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Old 17-11-2013, 10:54   #541
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I calculated for the true wind changes and they are shown by the changes in CTS. You will see the dashed red lines that represent the tacking angles for each current speed - see the numbers on the edges.

The shift in wind for 2,3 and 4 kts for this wind angles and current angle is 6,9 and 12 degrees respectively.

The shifts are all included for each segment in the CTS
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Old 17-11-2013, 11:04   #542
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Originally Posted by Dockhead View Post

If you imagine the tide cocking around off the perpendicular, I can see that on one favored (lifted) tack, the tide might be on your lee quarter or lee beam. But I'm struggling to imagine a case where you will have a favored tack -- a tack lifted by a tide change -- with the tide on the weather side of the boat at all. Maybe Angela, who has been a step ahead on this, could help.
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Originally Posted by SeaworthyLass
Here you go .

Port tack is lifted and current is on the windward side.

This is not some weird example at "the limits". A lifted tack with the current on the windward bow occurs for roughly half of all possible current directions. This is why the word "lee" needs to be removed. How does the word "lee" apply in the example I have shown? .
OK, but the current in your drawing is fine on the weather bow, and the boat in your drawing is at about 80 degrees off the true wind -- implying a tacking angle of 160 degrees. If we trim up and get that boat to 45 degrees, which should surely be much nearer to reality if she's not a square rigger, then the current will be on the lee bow.

You've certainly made a point -- for which I'm grateful -- that it is theoretically possible, especially for boats with very wide tacking angles, but even in that case, I don't see how even conceivably this could work in 50% of cases or even 20%. And also your drawing must be near the limits of the Effect, since the angle between ground wind and current is already getting pretty acute, which will really cut down on the wind shift.

Can you show ANY example of the Effect where the current is on the weather bow, the tide is less than 45 degrees skewed from perpendicular to the ground wind, and the boat is tacking through 90 degrees?

I could probably do this myself, but I'm being lazy -- since you have worked so hard on this, will be much easier for you
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Old 17-11-2013, 11:30   #543
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Here's a hypothesis:

We'll never see the Effect with the wind on the weather side of the boat, unless the current is skewed more than 45 degrees from the perpendicular and/or the boat tacks through more than 90 degrees.

The effect will work with the tide on the weather side of the boat only at the margins, defined as above.

They are "margins", because the Effect is greatest with perpendicular current. If you did an area map of quantum of Effect versus alignment of current, over a realistic set of variables for current and wind velocities, you would probably get -- I'm guessing -- no more than 10% of total aggregate amount of the effect under the "current alignment 0-45 degrees" half of the map.

Likewise with tacking angle. Not every boat can tack through 90 degrees, but even a square rigger can tack through 120 degrees. I guess the mean would be about 100 degrees.

I don't know if this is right or not, so far it's just a hunch. Looking forward to Seaworthy's calculations.
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Old 17-11-2013, 11:33   #544
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Originally Posted by europaflyer View Post
I hadn't really considered it in terms of maximising time in the lesser current.
Exactly!

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Originally Posted by europaflyer View Post
Which is funny, because it would be more obvious in real life, it's just that this theory is killing me.
Also dead on. It is very difficult to explain and demonstrate - but relatively easy to execute on the fly in the real world.

However I want to thank everyone as well as this has forced me to dig really into the details of this concept and I have to say i am surprised that the effect was not larger over a 30nm passage. I had assumed that in almost all situations that involved crossing tidal streams and being forced to go upwind and upcurrent that this strategy would produce significant results and in my own scenario it is shown to exist but be possibly not significant enough to warrant a cruiser using it.

And with the margin of error in the rounding I did it is possible the effect could be either cut in half or at best made to be 12 minutes gain. Good enough to win a race, but hardly gets you to the bar faster than your fellow cruiser.

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I'm pretty sure we are all in agreement here about what I shall carefully call 'the effect'. Staying on the favoured tack is what it boils down to. If the current can alter the favoured tack as it changes, then you need to stay on the tack with the current nearer the bow (thanks Angela)... or use the 'true wind current angle' formula I proposed earlier (same thing).
I can live with this - but change the idea of "effect" to "rule of thumb", since there are situations that can be demonstrated where it isnt optimal, but works well enough most of the time. Sailing the favored tack isn't just a cruising rule of thumb it is also a strategic one as well so we really are arguing details.


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It's been a great ride people. Thanks!
Very enjoyable, nice to really dig into things - even ones own preconceptions!

Thanks everyone.
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Old 17-11-2013, 11:39   #545
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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This thread has been a remarkable lesson on how much easier it is to solve complicated problems when you have several smart people all collaborating. For me, it's been a great leap forward in understanding these issues.

I think "Lee Bow Effect" is probably an ok is not totally precise term for the one flavor of it which, by consensus view, actually works. That's because of this:

Geometrically, the wind shift caused by a tide change is going to be greatest when the current is just perpendicular to the ground wind. When the current is aligned with the ground wind, there is zero wind shift.

When the current is perpendicular to the ground wind, then in fact keeping the lee bow into the current is what exactly does the trick. So I think it's not an accident that it was called like this. This covers every conceivable Channel crossing situation.

As the direction of the tide deviates from perpendicular to the ground wind, the magnitude of the wind shift falls off. Of course, the magnitude of the wind shift also depends on the relative velocities of wind and current, so the effect will be more profound at springs and light wind, than it will at neaps and strong wind.

If you imagine the tide cocking around off the perpendicular, I can see that on one favored (lifted) tack, the tide might be on your lee quarter or lee beam. But I'm struggling to imagine a case where you will have a favored tack -- a tack lifted by a tide change -- with the tide on the weather side of the boat at all. Maybe Angela, who has been a step ahead on this, could help.
Maximal when the tide is perpendicular to the ground wind? Hmmm... If I can reproduce Angela's old diagram, it shows that the greatest shift in true wind is when the current is perpendicular to the true wind. Agreed it is zero when the current is aligned with the ground wind, but note in this case that the ground wind direction is equal to the true wind. Splitting hairs though, because I agree with your basic point. I also think it isn't an accident it was named the lee bow effect, but shhh... this is a controversial point for some because it doesn't hold true for all cases.

As for your last point, there are situations where a tide coming from the windward side can produce a lift. These situations are where the tide is coming from a range of angles close to the true wind. I attach a typically shoddy diagram which attempts to show one of these situations. This is why people have an argument with calling it the 'lee bow effect', and they certainly have a point.
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Old 17-11-2013, 11:43   #546
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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I also think it isn't an accident it was named the lee bow effect, but shhh... this is a controversial point for some because it doesn't hold true for all cases.
I think the term just still doesn't sit well because it is related to the mythical Lee Bow Effect we were speaking about in the beginning and disproved...
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Old 17-11-2013, 11:49   #547
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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However I want to thank everyone as well as this has forced me to dig really into the details of this concept and I have to say i am surprised that the effect was not larger over a 30nm passage. I had assumed that in almost all situations that involved crossing tidal streams and being forced to go upwind and upcurrent that this strategy would produce significant results and in my own scenario it is shown to exist but be possibly not significant enough to warrant a cruiser using it.

And with the margin of error in the rounding I did it is possible the effect could be either cut in half or at best made to be 12 minutes gain. Good enough to win a race, but hardly gets you to the bar faster than your fellow cruiser.
Oh no...that's because you weren't looking at the effect as we were discussing it. Our effect involves at least one 180 degree change of tide resulting in a true wind shift to swap the favoured tack over. Yours is just as a result of some slightly different tide speeds at sides of the channel. Great calculations though.

If you work your magic for a scenario where the first half of the 'race' has a tide running one way through your channel, and the second half has it running the other, with a ground wind at 90 degrees to the channel and destination directly across it, then I think you would find a very significant difference indeed for boats taking the tides on different tacks. THAT is the essence of our 'bow effect'. That is exactly the sort of diagram you were badgering me for a few pages back, which I didn't have the ability to produce. You are miles ahead of any of us as far as producing accurate diagrams goes, so it would be really brilliant if you could do one for this classic '(lee) bow effect' scenario.
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Old 17-11-2013, 11:49   #548
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Something really weird going on.
I did not write post 542, Dockhead did. It has my name as poster.
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Old 17-11-2013, 11:56   #549
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Something really weird going on.
I did not write post 542, Dockhead did. It has my name as poster.
Thought you seemed to be getting a bit argumentative with yourself... Still, in a 37 page thread I suppose one bug is allowed to pop up in the forum software.
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Old 17-11-2013, 11:58   #550
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Oh no...that's because you weren't looking at the effect as we were discussing it. Our effect involves at least one 180 degree change of tide resulting in a true wind shift to swap the favoured tack over. Yours is just as a result of some slightly different tide speeds at sides of the channel. Great calculations though. If you work your magic for a scenario where the first half of the 'race' has a tide running one way through your channel, and the second half has it running the other, then I think you would find a very significant difference indeed for boats taking the tides on different tacks. THAT is the essence of our 'bow effect'.
How the hell did I miss that you all were referring to a change of tide?

I completely missed that somehow...

That makes alot more sense i will now have to go back and re read some stuff I think.

I wasnt refering to the lee bow effect - my main point is that you cant always use a favored tack rule of thumb and more importantly any rule of thimb that doest take into account the conditions as a whole.

For example the favored tack calcs you drew out do not take into account the fact that there is a lesser current and more relief across the channel, and therefore has no mecanism to decide the optimal tack as part of a whole solution - only for that moment assuming all things equal.
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Old 17-11-2013, 12:03   #551
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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How the hell did I miss that you all were referring to a change of tide?

I completely missed that somehow...

That makes alot more sense i will now have to go back and re read some stuff I think.

I wasnt refering to the lee bow effect - my main point is that you cant always use a favored tack rule of thumb and more importantly any rule of thimb that doest take into account the conditions as a whole.

For example the favored tack calcs you drew out do not take into account the fact that there is a lesser current and more relief across the channel, and therefore has no mecanism to decide the optimal tack as part of a whole solution - only for that moment assuming all things equal.
The prize for the best penny dropping moment of the thread goes to... foolishsailor!!

As I say, you have done by far the best diagrams here so it would be great if you could turn your hand to the classic scenario I describe in post #547. That would clear things up once and for all, I think.
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Old 17-11-2013, 12:05   #552
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The prize for the best penny dropping moment of the thread goes to... foolishsailor!! As I say, you have done by far the best diagrams here so it would be great if you could turn your hand to the classic scenario I describe in post #547. That would clear things up once and for all, I think.
I will, but not tonight.

Time for a glass of wine.

The fact that Angela is not chiming in means she is doing some serious work on these questions - and I am excited and a bit fearful of what she is going to produce.
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Old 17-11-2013, 12:07   #553
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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I will, but not tonight.

Time for a glass of wine.
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Old 17-11-2013, 12:08   #554
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Exactly! Also dead on. It is very difficult to explain and demonstrate - but relatively easy to execute on the fly in the real world. However I want to thank everyone as well as this has forced me to dig really into the details of this concept and I have to say i am surprised that the effect was not larger over a 30nm passage. I had assumed that in almost all situations that involved crossing tidal streams and being forced to go upwind and upcurrent that this strategy would produce significant results and in my own scenario it is shown to exist but be possibly not significant enough to warrant a cruiser using it. And with the margin of error in the rounding I did it is possible the effect could be either cut in half or at best made to be 12 minutes gain. Good enough to win a race, but hardly gets you to the bar faster than your fellow cruiser. I can live with this - but change the idea of "effect" to "rule of thumb", since there are situations that can be demonstrated where it isnt optimal, but works well enough most of the time. Sailing the favored tack isn't just a cruising rule of thumb it is also a strategic one as well so we really are arguing details. Very enjoyable, nice to really dig into things - even ones own preconceptions! Thanks everyone.
Dame ...... We got there in the end !
has been really good learning thread, as DH has said the effects are to many to calc with out a computer, he has one I don,t
Just to state the obvious as I think FS , this is not what he would do when sailing with his family, and in the real world, I would not want to be doing it after 14 hrs from Guernsey heading to the Hamble
Thank you to all, I have followed every post, some multiple times I can't say I absorbed it though all , but it will be a great reference point
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Old 17-11-2013, 12:13   #555
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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OK, but the current in your drawing is fine on the weather bow, and the boat in your drawing is at about 80 degrees off the true wind -- implying a tacking angle of 160 degrees. If we trim up and get that boat to 45 degrees, which should surely be much nearer to reality if she's not a square rigger, then the current will be on the lee bow.

You've certainly made a point -- for which I'm grateful -- that it is theoretically possible, especially for boats with very wide tacking angles, but even in that case, I don't see how even conceivably this could work in 50% of cases or even 20%. And also your drawing must be near the limits of the Effect, since the angle between ground wind and current is already getting pretty acute, which will really cut down on the wind shift.

Can you show ANY example of the Effect where the current is on the weather bow, the tide is less than 45 degrees skewed from perpendicular to the ground wind, and the boat is tacking through 90 degrees?

I could probably do this myself, but I'm being lazy -- since you have worked so hard on this, will be much easier for you
Yes, it easy now for me, as I know what parameters it works for, so I don't mind hunting up examples.

My tacking angle was not that appalling in the cone diagram. I measured it and I had drawn it 115 degrees between tacks relative to ground wind (it would be greatly reduced relative to apparent wind at 20 knots, as the wind due to boat speed would shift it around).

(What angle between tacks do you achieve on the charplotter? You must have a super boat if you can do dramatically better than 115, making my example a silly one ).

Anyway, here we go. An example for you .
There is a 20 knot headwind and 2 knots of current at an angle of 30 degrees off the ground wind.
The true wind is lifted by 4 degrees. How much do you want to shift the apparent wind? If you think 10 degrees is likely (making it 70 degrees between apparent wind on tacks), the current still falls on the windward side.

Now if it works for the current being 30 degrees off the ground wind (and it works even better for angles less than this) then that is already 60 degrees of current it works for, and that is without taking into account current 180 degrees off this. I will work out those angles in a moment, working out a percentage of angles it works for in these conditions.
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