps you say that because people have 'traditionally defined' the lee bow effect wrongly, or at least that is your understanding of it.

Referencing the website:
"Current is irrelevant because the boat only knows about the water and not the land."
Having to talk about the boat 'knowing' is a sign that they can't explain themselves properly. The boat is an inert object. The point here is that the boat 'knows' about the water, while the wind 'knows' about the land. The two move relative to each other, and the lee bow effect is the result.
"Wind direction changes if the current changes. Its just another wind shift, no different from any other wind change."
Yes - and wind changes affect your sailing decisions, just as this one does.

To me, their arguments appear confused. While the lee-bow effect is not relevant to tactical decision making if no current change is observed during racing (which is perhaps what they are thinking of), that does not mean it does not exist. In, say, a cross Channel race where the tide may change several times during the race, the boats which tack to exploit lee bow effect will win, because of the points I have made earlier.

Trying to think about cases where the tide is not at 90 degrees to the true (ground, whatever) wind confuses the issue, and it was a mistake on my part to try to put my thoughts in to words.

[europaflyer]

Originally Posted by Dockhead

Excellent stuff.

You are the first person I have ever talked to who actually seems to understand all this. You have an advantage over Seaworthy and me in that we started this discussion in a state of utter cluelessness.

The subtle points are not unimportant, because they illustrate the limits of the phenomenon and therefore give clues about its nature. Very good stuff.

Why don't you have a look at these posts:

http://www.cruisersforum.com/forums/...ml#post1388619

http://www.cruisersforum.com/forums/...ml#post1388677

and tell us if you think we're on the right track?


And next question -- do you have any opinion on whether it is ever possible to gain miles by actually pinching, in order to stay on the best tack for a while longer? If so, how can it work if you're giving up VMG to windward?

One of the puzzles is that your polars give you max VMG to windward, but that is to the True Wind, not to the Ground Wind. Maybe that makes no difference, but I haven't figured it out yet.

Dockhead, I'll have a think about all this and reply later when I have a little time! Oh, and please don't mistake me for someone who understands all this. It hurts my head just as much as anyone else

[Hoofsmit]

Originally Posted by europaflyer

Perhaps this is UK vs US terminology. I have always heard, and used, the phrase 'true wind' to describe the path and velocity the wind has over the ground. Calling it 'sailing wind' is just confusing because it isn't what you 'sail to' - that is apparent wind! I don't agree at all! Perhaps you say that because people have 'traditionally defined' the lee bow effect wrongly, or at least that is your understanding of it. Referencing the website: "Current is irrelevant because the boat only knows about the water and not the land." Having to talk about the boat 'knowing' is a sign that they can't explain themselves properly. The boat is an inert object. The point here is that the boat 'knows' about the water, while the wind 'knows' about the land. The two move relative to each other, and the lee bow effect is the result. "Wind direction changes if the current changes. Its just another wind shift, no different from any other wind change." Yes - and wind changes affect your sailing decisions, just as this one does. To me, their arguments appear confused. While the lee-bow effect is not relevant to tactical decision making if no current change is observed during racing (which is perhaps what they are thinking of), that does not mean it does not exist. In, say, a cross Channel race where the tide may change several times during the race, the boats which tack to exploit lee bow effect will win, because of the points I have made earlier. Trying to think about cases where the tide is not at 90 degrees to the true (ground, whatever) wind confuses the issue, and it was a mistake on my part to try to put my thoughts in to words.

My be that's why it's called the English language , ........ Sorry couldn't resist

But as I said earlier traditionally ...... The world was flat. ,

I know I add nothing........ Still trying to tame my missed placed humour ....

And hopefully learn something

[Dockhead]

Originally Posted by europaflyer

Perhaps this is UK vs US terminology. I have always heard, and used, the phrase 'true wind' to describe the path and velocity the wind has over the ground. Calling it 'sailing wind' is just confusing because it isn't what you 'sail to' - that is apparent wind!.

"True Wind" was traditionally whatever wind you have which is not influenced by boat motion. Only when we got our own precise anemometers did the problem arise about how to distinguish land-referenced from water-referenced "true wind". So the instrument makers invented the term "ground wind".

All modern instrument systems use this term, so maybe it's worth living with it. In a discussion like this, its incredibly useful, because its really all about how the wind shifts with the current. What do you call the ground refernced wind? It's not the same as True Wind your instruments tell you about.

And you get a grandiose porridge if you try to shove that into Apparent Wind.

[Paul Elliott]

Originally Posted by europaflyer

[...]
I don't agree at all! Perhaps you say that because people have 'traditionally defined' the lee bow effect wrongly, or at least that is your understanding of it.
[...]

So can you define the lee bow effect for me? No doubt I will agree with your definition. The explanations I have seen in the literature are either wrong, or ridiculously obvious, not worthy of the dignifying label "effect".

"True Wind" is context dependent. On land or in weather reports it means one thing, but on the water while sailing it means another. I wish this weren't the case, but it is, largely as an artifact of historical sailing instrumentation. Until we can agree on better terms, I'm going to stick with the traditional sailing ones, and define them as necessary. And I realize that not everybody agrees with these definitions, which is why I define them in these types of discussion.

At least we agree on the definition of Apparent Wind, but this doesn't help the resolve the debate(?) I actually think we are debating terminology without knowing it.

[Hoofsmit]

Originally Posted by Paul Elliott

So can you define the lee bow effect for me? No doubt I will agree with your definition. The explanations I have seen in the literature are either wrong, or ridiculously obvious, not worthy of the dignifying label "effect". "True Wind" is context dependent. On land or in weather reports it means one thing, but on the water while sailing it means another. I wish this weren't the case, but it is, largely as an artifact of historical sailing instrumentation. Until we can agree on better terms, I'm going to stick with the traditional sailing ones, and define them as necessary. And I realize that not everybody agrees with these definitions, which is why I define them in these types of discussion. At least we agree on the definition of Apparent Wind, but this doesn't help the resolve the debate(?) I actually think we are debating terminology without knowing it.

We have been through this before...... They won't be happy !

[Seaworthy Lass]

Originally Posted by europaflyer

In my mind it works for any angle of current, other than two points (roughly, to windward and to leeward) where it makes no difference to the direction of the apparent wind vector. At these points, it switches from helping to on one tack (coming from your leeward side) to hindering you (coming from windward).

It is a very, very subtle point, that I can picture but not quite explain properly (yet), that these 'switchover points' are not exactly to windward and leeward. Picturing it using my 'sum of vectors' approach for the wind as above, it can be seen that these points where the 'current induced wind vector' does not affect the 'apparent wind vector' are when the tide is just slightly to port of windward and leeward when on starboard tack, and just slightly to starboard when on port tack. As I say, this is a VERY subtle (ie. totally unimportant!) point, that I may try to support with a diagram if I have the time later. No, I'm not sure if I'm right either.

Hi Europlayer
Great to have some more people chiming in. It is not quite the "fray", I think Dockhead and I are really enjoying tossing ideas back and forth and poking holes and challenging thoughts (at least I am LOL).

Your comment about the effect working a full 180 degrees has made me stop and think. It only works for 135 odd degrees on the lee side. But it seems to work on the windward side when the current is 0-45 degrees, completing the full 180. So it is not really a lee bow effect at all, but what side of the ground wind the current wind is on.

Wow, this is ground breaking stuff. This I have never heard of and I am feeling a bit (OK more than a bit) nutty writing it. But weirdly it makes sense. Why should the effect be different depending on a boat's pointing ability? It has nothing to do with the boat. Really going out on a limb here LOL. I need help here folks with someone to point out why this is not so .

By the way, this leads to all sort of other things .....

Originally Posted by europaflyer

Oh and by the way Seaworthy, what is this favourable vs advantageous tack issue? Probably being thick but I don't quite understand the difference...

Favourable tack (a common term) is the tack that is closest to your CTS. In a perfect world where you could predict things perfectly and tack at exactly the perfect point for the final tack, then sailing the favourable tack first does not result in getting to the mark more quickly. Unless your true wind shifts, you need to sail both tacks. In practice that perfect point to tack is more difficult to judge the further away you are from the mark, so taking the favourable tack first is sensible.

The advantageous tack is what I would call the tack that there is a real advantage being on as it will shorten your time to reach the mark (possibly substantially.) This situation will only occur if the true wind is variable along that leg. It is in common practice to utilise this when the predicted change to come in true wind is due to variable ground wind. It also applies if there is variable current though. No one seems to mention this, but I am sure this is used by tacticians. There must be dozens of good books written about this.

Anyone here like to comment on that?

[IdoraKeeper]

Originally Posted by AndytheSailor

My only comment, while agreeing with Seaworthy, is that if you are crossing a tide on a tack, if the tide is on the lee bow then you can lay you course less close to the wind, thus sail faster and still make your destination. I know this doesn't deal with the maths but it does offer the logic.

I totally agree with this.
Would like to add that when the current is on the Lee bow it cancels out some or all of the drift depending on strength of the current. More of the energy goes into vmg toward whatever point you have chosen. If that energy is not directed at the correct spot you are proceeding more rapidly in the wrong direction and will not be able to make it up on the next tack.
I doubt that in a pinching ( ouch ) can over come the inherent losses in boat speed. This will be better worse depending on your rig. My brothers sloop beats my ketch at this game 100% of the time. I can only keep up when I can get a nose Lee bowing on a broad reach.

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

My theory is: There is tack that is advantageous when tacking in a situation with variable cross current.

Following the tack that gives you a heading closest to the calculated CTS (and tacking on this criteria) will give you the shortest time taken for the journey.

This tack is also with the current on the lee bow I think, but I have not had the time to think about if this applies universally (it certainly does for all situations I can think of, just try it out).

Just looked back at my theory (from what seems like eons ago, but it was just over a day).

I still hold with the first part.
The second part only applies in certain circumstances.
The third part is totally incorrect.

Making progress slowly .

[Dockhead]

Originally Posted by IdoraKeeper

Would like to add that when the current is on the Lee bow it cancels out some or all of the drift depending on strength of the current. More of the energy goes into vmg toward whatever point you have chosen. If that energy is not directed at the correct spot you are proceeding more rapidly in the wrong direction and will not be able to make it up on the next tack.
.

If you have correctly calculated your Course to Steer, then you don't have this issue, which is not related to lee bow effect.

This is why Seaworthy and I dealt with that issue separately and got it out of the way.

You will never be "fighting the tide" if you are sailing in, or tacking towards, the right direction in the first place. It's the same challenge as when you cross changing current waters (like the English Channel) without tacking.

Tacking adds a whole different layer of complexity. Get those layers confused, and you're really scrod.

[Dockhead]

Originally Posted by Seaworthy Lass

Favourable tack (a common term) is the tack that is closest to your CTS. In a perfect world where you could predict things perfectly and tack at exactly the perfect point for the final tack, then sailing the favourable tack first does not result in getting to the mark more quickly. Unless your true wind shifts, you need to sail both tacks. In practice that perfect point to tack is more difficult to judge the further away you are from the mark, so taking the favourable tack first is sensible.

The advantageous tack is what I would call the tack that there is a real advantage being on as it will shorten your time to reach the mark (possibly substantially.) This situation will only occur if the true wind is variable along that leg. It is in common practice to utilise this when the predicted change to come in true wind is due to variable ground wind. It also applies if there is variable current though. No one seems to mention this, but I am sure this is used by tacticians. There must be dozens of good books written about this.

Anyone here like to comment on that?

Yes, this is very basic sailing. If you can almost but not quite lay your mark in one tack, don't sail the short (unfavorable) tack first. On paper - geometrically - it doesn't matter, but in real life, there will be at least little wind shifts which will nullify what you've done, plus you just can't judge that accurately.

Another way to describe lee bow effect is that the wind shift from the current change could flip around the favorable tack to the other side (making it an "advantageous tack"? I still don't quite understand the terminology). Then the situation looks like my drawings. It doesn't have to be like that to get a lift, but the less dead upwind the situation is in relation to ground wind, the less benefit, as you will not be able to tack every time the current changes - there will be one favorable tack which stays favorable despite the wind shifts.

So we have been discussing an idealized situation.

What if our CTS is not always dead on the ground wind?

Also, the strength of the current varies during the tide cycle.

And, the stronger the wind is, and the weaker the current, the less the true wind direction will be changed.

With 30 knots of wind, like during my last crossing, and at neaps, I bet there is no effect at all.

These factors will define the limits of the effect.



I bet that 90% or more of people who think they understand lee bow effect, actually only understand that their ground track will be shorter if they tack with the current, keeping it on the lee bow. Now we know for sure that this in itself brings no benefit whatsoever.

[Seaworthy Lass]

Originally Posted by europaflyer

I think people complicate the lee-bow effect somewhat. The apparent wind is the vector sum of the true wind, the headwind (equal and opposite to the boats speed) and the current induced wind (equal and opposite to the current). By putting the 'current induced wind' on the correct side of the boat, you will be able to point closer to the true wind (but, of course, no closer to the apparent wind... which is where a lot of the confusion comes from I think).

The effect is most pronounced in light airs, as the 'current induced wind' represents a far greater component of the 'apparent wind' than it would if the true wind were stronger. Take for example a scenario where true wind is 10kt, at 90 degrees to a 5 knot current. Boat speed is 5 knots. (diagram below... hopefully)

It can be seen that the 'lee bow' boat points much closer to the true wind than the 'weather bow' boat, perhaps by around 60 degrees. This results in a course through the water much more direct to windward, and therefore substantially greater VMG to windward for the lee-bow boat. This is what the lee bow effect is all about. It is worth noting that this difference would reduce as true windspeed increases, but would still remain significant.

So... even though the motion of the tide does not itself contribute to the VMG to windward of either boat, the change in apparent wind it produces makes a substantial difference in how close the boats sail to the 'true wind', and therefore their VMG to windward.

Hi Europaflyer
Just a comment leading on from your definition of the lee bow effect (which is a nice clear simple one):

When beating upwind, the tack with current on the lee bow is only the favourable tack if the the heading is closer to the CTS (note this is not necessarily the rhumb line) than the other tack. In my ignorance, I was assuming the lee bow effect was somehow universal for situations where you needed to tack. Just pointing this out in case others had not thought of this too.

Given that it is now simply a "lee bow effect" if my post #174 is correct (it occurs from 0 degrees your angle for best VMG to windward also), perhaps the name should be changed?

I really must stress that all these decisions are made relative to the Course to Steer (CTS) NOT the rhumb line. Your charplotter/GPS will not give you this information. You need to make calculations (programs are now available for this). Choosing to make your decisions based on the rhumb line can be very detrimental.

The two CTS threads from back in Feb of this year are a heap to wade through, but explain al of this well. Maybe at some stage I should copy and paste all the relevant important posts in one thread so people don't have to wade through hundreds of posts. Would anyone find this useful?

[Dockhead]

Originally Posted by Paul Elliott

So can you define the lee bow effect for me? No doubt I will agree with your definition. The explanations I have seen in the literature are either wrong, or ridiculously obvious, not worthy of the dignifying label "effect".

"True Wind" is context dependent. On land or in weather reports it means one thing, but on the water while sailing it means another. I wish this weren't the case, but it is, largely as an artifact of historical sailing instrumentation. Until we can agree on better terms, I'm going to stick with the traditional sailing ones, and define them as necessary. And I realize that not everybody agrees with these definitions, which is why I define them in these types of discussion.

At least we agree on the definition of Apparent Wind, but this doesn't help the resolve the debate(?) I actually think we are debating terminology without knowing it.

We defined wind terms at the very beginning of the thread. This terminology corresponds to the way your instruments read, so maybe not worth fighting it? My instruments will give me "ground wind", for example.

I agree that "true wind" is a wide tent, into which different ideas can and have been shoved. If you're uncomfortable with our using "true wind" for the water-referenced flavor of true wind, why don't you use that? Instead of "ground wind", you could say "ground-referenced true wind", and instead of "true wind" you could say "water-referenced true wind". Bit of a mouthful, and it deviates from the terminology used in your instruments, but will be extremely clear to everyone what you mean, which is good.

The only thing I would object to and be unable to accept would be a description of the effect of current as creating "apparent wind", as someone above did. This is false. If the boat is dead in the water, and there is zero ground wind, but you can feel 4 knots of wind because a current is running, it would be a perversion to call this "apparent wind". We can't get around the fact that there are two flavors of what used to be called true wind.