Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 11-11-2013, 07:13   #106
Registered User
 
Hoofsmit's Avatar

Join Date: Jul 2013
Location: cornwall uk
Posts: 569
Ok .... Got it know....... Just because !
Let's call it hyperthetical ( can't spell or do algebra )
Because we can disregard something's and make others constant ( even though we know they never are) to come up with a answer using some sums.

I prefer experience ( even gut feeing )and standing on the winners podium as proof that lee bow works ( in the right conditions )

But it's still an interesting thread
__________________

__________________
Hoofsmit is offline   Reply With Quote
Old 11-11-2013, 08:06   #107
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Dockhead View Post
The effect of lee bowing on wind speed ranges from nil to very slight -- see the post above. It's best to leave this aside for a moment so that we can concentrate on figuring out whether it is possible to exploit the change in wind direction to get a lift, by lee bowing. The jury is still out. Seaworthy is struggling manfully to put together a case that you can. I am blundering around a bit but seem to be stumbling towards the conclusion that you can't. Stay tuned. When we make a little more progress with the abstract principles, then we will start doing maths, I think.
Yes, sorry Estarzinger and Transmitterdan, you are not up to speed here. Wait until the penny has sunk in for Dockhead on this issue and then we can both explain this LOL. Understanding this statement that Dockhead correctly made is the starting point:

Quote:
Originally Posted by Dockhead View Post
Thesis: sailing dynamics cannot possibly be affected by any constant current.

Dockhead, you are just so tantalisingly close to getting that there is an advantage on a particular tack with current reversing direction. (I will poke lots of holes in some of what you have said in my next post).
I think the key for you is to understand that "yes, you do feel the change in true wind when the current changes". Think of sitting on a magic carpet that an unknown force (current) is moving all over the place. If you sit in one spot and the rug that was moving left relative to you now starts to move right, you will feel the wind on you face move from one side to the other. If you could sail on the rug, you could move anywhere on the rug as long as it kept moving. All this is with zero ground wind, but the same principle applies to any ground wind - the true wind, ie the wind you feel on the water or on the magic carpet, is just the sum of ground and current (and leeway, but forget about this for now).
__________________

__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Old 11-11-2013, 08:13   #108
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Seaworthy Lass View Post
Yes, sorry Estarzinger and Transmitterdan, you are not up to speed here. Wait until the penny has sunk in for Dockhead on this issue and then we can both explain this LOL. Understanding this statement that Dockhead's correctly made is the starting point:



Dockhead, you are just so tantalisingly close to getting this. (I will poke lots of holes in some of what you have said in my next post).
I think the key for you is to understand that "yes, you do feel the change in true wind when the current changes". Think of sitting on a magic carpet that an unknown force (current) is moving all over the place. If you sit in one spot and the rug that was moving left relative to you now starts to move right, you will feel the wind on you face move from one side to the other. If you could sail on the rug, you could move anywhere on the rug as long as it kept moving. All this is with zero ground wind, but the same principle applies to any ground wind - the true wind, ie the wind you feel, is just the sum of ground and current (and leeway, but forget about this for now).
I'm with you so far. Looking forward to seeing poked holes.

So have we agreed that a non-changing current cannot create any Lee Bow effect?

So are we now concentrating on the changing current -- i.e., tacking across the English Channel scenario?

I see two different ways that Lee Bow effect doesn't exist in a changing current situation. The second way is somewhat doubtful, but the first way remains.

So why don't you construct a complete theory of why it does exist in a changing current situation, and I'll try to poke holes this evening when I come home from work.

But tell me first -- have you formed this view? Do you believe it exists? I have an open mind, but tend to think, so far, that it does not.

Then hopefully we will reach some consensus on the theory and can go on to run some scenarios with maths to verify.
__________________
Dockhead is offline   Reply With Quote
Old 11-11-2013, 08:23   #109
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Dockhead, you wanted holes poked in your posts. Firstly let me preface it, by saying that I agree with a lot you have said.
Here are a few holes though .

You are constantly mixing up frames of reference, bringing up VMG and COG etc. These are land based.
Your use of the term wind seems to mainly refer to ground wind not true wind.

eg "We get to the point in the water which is at the end of that line by sailing so as to maximize VMG, that is maximum VMG to windward if we can't lay it in one tack."

And: "So which tack we are at what point cannot matter if we assume the wind doesn't change. That is mathematically definite if we make the same VMG to windward on both tacks -- the ONLY thing which matters is VMG to windward."

That leads you to make statements like this:
"Leaving out affects of changing wind, no -- I can say with robust confidence -- it does not matter which tack you are on when."

When you made the above statement you had just been discussing ground wind, so I presume your use of the word "wind" in this sentence was meant to mean ground wind?

If not, you can't "leave out the effects of changing wind". It is because the true wind is changing as the current changes that this whole phenomenon exists.

Quote:
Originally Posted by Dockhead View Post
Yes, I agree, absolutely if you need to be on one tack rather than the other from point of view of changing wind, then this is valid.

If you need more wind, then lee bow. If you need less wind, windward bow. This effect is real. But this is the only real part of it -- the rest is pure confusion.
Nonsense . Think about what you are saying here. You are not sailing to ground wind (which is what I presume you mean by "changing wind").
You cannot induce more true wind by what tack you are on. And your speed relative to the water is the same on each tack (unless your boat is not balanced well), so your apparent wind should be the same on each tack. So you cannot induce more or less true wind OR apparent wind according to what tack you are on. (And I presume you are not saying you can alter the ground wind by what tack you are on )

Quote:
Originally Posted by Dockhead View Post
I earlier said that you should lee bow to increase the true wind, windward bow to reduce it. The idea being that if you need more wind, you'll get a lift by lee bowing.

But that is only really true in a very limited set of circumstances. First of all -- if the ground wind direction is perpendicular to the current, the current will have no effect on true wind speed.
Huh? If the ground wind is perpendicular to the current, the effective change in true wind is maximum not zero (true wind is the sum of ground wind plus wind from current movement).

Quote:
Originally Posted by Dockhead View Post
When the lee bow is in the current, the true wind will be freerer than the ground wind.

So you're getting a lift from a ground-referenced point of view:
Why reference anything to the ground? This is where you are going wrong.

Quote:
Originally Posted by Dockhead View Post
Here is a precise formulation of the problem:
True wind direction changes when the current changes, so why not get a lift by tacking with the current, to catch the freerer wind every time.

Problem with this is that it assumes that the boat is moving along a constant COG. Since we will moving along (or tacking towards) a point along a constant heading, our COG changes with the changes in the current and nullifies the Lee Bow Effect.
Why are you bringing COG into the equation? You are discussing true wind, which is water not land referenced. This is where you are going wrong, you are constantly mixing up your frames of reference. COG changes have nothing to do with nullifying the effect.

Quote:
Originally Posted by Dockhead View Post
At a given constant heading, our boat's keel will be aligned at a constant angle to the True Wind, not a constant angle to the Ground Wind, because the direction at which the boat moves in relation to the land is changed by the same current vectors which change the direction the wind moves in relation to the land.

So the most efficient path through the water will always have the same angle to the True Wind, notwithstanding the changes in current. The True Wind will change compass direction, but will not change direction in relation to our boat's keel (and therefore her sails) while we are sailing on a constant heading.

To say it another way -- and this is really funny!!! The True Wind will change when the tide changes. But the Apparent Wind Will Not change!!! Because the current imparts the same vectors to our boat's motion in relation to land, as it does to the wind.
Of course the apparent wind will change LOL. The change in direction and changing amount of current will alter the true wind and therefore boat speed, so the apparent wind WILL alter.

Quote:
Originally Posted by Dockhead View Post
QED. This version of Lee Bow Effect is obliterated, as far as I'm concerned. The problem is caused by failing to compare apples to apples -- by confusing ground and water referenced vectors. It would be right to now run some scenarios and do some numbers to double check.
I have to laugh that you are telling me my theory is wrong because I am confusing ground and water referenced vectors when this is precisely the reason you are going wrong, over and over.

OK, enough holes for now. I need some response before I poke more holes LOL.

Note : So far not one single hole you have tried to induce in my theory made it drip, let alone leak .
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Old 11-11-2013, 08:42   #110
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Dockhead View Post
I'm with you so far. Looking forward to seeing poked holes.

So have we agreed that a non-changing current cannot create any Lee Bow effect?
Yes, we agree. You don't have to rub in in LOL, I am still licking my wounds, although I have managed to get all the egg off my face LOL.

Quote:
Originally Posted by Dockhead View Post
So are we now concentrating on the changing current -- i.e., tacking across the English Channel scenario?
Correct. Changing in direction is the key here though, not just changing amount.

Quote:
Originally Posted by Dockhead View Post
I see two different ways that Lee Bow effect doesn't exist in a changing current situation. The second way is somewhat doubtful, but the first way remains.

So why don't you construct a complete theory of why it does exist in a changing current situation, and I'll try to poke holes this evening when I come home from work.
My complete theory is that the CTS is the most efficient way of sailing. You cannot do this if you are tacking, so the next best thing is the tack that gives you a heading closest to this.

I have shown a clear example of this working, and by a huge margin (see my only diagram on this thread).

Quote:
Originally Posted by Dockhead View Post
But tell me first -- have you formed this view? Do you believe it exists? I have an open mind, but tend to think, so far, that it does not.
Yes, I think it exists. Lots of people must have thought this through (it is just an extension of the CTS), and I think that is the real "lee bow effect". Pinching to achieve this will not work though. You must sail with the sails perfectly trimmed close hauled when going to windward (with possibly at times the exception of doing this when you have just misjudged when to tack to reach the mark with your sails perfectly trimmed the whole way).

Quote:
Originally Posted by Dockhead View Post
Then hopefully we will reach some consensus on the theory and can go on to run some scenarios with maths to verify.
Hope so too .
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Old 11-11-2013, 08:55   #111
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

By the way, this is going to the next step, but is something to think about (I am not focussing on this at the moment, but still have vague thoughts whirling around that need to be worked on).

It is regarding my comment about what to do with oblique current where you did not have to tack when the current changed sufficiently in amount or direction to enable you to lay your CTS. What is needed is a new CTS calculation and you simply follow that.

The nice thing in this case though is this will occur the most when the current is very oblique and in this case the CTS will not be affected much anyway.
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Old 11-11-2013, 09:38   #112
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

OK, I think there are a few valid holes found, which is good However, many, maybe most of the holes are not holes, but the result of poor expression on my part, poor comprehension on your part, or a combination.

Quote:
Originally Posted by Seaworthy Lass View Post
Dockhead, you wanted holes poked in your posts. Firstly let me preface it, by saying that I agree with a lot you have said.
Here are a few holes though .

You are constantly mixing up frames of reference, bringing up VMG and COG etc. These are land based.
Your use of the term wind seems to mainly refer to ground wind not true wind.

eg "We get to the point in the water which is at the end of that line by sailing so as to maximize VMG, that is maximum VMG to windward if we can't lay it in one tack."

And: "So which tack we are at what point cannot matter if we assume the wind doesn't change. That is mathematically definite if we make the same VMG to windward on both tacks -- the ONLY thing which matters is VMG to windward."

That leads you to make statements like this:
"Leaving out affects of changing wind, no -- I can say with robust confidence -- it does not matter which tack you are on when."

When you made the above statement you had just been discussing ground wind, so I presume your use of the word "wind" in this sentence was meant to mean ground wind?

If not, you can't "leave out the effects of changing wind". It is because the true wind is changing as the current changes that this whole phenomenon exists.
Sorry, what I wrote there was concerning a non-changing current scenario. We had not gotten to changing currents yet, so we were dealing without any effects of any changing wind. No change of current = constant deviation between Ground Wind and True Wind = no changing wind effects.

A few points:

1. Here is an important mistake: VMG is not ground-referenced. VMG, as a general concept, means Velocity Made Good, which can be referenced to anything, ground, water, my boat, the moon if you like. We are talking here about VMG to windward, which is referenced to the source of the wind, the True Wind. This is a key concept in sailing dynamics because whenever you are trying to go somewhere which you can't lay in one tack, then you will lay that place (notice I said "place" not "mark", because we will sailing to a point in the water via a CTS calculation, not to the mark) best and fastest by sailing to highest VMG to windward according to your polars.

This is crucial to the discussion since we are tacking by definition of the problem.


2. Other stuff, like this: "So which tack we are at what point cannot matter if we assume the wind doesn't change. That is mathematically definite if we make the same VMG to windward on both tacks -- the ONLY thing which matters is VMG to windward." Is all valid for the non-changing current scenario, which is what I was talking about. It is not relevant to the changing current situation. In the non-changing current scenario, neither Ground Wind nor True Wind changes during the leg.



Quote:
Originally Posted by Seaworthy Lass View Post
Nonsense . Think about what you are saying here. You are not sailing to ground wind (which is what I presume you mean by "changing wind").
You cannot induce more true wind by what tack you are on. And your speed relative to the water is the same on each tack (unless your boat is not balanced well), so your apparent wind should be the same on each tack. So you cannot induce more or less true wind OR apparent wind according to what tack you are on. (And I presume you are not saying you can alter the ground wind by what tack you are on )
Right! That's a good insight. My mistake. Wow, that's interesting. That's a fallacy I seem to have somehow gotten infected by from Lee Bow Effect Cultists. They often explain this effect, and I seem to have fallen for it. You're right, it can't be. True Wind will always be the same no matter what tack you are on -- duh, of course. Good one.


Quote:
Originally Posted by Seaworthy Lass View Post
Huh? If the ground wind is perpendicular to the current, the effective change in true wind is maximum not zero (true wind is the sum of ground wind plus wind from current movement).
Sorry, the effect of current on True Wind Speed is minimum at 90 degrees angle between Ground Wind and current. Not zero -- you are technically correct. But at its minimum, and a very small value in a typical case with say 3 knots of current and 20 knots of Ground Wind (typical English Channel at peak tidal flow), where you get 20.224 knots of True Wind out of 20 knots of Ground Wind. So we can ignore this for these purposes at 90 degrees. If the wind direction is parallel to the current direction, the effect is at its maximum -- 23 knots if opposite directions, 17 knots if same direction between Ground Wind and current. So the effect on True Wind speed of a 3 knot current will vary from 0.224 knots when the angle is 90 degrees, to 3 knots (either plus 3 or minus 3) when the angle is either 0 or 180.

However, the relevance of this is suddenly much less than I had thought.


Quote:
Originally Posted by Seaworthy Lass View Post
Why reference anything to the ground? This is where you are going wrong.
I have found, thanks to you, one place where I might have made this mistake, but it's not here.

I said "you're getting a lift from a ground-referenced point of view" to underline how a mistake might be being made. I go on to show that from a water-referenced point of view, there's no lift (and this is the unchanging current scenario which we seem to agree on).

Quote:
Originally Posted by Seaworthy Lass View Post
Why are you bringing COG into the equation? You are discussing true wind, which is water not land referenced. This is where you are going wrong, you are constantly mixing up your frames of reference. COG changes have nothing to do with nullifying the effect.
Yes, this was Thesis #2 of mine, which is now in doubt. I will go back through it and try to decide whether it was pure bullocks from the beginning, or whether there is something in it worth salvaging, and get back to you.


Quote:
Originally Posted by Seaworthy Lass View Post
Of course the apparent wind will change LOL. The change in direction and changing amount of current will alter the true wind and therefore boat speed, so the apparent wind WILL alter.
Ditto.

Quote:
Originally Posted by Seaworthy Lass View Post
I have to laugh that you are telling me my theory is wrong because I am confusing ground and water referenced vectors when this is precisely the reason you are going wrong, over and over.
Huh? I don't recall ever saying your theory is wrong. I don't even have a complete statement of your theory -- eagerly awaiting it. When I see it, I will tell you why it's wrong If it is, of course

I think I've shown that there is one possible case of ground-water confusion, in my probably flawed Thesis #2, but that's the only one you've found! (Not over and over ) Still, it's a great service, for which I am grateful Possibly saved me from being stuck with several more pages of pure bullocks with my name on it living forever in the internet

The other great service you did me, was to relieve me of the one bit of Lee Bow Effect Cult nonsense which I had accepted, concerning wind speed lift. Duh, I can't believe I fell for that. Thank you.



Quote:
Originally Posted by Seaworthy Lass View Post
Note : So far not one single hole you have tried to induce in my theory made it drip, let alone leak .
I haven't had a chance to poke any holes in your theory, because I don't have any statement of it. I will do you the same kindness you have just done me, if you'll give me a comprehensive statement of your theory. I have only been poking holes at a straw man -- my own vague idea of what I suppose the Lee Bow Effect Cult theory says. It's a weak straw man because I'm not a Cultist and therefore don't have a vision. That's why we need your theory, something one can get one's teeth into without any straw aftertaste
__________________
Dockhead is offline   Reply With Quote
Old 11-11-2013, 09:54   #113
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,867
Images: 4
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Seaworthy Lass View Post
Yes, sorry Estarzinger and Transmitterdan, you are not up to speed here. [...]
I think when you throw in the polars, the continuously changing current, and so the continuously changing wind, then differential equations are required for the mathematically correct solution. I'm not saying that this is a practical method, but the optimum AWA will change with the wind speed.

And for the record, I was the first to mention polars in this thread!

But back to the Lee-Bow effect, why do we even need to consider the changing-current case? We can show that the Lee-Bow "rule" does not always result in an optimum solution and in truth does not bring anything useful to the course to steer problem. If this holds in the fixed-current case, it surely holds in the changing-current one.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline   Reply With Quote
Old 11-11-2013, 10:20   #114
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Paul Elliott View Post
I think when you throw in the polars, the continuously changing current, and so the continuously changing wind, then differential equations are required for the mathematically correct solution. I'm not saying that this is a practical method, but the optimum AWA will change with the wind speed.

And for the record, I was the first to mention polars in this thread!

But back to the Lee-Bow effect, why do we even need to consider the changing-current case? We can show that the Lee-Bow "rule" does not always result in an optimum solution and in truth does not bring anything useful to the course to steer problem. If this holds in the fixed-current case, it surely holds in the changing-current one.
Seaworthy has just blown out of the water the old idea that there is any lift from wind force resulting from any different tacking strategy in changing currents.

She and I have agreed that there is no lee bow effect at all if the current is not changing.

I think she is putting together a defense of the idea that you can get a lift by exploiting the changing wind angle in a changing current situation with your tacking strategy. That is, if you tack every time the current changes direction, so that you keep your lee bow into the current, the wind will be freerer and you get a lift -- that's the only formulation of the Lee Bow Effect Theory I have, and it may be a poor one; I think someone who believes in it might state it more powerfully than that. But in any case, I think it is safe to say that it is a situation where it is the changing current which does the trick.

If it works the way it's supposed to, you can really ignore your polars, I think. In a classic situation of 000 wind and current alternating between 90 and 270, you will only get a maximum 0.22 knot increase of True Wind if the Ground Wind is 20 knots, around 1%, absolutely out of any possible level of accuracy of your polars. That would be at peak flow of 3 knots, and that only occurs once every 6 hours. So really, simple trig will do it.

If Seaworthy can formulate the theory, then we'll run some scenarios and calculate the time.
__________________
Dockhead is offline   Reply With Quote
Old 11-11-2013, 10:44   #115
CF Adviser

Cruisers Forum Supporter

Join Date: Aug 2009
Boat: Custom Van De Stadt 47 Samoa
Posts: 3,744
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Dockhead View Post
If it works the way it's supposed to, you can really ignore your polars, I think. In a classic situation of 000 wind and current alternating between 90 and 270, you will only get a maximum 0.22 knot increase of True Wind if the Ground Wind is 20 knots, around 1%, absolutely out of any possible level of accuracy of your polars.
Dockhead, you are obviously not a racer. You win if you are only one meter ahead of the other boat. Very small differences often make the difference in winning vs. not. And modern slow race boats sail at (boat speed) around the true wind speed. The AC boats sailed downwind at about 2.5x the true wind speed and upwind 1.8x. so .22kts is absolutely important to them.

As to the alternating current situation you can obviously gain by being on the right side of each shift. Its just like playing regular oscillating wind shifts. Any racer would know that there is a correct way/side (the lifted side) to be on those shifts.
__________________
estarzinger is online now   Reply With Quote
Old 11-11-2013, 10:56   #116
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Dockhead View Post
1. Here is an important mistake: VMG is not ground-referenced. VMG, as a general concept, means Velocity Made Good, which can be referenced to anything, ground, water, my boat, the moon if you like. We are talking here about VMG to windward, which is referenced to the source of the wind, the True Wind. This is a key concept in sailing dynamics because whenever you are trying to go somewhere which you can't lay in one tack, then you will lay that place (notice I said "place" not "mark", because we will sailing to a point in the water via a CTS calculation, not to the mark) best and fastest by sailing to highest VMG to windward according to your polars.

This is crucial to the discussion since we are tacking by definition of the problem.
Yes, you are right, my error, I had a brain freeze. VMG is not necessarily ground related. So point 2 is valid too.

Quote:
Originally Posted by Dockhead View Post
Sorry, the effect of current on True Wind Speed is minimum at 90 degrees angle between Ground Wind and current. Not zero -- you are technically correct. But at its minimum, and a very small value in a typical case with say 3 knots of current and 20 knots of Ground Wind (typical English Channel at peak tidal flow), where you get 20.224 knots of True Wind out of 20 knots of Ground Wind. So we can ignore this for these purposes at 90 degrees. If the wind direction is parallel to the current direction, the effect is at its maximum -- 23 knots if opposite directions, 17 knots if same direction between Ground Wind and current. So the effect on True Wind speed of a 3 knot current will vary from 0.224 knots when the angle is 90 degrees, to 3 knots (either plus 3 or minus 3) when the angle is either 0 or 180.

However, the relevance of this is suddenly much less than I had thought.
We are discussing different things here - you are looking at the effect of current altering true wind speed, I am looking at the affect of current altering true wind direction. Yes, the affect on amount of true wind is maximum when the current is parallel. The affect on true wind direction is maximum when the current is perpendicular.

I don't think we disagree here. The reason why I am focussed on change in wind direction though, is that this is what affects the tack selection. A change in wind amount will not do this. So the effect of choosing the correct tack is maximum when the current is perpendicular (and so this is particularly relevant to a Channel crossing).

Quote:
Originally Posted by Dockhead View Post
I think I've shown that there is one possible case of ground-water confusion, in my probably flawed Thesis #2, but that's the only one you've found! (Not over and over ) Still, it's a great service, for which I am grateful Possibly saved me from being stuck with several more pages of pure bullocks with my name on it living forever in the internet

The other great service you did me, was to relieve me of the one bit of Lee Bow Effect Cult nonsense which I had accepted, concerning wind speed lift. Duh, I can't believe I fell for that. Thank you.
Glad to help. Few things made clear for me too, thanks to you. That is the great thing about these discussions. Although not possible, they would be a million times easier face to face though. This would all take a fraction of the time to sort out and maybe lead to all sorts of new ideas. Will have to at least be thankful for the internet .

Quote:
Originally Posted by Dockhead View Post
I don't recall ever saying your theory is wrong. I don't even have a complete statement of your theory -- eagerly awaiting it. When I see it, I will tell you why it's wrong If it is, of course
And:
Quote:
Originally Posted by Dockhead View Post
I haven't had a chance to poke any holes in your theory, because I don't have any statement of it. I will do you the same kindness you have just done me, if you'll give me a comprehensive statement of your theory. I have only been poking holes at a straw man -- my own vague idea of what I suppose the Lee Bow Effect Cult theory says. It's a weak straw man because I'm not a Cultist and therefore don't have a vision. That's why we need your theory, something one can get one's teeth into without any straw aftertaste
Ah, such a simple thing I haven't stressed then .
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).

I thought you disagreed with this (or maybe we have progressed and you have an open mind regarding this LOL ).
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Old 11-11-2013, 11:01   #117
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,744
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by estarzinger View Post
Dockhead, you are obviously not a racer. You win if you are only one meter ahead of the other boat. Very small differences often make the difference in winning vs. not. And modern slow race boats sail at (boat speed) around the true wind speed. The AC boats sailed downwind at about 2.5x the true wind speed and upwind 1.8x. so .22kts is absolutely important to them.

As to the alternating current situation you can obviously gain by being on the right side of each shift. Its just like playing regular oscillating wind shifts. Any racer would know that there is a correct way/side (the lifted side) to be on those shifts.
I might be a racer

What we are trying to do is to understand the essence of this supposed Lee Bow Effect, to find its first principles and decide whether it is nonsense or not.

In order to do that, it is correct experimental procedure to eliminate as many variables as possible.

Therefore, we absolutely ignore shifts in the Ground Wind, although in real life they happen, and in real life, racers are playing them.

We absolutely ignore changes in Ground Wind speed -- also exist in real life, but they are not related to any supposed Lee Bow Effect and thus only muddy the water, so to speak.

Likewise with your polars -- even the hottest racer in the world cannot calculate a change of angle to the wind needed to respond to a 1% change in wind speed -- this is far out of any possible accuracy of helming, or of your polars themselves. Thus, it is absolutely correct to throw out the polars and assume a constant angle to the wind. Like that it will be much easier to see whether the Lee Bow Effect exists, or whether it is nonsense, based on first principles.

By the way, do you have an opinion on whether it exists or not? Would you care to throw your hat in the ring?
__________________
Dockhead is offline   Reply With Quote
Old 11-11-2013, 11:02   #118
Registered User
 
Hoofsmit's Avatar

Join Date: Jul 2013
Location: cornwall uk
Posts: 569
Quote:
Originally Posted by Paul Elliott View Post
I think when you throw in the polars, the continuously changing current, and so the continuously changing wind, then differential equations are required for the mathematically correct solution. I'm not saying that this is a practical method, but the optimum AWA will change with the wind speed. And for the record, I was the first to mention polars in this thread! But back to the Lee-Bow effect, why do we even need to consider the changing-current case? We can show that the Lee-Bow "rule" does not always result in an optimum solution and in truth does not bring anything useful to the course to steer problem. If this holds in the fixed-current case, it surely holds in the changing-current one.
So true

Let's throw out the changing current direction, unless you are going to work with a a scenario changing by every five degrees off the bow , each current variant having a five knot change in ground wind speed,

The objective of lee bowing is to get from a static point (mark of course , harbour entrance whatever ) to another Static point , not some point in the water which may well have moved by the time you get there . To prove lee bowing the current spinning around on a magic carpet or table cloth is not the point , give the workings a constant then work on the variables.
__________________
Hoofsmit is offline   Reply With Quote
Old 11-11-2013, 11:05   #119
Registered User
 
Hoofsmit's Avatar

Join Date: Jul 2013
Location: cornwall uk
Posts: 569
Quote:
Originally Posted by estarzinger View Post
Dockhead, you are obviously not a racer. You win if you are only one meter ahead of the other boat. Very small differences often make the difference in winning vs. not. And modern slow race boats sail at (boat speed) around the true wind speed. The AC boats sailed downwind at about 2.5x the true wind speed and upwind 1.8x. so .22kts is absolutely important to them. As to the alternating current situation you can obviously gain by being on the right side of each shift. Its just like playing regular oscillating wind shifts. Any racer would know that there is a correct way/side (the lifted side) to be on those shifts.
Yep
Preach it Bro
__________________
Hoofsmit is offline   Reply With Quote
Old 11-11-2013, 11:09   #120
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,815
Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by Paul Elliott View Post
I think when you throw in the polars, the continuously changing current, and so the continuously changing wind, then differential equations are required for the mathematically correct solution. I'm not saying that this is a practical method, but the optimum AWA will change with the wind speed.

And for the record, I was the first to mention polars in this thread!
The polars are extremely important for showing that pinching does not help. Aside from that they are irrelevant, as maximum performance occurs at their peak and that is what we should aim for. Sails perfectly trimmed close hauled so that you are at the maximum point of the polar is the only configuration Dockhead and I are discussing.

Quote:
Originally Posted by Paul Elliott View Post
But back to the Lee-Bow effect, why do we even need to consider the changing-current case? We can show that the Lee-Bow "rule" does not always result in an optimum solution and in truth does not bring anything useful to the course to steer problem. If this holds in the fixed-current case, it surely holds in the changing-current one.
The reason we need to consider changing current (with a component of the current perpendicular to the ground wind) is because it is the only one where it makes a difference what tack you are on. There is otherwise absolutely no advantage to one tack over the other.

Changing direction of the component of current perpendicular to ground wind alters true wind direction and this is why there is a tack that is advantageous.
__________________

__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 06:58.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.