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

[Dockhead]

Originally Posted by goboatingnow

Ok let me explain

(1) LBE in my mind has never had anything to do with pushing the keel to weather , it has to do with the improvement in wind angles ratios as a result of a particular current

(2) the effect is that , the reduction in apparent wind angle lost that occurs for a given windward heading change over the same apparent wind angle loss due to windward heading change without any current

(3) we all agree that that effect occurs elsewhere on the compass , and does not " require" the current to end up on the lee side, nor is there any abrupt change as the current crosses the bow ( which would be nonsense if it did )

(4) however the effect DOES occur in the situation where the current " ends" up fine on the lee bow. That alone removes the debunking of the myth.

(5) The effect does occur , the fact that it occurs elsewhere on other headings does not invalidate the fact that it occurs in that specific case

(6) pinching is not germane to the case , the effect occurs even when not pinching

(7) the total effect and ability to use the effect to gain advantage is not being determined here, it's not called the lee bow advantage


Dave

I don't understand any of this.

In a steady current, there can't be any Effect of any kind -- you've got one true wind, and you sail as usual in it. No effects of any kind. Just because a current is running, doesn't change in any way how your boat responds to wind angle and so forth. A steady current running only affects navigation -- how you lay your mark (ground attached) considering you are sailing in moving water. Has nothing to do with sailing dynamics.

In a changeable current it's a different story, and that's the main version of the Effect we've been discussing. But what you're talking about here does not seem to relate to changing currents, so can't -- as far as I can see -- exist at all.

To say this another way -- in a steady current, the true wind is changed only once. The true wind will now be the same irrespective of what tack you are on, where your bow is in relation to the current, etc. And the apparent wind will vary purely according to your boat speed and heading, just like it does in still water.

Can we agree on that? That would be quite fundamental and basic stuff.

So how can you exploit any shift in apparent wind, from the current? The current doesn't change the apparent wind; it changed the true wind, and won't change it again, no matter how you sail your boat, unless the current changes.

[goboatingnow]

Originally Posted by Dockhead

I don't understand any of this.

In a steady current, there can't be any Effect of any kind -- you've got one true wind, and you sail as usual in it. No effects of any kind. Just because a current is running, doesn't change in any way how your boat responds to wind angle and so forth. A steady current running only affects navigation -- how you lay your mark (ground attached) considering you are sailing in moving water. Has nothing to do with sailing dynamics.

In a changeable current it's a different story, and that's the main version of the Effect we've been discussing. But what you're talking about here does not seem to relate to changing currents, so can't -- as far as I can see -- exist at all.

This is what I call the dave perry error.

We are talking about LBE in conjunction with racing , i.e. as my diagram showed a "mark" to weather. The boat is trying to make a destination that is attached to the ground, ( and I think you'll agree)

it is affected by the ground wind generated by the current ( it being the same but opposite to the current vector )

it is affected by the ground wind from atmospherics.

It is affected by the wind causes by the boat moving


The vector sum represents the apparent wind that the boat is sailing in , ( leaving aside extraneous effects like lee way etc)

Hence any variation in current , ground wind, or boat speed causes the apparent wind to change , albeit in the vast majority of cases the current wind, is the far smaller of any of the effects


The LBE effect , shows that a current on the bow, provides greater wind angle then without such current ( irrespective of the merits of progression to the mark )

Hence equally , upon heading up , the reduction in apparent wind angle that occurs when you do so , is reduced by the presence of that current , which "CAN" appear find on the lee bow.

Hence the gain is that for a given turn to weather , apparent wind angle reduces less

dave

[Dockhead]

Originally Posted by goboatingnow

This is what I call the dave perry error.

We are talking about LBE in conjunction with racing , i.e. as my diagram showed a "mark" to weather. The boat is trying to make a destination that is attached to the ground, ( and I think you'll agree)

it is affected by the ground wind generated by the current ( it being the same but opposite to the current vector )

it is affected by the ground wind from atmospherics.

It is affected by the wind causes by the boat moving


The vector sum represents the apparent wind that the boat is sailing in , ( leaving aside extraneous effects like lee way etc)

Hence any variation in current , ground wind, or boat speed causes the apparent wind to change , albeit in the vast majority of cases the current wind, is the far smaller of any of the effects


The LBE effect , shows that a current on the bow, provides greater wind angle then without such current ( irrespective of the merits of progression to the mark )

Hence equally , upon heading up , the reduction in apparent wind angle that occurs when you do so , is reduced by the presence of that current , which "CAN" appear find on the lee bow.

Hence the gain is that for a given turn to weather , apparent wind angle reduces less

dave

Yes, but you are stating a navigation problem, not a sailing problem.

If the current is not changing, it's irrelevant to sailing, but not irrelevant to navigation. If the current is not changing, there are no wind effects at all. The true wind doesn't behave any differently from the true wind does at slack tide.

The navigation task is to calculate that spot in the water you have to sail towards, advance towards if you are tacking, which will put you at the land-tied mark at the end of the leg. In a non-changing current this is quite simple -- just add the current vector and you get corrected time and course.

Then comes the totally separate sailing problem -- get to that spot as fast as you can. If you can't lay that spot in one tack, then you have to tack. Once you are tacking, all your tacks must be made at maximum VMG to weather.

Therefore, it can never be that you get any kind of gain from pinching, unless you screwed up and ended up slightly downtide of your mark, and tacking again would consume too much overhead. But again, that is not a sailing problem.

This all seems extremely logical to me.

With respect, it looks to me you've got a muddle of confusion between sailing and navigation problems which really ought to be separated. Maybe I'm still missing something, but I've tried awfully hard to find it, and I don't think so.

[goboatingnow]

Jeepers, Again the LBE has never stated you get an overall advantage to the mark

If the current is not changing, it's irrelevant to sailing, but not irrelevant to navigation.

yes it is , because there is a current related wind, that affect s your ability to head in a particular direction , try sailing on a 8 knot ebb to see what I mean . All our destinations are ground based not water referenced. hence the way we head into or away from the current affects our ability to lay a mark.

Forget pinching , the effect is there even if you don't pinch.

This is a sailing issue

with a current on the bow ( and SL ha determined the range of effects around the compasses, so I'm only focusing on one specific area) ,

what is says is that turning to weather with the current fine on the bow, results in an improvement in the reduction of apparent wind angle over the reduction in apparent wind angle with no current , for a given heading change to weather,

Whether that results in a "navigation" advantage ( and I don't fully understand your use of the term) I don't know , for all I know the boat could be going back wards over the ground !!!

Forget "pinching" of for the moment,

forget VMG ( because there is a whole host of issues with VMG and peoples understand of it and in reality there are oddles of issues in determining if max VMG will get you to the mark, {because VMG cant see into the future ) any racer will tell you VMG tacking is not necessary the best way to the mark.

if the effect Ive described , as you head up , puts the current on the lee bow, then you have a a lee bow effect. If it didn't yout had a "bow effect" I suppose

Dave

[I.Grind]

Originally Posted by goboatingnow

it is affected by the ground wind generated by the current ( it being the same but opposite to the current vector ) it is affected by the ground wind from atmospherics. It is affected by the wind causes by the boat moving

So they are calling this first wind you describe "true wind".
This second one is "ground wind"
And lastly "apparent wind"

I do not believe that current generates ground wind. It may effect the way the ground referenced wind appears because the boat is moving with the current (which to me makes it apparent wind, because the boat is moving, but I'm going with the flow on this one just to see this thing resolved) which is being referred to in this thread as true wind, the direction the wind feels like it comes from when the boat is static in relation to the surface of the water.

Just trying to keep the terminology consistent so the rest of us have some hope of following this.

As you were.

[goboatingnow]

Originally Posted by I.Grind

So they are calling this first wind you describe "true wind".
This second one is "ground wind"
And lastly "apparent wind"

I do not believe that current generates ground wind. It may effect the way the ground referenced wind appears because the boat is moving with the current (which to me makes it apparent wind, because the boat is moving, but I'm going with the flow on this one just to see this thing resolved) which is being referred to in this thread as true wind, the direction the wind feels like it comes from when the boat is static in relation to the surface of the water.

Just trying to keep the terminology consistent so the rest of us have some hope of following this.

As you were.

ground wind, is the mariners term for land TRUE wind,

current generates a sailing wind. that wind is ground referenced as current is ground reference, sit in your boat , with no sails, being flush down a river on an 8 knot ebb and tell me theres no wind.

The vectors of ground ( land TRUE) wind) , current wind, and wind due to boat heading , can be added together to give the apparent wind , i.e. that wind is affected by three things, ground ( land TRUE) wind, current wind and boat speed ( i.e. the wing effect) which generates an opposing wind.

[Dockhead]

Originally Posted by goboatingnow

Jeepers, Again the LBE has never stated you get an overall advantage to the mark



yes it is , because there is a current related wind, that affect s your ability to head in a particular direction , try sailing on a 8 knot ebb to see what I mean . All our destinations are ground based not water referenced. hence the way we head into or away from the current affects our ability to lay a mark.

Forget pinching , the effect is there even if you don't pinch.

This is a sailing issue

with a current on the bow ( and SL ha determined the range of effects around the compasses, so I'm only focusing on one specific area) ,

what is says is that turning to weather with the current fine on the bow, results in an improvement in the reduction of apparent wind angle over the reduction in apparent wind angle with no current , for a given heading change to weather,

Whether that results in a "navigation" advantage ( and I don't fully understand your use of the term) I don't know , for all I know the boat could be going back wards over the ground !!!

Forget "pinching" of for the moment,

forget VMG ( because there is a whole host of issues with VMG and peoples understand of it and in reality there are oddles of issues in determining if max VMG will get you to the mark, {because VMG cant see into the future ) any racer will tell you VMG tacking is not necessary the best way to the mark.

if the effect Ive described , as you head up , puts the current on the lee bow, then you have a a lee bow effect. If it didn't yout had a "bow effect" I suppose

Dave

I suspected this confusion, and now I'm absolutely sure of it.

Look here --

A non-changing current has no affect on the wind whatsoever. It changes one ground wind to another true wind. But you are sailing in true wind; you don't feel the ground wind on the boat. So you've got one heading which is dead upwind and your whole polars around that, period -- just as if you were sailing at slack tide, except that the wind direction is different, but that's irrelevant if the current is not changing.

So when you're working out how to tack, how close to the wind to sail, etc. -- you are doing all of this around one heading which is just like you were sailing at slack tide. You don't care any more whether the current is on your lee bow or up your bum, than you would care whether you are sailing in the direction of the rotation of the earth or against it (that by the way is not a trivial point).

The fact that all your sailing is in regard to a point in the water which is moving in relation to your mark, presents a different problem -- namely the problem of how to figure out which way you need to head, or what line you need to advance down, to lay your mark most efficiently, which is basically a CTS computation.

If you do your CTS computation right, then you've got one heading to sail towards and you can tack any way you want to -- in relation to that one heading (which is slightly more complex than that, since your computation assumes a certain boat speed, and will be off if your factual boat speed deviates, but that's a different conversation).

Once you have figured out that you need to head 030, for example, to lay your mark which is at 000, for example, you can forget about the mark (unless you need to correct your computations, of course, but that is different). You tack straight down along 030 choosing either tack according to your tactics and getting there as fast as you can-- it matters not one whit, in a non-changing current, how the bow is aligned or not aligned with the current.

If you haven't figured out that you are trying to advance down 030, and you are making the mistake of trying to sail while looking at the mark, then of course -- a rule of thumb like this might lead to less error than you might otherwise have, because it allows you to work back in the effects of current. But this is not an "effect".

This is properly called a "fudge". Might actually be useful in the heat of battle when no one has time to work out a proper CTS, but it is still just a fudge which is a mishmash of sailing and navigation (meaning -- figuring how where to go, as opposed to sail towards a given direction as fast as you can -- different things altogether).


Extremely useful for clearing up this kind of confusion is to imagine, as I proposed way back earlier in the thread, that the water is stationary, and it is the mark which is moving. You can predict how the mark will move, and where it will be when. Therefore, you identify that spot on your non-moving water, and sail towards it.

So you are sailing on motionless water, and have identified a "mark" in the water, which is likewise stationary -- it is that spot which you have identified where your real mark will be when you finish your leeg.

You are sailing on motionless water just like at slack tide -- you have one true wind to sail in. So the sailing problem is now absolutely ordinary -- just like slack tide -- just get to your imaginary mark as fast as you can, sailing for maximum VMG to windward if you are tacking. The fact that your real mark is bobbing and weaving makes no difference -- the only significance is that you need to have accurately predicted where that bobbing and weaving land-tied mark, which is being dragged through the water by the moving land, will be when you finish your leg, and no doubt you will have to make some mid-course correction, but again -- it is not any kind of sailing problem.

[Hoofsmit]

Originally Posted by goboatingnow

I have not mixed up any winds , in my last diagram , all winds are ground referenced. I specifically computed the apparent wind vector and showed that as a ratio of heading change there is a net benefit with current on the lee bow and without current


Arguments about VMG are completely an aside. It's an " effect" it's not called the lee bow benefit.

Dave

actually...... the true term is

Lee bowing the tide both traditionally and
( if you are RYA trained ) It is described by Tim Bartlett ( famed for his bean induced wind by swl )
" The Lee bow strategy is intended to make sure that the shifts in apparent wind caused by tidal streams are always in your favour
He also says it should be applied to use it with cautions, that it should not be used if a wind shift is forecast

[Hoofsmit]

Originally Posted by goboatingnow

ground wind, is the mariners term for land TRUE wind,

current generates a sailing wind. that wind is ground referenced as current is ground reference, sit in your boat , with no sails, being flush down a river on an 8 knot ebb and tell me theres no wind.

Now.... this is a problem I see with the Dead reckoning format being used here ( and we all know how accurate that is in real life)

This tidal wind vector is being included in the equation as the true force created by the stream of the tide not allowing for resistance ,
(now i'm afraid you lot can not have it both ways) as Dockhead says "the current does not effect the wetted area"
polars allow for apparent ( sailing )wind resistance by the keel hull and rudder and show between a 4 and 8 degree lift to windward when close hauled
if the tidal wind produces an equal force on lee bow or windward bow, we have no problem ... but It clearly does not

the other problem is wind sheer, if we see this tidal wind fill our sails ( I have never but that's me , I am excepting the fact) now, correct me if I'm wrong I think the reference is water referenced wind we would have very noticeable high wind sheer especially if the difference in water temp. to wind temp is high this would be a further error of the applied current vector

I am ready for some more correction for the thicko

[Hoofsmit]

Originally Posted by Dockhead

I suspected this confusion, and now I'm absolutely sure of it.

Look here --

A non-changing current has no affect on the wind whatsoever. It changes one ground wind to another true wind. But you are sailing in true wind; you don't feel the ground wind on the boat. So you've got one heading which is dead upwind and your whole polars around that, period -- just as if you were sailing at slack tide, except that the wind direction is different, but that's irrelevant if the current is not changing.

So when you're working out how to tack, how close to the wind to sail, etc. -- you are doing all of this around one heading which is just like you were sailing at slack tide. You don't care any more whether the current is on your lee bow or up your bum, than you would care whether you are sailing in the direction of the rotation of the earth or against it (that by the way is not a trivial point).

The fact that all your sailing is in regard to a point in the water which is moving in relation to your mark, presents a different problem -- namely the problem of how to figure out which way you need to head, or what line you need to advance down, to lay your mark most efficiently, which is basically a CTS computation.

If you do your CTS computation right, then you've got one heading to sail towards and you can tack any way you want to -- in relation to that one heading (which is slightly more complex than that, since your computation assumes a certain boat speed, and will be off if your factual boat speed deviates, but that's a different conversation).

Once you have figured out that you need to head 030, for example, to lay your mark which is at 000, for example, you can forget about the mark (unless you need to correct your computations, of course, but that is different). You tack straight down along 030 choosing either tack according to your tactics and getting there as fast as you can-- it matters not one whit, in a non-changing current, how the bow is aligned or not aligned with the current.

If you haven't figured out that you are trying to advance down 030, and you are making the mistake of trying to sail while looking at the mark, then of course -- a rule of thumb like this might lead to less error than you might otherwise have, because it allows you to work back in the effects of current. But this is not an "effect".

This is properly called a "fudge". Might actually be useful in the heat of battle when no one has time to work out a proper CTS, but it is still just a fudge which is a mishmash of sailing and navigation (meaning -- figuring how where to go, as opposed to sail towards a given direction as fast as you can -- different things altogether).


Extremely useful for clearing up this kind of confusion is to imagine, as I proposed way back earlier in the thread, that the water is stationary, and it is the mark which is moving. You can predict how the mark will move, and where it will be when. Therefore, you identify that spot on your non-moving water, and sail towards it.

So you are sailing on motionless water, and have identified a "mark" in the water, which is likewise stationary -- it is that spot which you have identified where your real mark will be when you finish your leeg.

You are sailing on motionless water just like at slack tide -- you have one true wind to sail in. So the sailing problem is now absolutely ordinary -- just like slack tide -- just get to your imaginary mark as fast as you can, sailing for maximum VMG to windward if you are tacking. The fact that your real mark is bobbing and weaving makes no difference -- the only significance is that you need to have accurately predicted where that bobbing and weaving land-tied mark, which is being dragged through the water by the moving land, will be when you finish your leg, and no doubt you will have to make some mid-course correction, but again -- it is not any kind of sailing problem.

yes

told you it would not work until you tuned it upside down equation, you have now summed up why everything that has been done is full of variables that cannot be truly plotted with a cts

you now have to work in the tidal forces on that mark to get your answer
and then relate that back to your hull for hydrodynamic forces

off you go

told you

By the way I have the next bit aready done ...... I think

[Paul Elliott]

Originally Posted by goboatingnow

[...]
it is affected by the ground wind generated by the current ( it being the same but opposite to the current vector )

it is affected by the ground wind from atmospherics.

It is affected by the wind causes by the boat moving


The vector sum represents the apparent wind that the boat is sailing in , ( leaving aside extraneous effects like lee way etc) [...]

Dave, you've said this many times before (the Ground Wind being generated in part by the current). It was wrong the first time, and it's always going to be wrong. Ground Wind is the wind measured by a device attached to the ground. Current has absolutely no effect on Ground Wind.

Combine the Ground Wind and the current vector and you get True Wind (water-referenced). These are the *only* factors in True Wind. Boat motion or heading have no affect on True Wind.

On your boat you directly measure Apparent Wind. This is True Wind combined with the boat motion vector. If you like, you can also work from Apparent Wind to Ground Wind (or the other direction) by factoring in current and boat motion.

Can we agree on these terms? I'm having a very difficult time understanding your diagrams, and this might be one of the reasons.

[Paul Elliott]

Originally Posted by Hoofsmit

yes

told you it would not work until you tuned it upside down equation, you have now summed up why everything that has been done is full of variables that cannot be truly plotted with a cts

you now have to work in the tidal forces on that mark to get your answer
and then relate that back to your hull for hydrodynamic forces

off you go

told you

By the way I have the next bit aready done ...... I think

WHAT current-related hydrodynamic forces? There are none, unless you are merely talking about there being a down-current vector to your speed/course over ground, or something about the True Wind being affected by the current. The boat is sailing through the water. There is no additional current-related force on the hull. Please explain what you are talking about.

[Hoofsmit]

Originally Posted by Paul Elliott

WHAT current-related hydrodynamic forces? There are none, unless you are merely talking about there being a down-current vector to your speed/course over ground, or something about the True Wind being affected by the current. The boat is sailing through the water. There is no additional current-related force on the hull. Please explain what you are talking about.

#

What a daft statement
why would we work in current wind effect if there was not
the problem is how I have seen if from the start
your are turning a current into wind and that is inaccurate because of the true wind being an equation of hydrodymaic and aerodynamic forces which unless referenced to gps are not real.
the real affect is the current moving past your hull with a seabed fixed point and a ground fixed a wind point, after all this is the true affect the hull reacts to, we are so ingrained at working courses to steer and VMG that sailers look at it the other way because the variables are so complex
As I have said a CTS or actually DR which what you are all doing (are inherently inaccurate as we all know) and you have to be amazingly accurate to work of the effects of current on the lee bow
I think we are getting there !

[Hoofsmit]

if you disagree with me
I can justify my statement about true wind
Read a manual on how to set up an electronic wind instrument and work out the variables of uplift and wind sheer and the affect of water velocity hull senders both are effected by current
that's why they say to do it in non current situation as if you tried to go on both tacks and divide by 2 to get the average it will miss read

[DumnMad]

Originally Posted by Hoofsmit

#

What a daft statement
why would we work in current wind effect if there was not
the problem is how I have seen if from the start
your are turning a current into wind and that is inaccurate because of the true wind being an equation of hydrodymaic and aerodynamic forces which unless referenced to gps are not real.
the real affect is the current moving past your hull with a seabed fixed point and a ground fixed a wind point, after all this is the true affect the hull reacts to, we are so ingrained at working courses to steer and VMG that sailers look at it the other way because the variables are so complex
As I have said a CTS or actually DR which what you are all doing (are inherently inaccurate as we all know) and you have to be amazingly accurate to work of the effects of current on the lee bow
I think we are getting there !