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

[Dockhead]

Originally Posted by DumnMad

Thanks Hoofsmit. The hydraulic force perpendicular to current flow can be massive. I've seen heavy walled 600 dia steel piles break perpendicular to the flow due to eddie effects.
Changing course to account for current must have an affect on lift.

One more time, with feeling

The difference is that the steel piles are attached to the ground. Your boat is not attached to anything except the water. Very different situation.

If you really think the current is pushing against your boat, try this:

Sailing in the strongest current you can find, take the sails down and let all way come off until you have 0 boat speed.

Now do this -- turn the wheel. What, nothing happened? Right.

If the current is pushing against your keel, then surely it's pushing the rudder too, wouldn't you think? But it does absolutely nothing, because the only thing that matters is relative speed between your boat and the water. What the ground is doing underneath is irrelevant. Speed over ground will not produce any ounce of hydrodynamic pressure. Only speed through the water will. Just because you're flying over the seabed with a current doesn't make the slightest difference to what your keel and rudder feel -- they feel only speed through the water.

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

I have a hell of a case LOL. That is why I am jumping up and down about it .

Give me any ground wind amount and any current amount and the angle possible between tacks relative to the ground wind (it is a ground referenced diagram) and I will give you an approximate percentage of current directions it works for.

Originally Posted by Dockhead

OK, cool. Let's take a 90 degree tacking boat (for simplicity's sake; can expand to 100 later if we want). Let's take current range from 0 slack tide going up to 4 peak tide. Let's take 6 knots boat speed (Effect will be greater with less boat speed).

Let's take ground wind speed range from 15 to 20 knots.

Do we agree that a skew from perpendicular of more than 45 degrees between ground wind and current axis is already "in the margins"?

So, within 45 degrees or less of skew, are there any cases, playing around with the other parameters, of the Effect working with the current on the weather side of the bow? If there are some cases, what percent, roughly?

I would be very surprised if it is more than 15%.

It will be somewhat greater with a 100 degree tacking angle, but why don't we look at this first.

OK, here is an example based on your figures.
I need one set of figures per cone.
I took ground wind speed as 15 knots and current amount as 3 knots.
Tacking angle is 90 degrees (ground based diagram, so ground wind used).
I am happy to do another cone if you pick another set of figures .

Dotted region has current closer to the lee bow on the lifted tack.
Yellow region has current closer to the windward bow on the lifted tack.

Relative size in total of lee: windward is roughly 182 degrees: 178 degrees.
(I would say my accuracy is +/- 2 degrees).

% of current directions where the lifted tack is the one with the current closer to the bow on the windward bow for this cone is about 49%.

Now can we please, please, please stop putting the word "lee" in when describing how current lifts a tack .

Edited to add: Europaflyer, I hope this convinces you that the effect is centred on ground wind, not true or apparent wind. Look how beautifully symmetrical this diagram is.
I also hope it convinces you too to drop the word "lee" when describing how current lifts a tack (with no whistling involved LOL).

[Dockhead]

Originally Posted by Seaworthy Lass

OK, here is an example based on your figures.
I need one set of figures per cone.
I took ground wind speed as 15 knots and current amount as 3 knots.
Tacking angle is 90 degrees (ground based diagram, so ground wind used).
I am happy to do another cone if you pick another set of figures .

Dotted region has current closer to the lee bow on the lifted tack.
Yellow region has current closer to the windward bow on the lifted tack.

Relative size in total of lee: windward is roughly 182 degrees: 178 degrees.
(I would say my accuracy is +/- 2 degrees).

% of current directions where the lifted tack is the one with the current closer to the bow on the windward bow for this cone is about 49%.

Now can we please, please, please stop putting the word "lee" in when describing how current lifts a tack .

You have a great talent for visualizing these problems in drawings

But unless I'm missing something, your drawing exactly proves my point. The yellow parts on your drawing are all situations where the current is skewed more than 45 degrees from the perpendicular, where the Effect disappears or at least, sinks into rapidly diminishing returns.

My hypothetical, which I thought you accepted, was what percentage of situations with skew of less than 45 degrees will the wind be on the weather bow? According to your drawing, it approaches zero.

Now we could argue whether situations with skew of more than 45 degrees are entirely useless. Probably there are some cases where it's not entirely useless. However, I think highly marginal is just about the best we can say about the Effect when the current is on the weather side of the boat.

So, in my opinion, you've made the extremely valuable observation that the Effect is not absolutely defined by current being on the lee bow, because you showed that there is at least one case where you can have the current on the weather bow but the Effect is still working. This is especially valuable because it proves that the Effect is not caused by the current's being on the lee bow.

However, I am now convinced by your drawing that my hunch was correct, and that the Effect overwhelmingly coincides with the current's being on the lee side of the boat, with only theoretical and extremely marginal deviations, however interesting these deviations are.

Which convinces me that the salty old Channel sailors who described the Effect as the Lee Bow Effect (their flavor of it) were not idiots, even if they were not speaking with total precision.

[Hoofsmit]

Originally Posted by Dockhead

You have a great talent for visualizing these problems in drawings But unless I'm missing something, your drawing exactly proves my point. The yellow parts on your drawing are all situations where the current is skewed more than 45 degrees from the perpendicular, where the Effect disappears or at least, sinks into rapidly diminishing returns. My hypothetical, which I thought you accepted, was what percentage of situations with skew of less than 45 degrees will the wind be on the weather bow? According to your drawing, it approaches zero. Now we could argue whether situations with skew of more than 45 degrees are entirely useless. Probably there are some cases where it's not entirely useless. However, I think highly marginal is just about the best we can say about the Effect when the current is on the weather side of the boat. So, in my opinion, you've made the extremely valuable observation that the Effect is not absolutely defined by current being on the lee bow, because you showed that there is at least one case where you can have the current on the weather bow but the Effect is still working. This is especially valuable because it proves that the Effect is not caused by the current's being on the lee bow. However, I am now convinced by your drawing that my hunch was correct, and that the Effect overwhelmingly coincides with the current's being on the lee side of the boat, with only theoretical and extremely marginal deviations, however interesting these deviations are. Which convinces me that the salty old Channel sailors who described the Effect as the Lee Bow Effect (their flavor of it) were not idiots, even if they were not speaking with total precision.

I am reading all this with great appreciation of the work being applied to drawing these variables

I would humbly ask purely to bring something of substance to this thread and not to undermine your work but maybe to build on it, if I can bring my findings as a workable equation

Can you tell me how leeway is applied to your calculation ..... You know where am heading !

It will only be applied in a workable and understandable equation if I can't do that, then I will start another thread to apply its affects from a proven perspective , it may take sometime but bare with me.

[Seaworthy Lass]

Originally Posted by Dockhead

You have a great talent for visualizing these problems in drawings

But unless I'm missing something, your drawing exactly proves my point. The yellow parts on your drawing are all situations where the current is skewed more than 45 degrees from the perpendicular, where the Effect disappears or at least, sinks into rapidly diminishing returns.

My diagram shows that for this set of figures the current is more on the windward bow for current directions from 0-36 then from 127 all the way around to 233 degrees, than from 324 to 360. This is 49% of the time. This is not insubstantial.

Originally Posted by Dockhead

My hypothetical, which I thought you accepted, was what percentage of situations with skew of less than 45 degrees will the wind be on the weather bow? According to your drawing, it approaches zero.

Now we could argue whether situations with skew of more than 45 degrees are entirely useless. Probably there are some cases where it's not entirely useless. However, I think highly marginal is just about the best we can say about the Effect when the current is on the weather side of the boat.

What if you were going Plymouth to Cherbourg? How many Plymouth sailors discuss the LBE when going to Cherbourg. I bet only the really, really really slow ones.

Why on earth are you fixated on the current having to come roughly in the 45-135 and 225-315 degree segments?

Originally Posted by Dockhead

However, I am now convinced by your drawing that my hunch was correct, and that the Effect overwhelmingly coincides with the current's being on the lee side of the boat, with only theoretical and extremely marginal deviations, however interesting these deviations are.

Marginal deviations? 49% of the time is "marginal"?

Originally Posted by Dockhead

Which convinces me that the salty old Channel sailors who described the Effect as the Lee Bow Effect (their flavor of it) were not idiots, even if they were not speaking with total precision.

I have never denied that it is the lee bow when crossing the Channel, but that is purely because the current is nearly perpendicular to the course. If the current was not perpendicular, half the time the current would be coming closer to the windward bow on the lifted tack. Why use "lee" to describe a universal effect just because it applies for the English Channel? Is it the epicentre of the universe or something like that?

[Dockhead]

Originally Posted by Hoofsmit

I am reading all this with great appreciation of the work being applied to drawing these variables

I would humbly ask purely to bring something of substance to this thread and not to undermine your work but maybe to build on it, if I can bring my findings as a workable equation

Can you tell me how leeway is applied to your calculation ..... You know where am heading !

It will only be applied in a workable and understandable equation if I can't do that, then I will start another thread to apply its affects from a proven perspective , it may take sometime but bare with me.

Leeway is included in the tacking angle. Boat tacks through 90 degrees over ground, for example - includes apparent to true and leeway.

[Dockhead]

Originally Posted by Seaworthy Lass

My diagram shows that for this set of figures the current is more on the windward bow for current directions from 0-36 then from 127 all the way around to 233 degrees, than from 324 to 360. This is 49% of the time. This is not insubstantial.

What if you were going Plymouth to Cherbourg? How many Plymouth sailors discuss the LBE when going to Cherbourg. I bet only the really, really really slow ones.

Why on earth are you fixated on the current having to come roughly in the 45-135 and 225-315 degree segments?

Marginal deviations? 49% of the time is "marginal"?

I have never denied that it is the lee bow when crossing the Channel, but that is purely because the current is nearly perpendicular to the course. If the current was not perpendicular, half the time the current would be coming closer to the windward bow on the lifted tack. Why use "lee" to describe a universal effect just because it applies for the English Channel? Is it the epicentre of the universe or something like that?

The point was explained somewhere above. The Effect is useful somewhere around current=perpendicular to ground wind. Somewhere around current=parallel ground wind, the Effect does not exist.

So your "49% of the time" is of purely theoretical interest, because that is exactly the 49% percent where it either totally does not work, or practically does not.

[europaflyer]

Originally Posted by Seaworthy Lass

OK, here is an example based on your figures.
I need one set of figures per cone.
I took ground wind speed as 15 knots and current amount as 3 knots.
Tacking angle is 90 degrees (ground based diagram, so ground wind used).
I am happy to do another cone if you pick another set of figures .

Dotted region has current closer to the lee bow on the lifted tack.
Yellow region has current closer to the windward bow on the lifted tack.

Relative size in total of lee: windward is roughly 182 degrees: 178 degrees.
(I would say my accuracy is +/- 2 degrees).

% of current directions where the lifted tack is the one with the current closer to the bow on the windward bow for this cone is about 49%.

Now can we please, please, please stop putting the word "lee" in when describing how current lifts a tack .

Edited to add: Europaflyer, I hope this convinces you that the effect is centred on ground wind, not true or apparent wind. Look how beautifully symmetrical this diagram is.
I also hope it convinces you too to drop the word "lee" when describing how current lifts a tack (with no whistling involved LOL).

Angela, I'm glad we finally managed to reconcile our opinions yesterday, but you've really lost me with your last few posts.

Firstly, any diagram calculating whether something is 'on the windward side' is looking at the wind relative to the boat, so must necessarily be looking at apparent wind. Yours doesn't - it looks at true wind. Adding the headwind in, as I did with my diagram, makes it very difficult indeed to demonstrate a significant zone where a current to windward can lift the boat. Not saying it doesn't exist, but it is pretty small in most situations. My diagram just about demonstrates it, but it is highly marginal. Much easier to demonstrate for boats with poor pointing ability I think. This zone exists when the current is higher than the bow, but lower than the zone where both boats are being knocked by it (and draw yourself some diagrams which include headwind, this zone does exist). In most practical situations this will be a very thin sliver.

I can't picture where the yellow sector at the lower part of the diagram comes from. Probably my fault, but I just don't get it... This is a closehauled effect, and in this zone the current is on the leeward side of a boat on either tack... isn't it?

To reiterate what I think I have said before: I agree that there is nothing special about the current coming from the lee side, or indeed the lee bow. However, in the real world, this is the situation where the effect is strongest and I feel the name is not necessarily a bad one.

I'm not trying to say the effect is 'centred' on anything, it incorporates multiple vectors from ground, true, head, current and apparent winds. It certainly isn't centred on the lee bow!

[Seaworthy Lass]

Originally Posted by Dockhead

The point was explained somewhere above. The Effect is useful somewhere around current=perpendicular to ground wind. Somewhere around current=parallel ground wind, the Effect does not exist.

So your "49% of the time" is of purely theoretical interest, because that is exactly the 49% percent where it either totally does not work, or practically does not.

Light winds travelling from Plymouth to Cherbourg and you DO want to know that you should keep the current closer to your windward bow and that you should tack to achieve this. If you had a Portsmouth sailor on board what do you think he would do the whole way?

And if you are racing it would make a huge difference to you knowing the correct tack to be on at any particular time. The effect may be less in the sectors where the the current is closer to the windward bow, but it does exist.

[europaflyer]

Originally Posted by europaflyer

This zone exists when the current is higher than the bow, but lower than the zone where both boats are being knocked by it (and draw yourself some diagrams which include headwind, this zone does exist). In most practical situations this will be a very thin sliver.

Sorry, I confused myself here. Yes, the yellow zone at the top is where one boat can be lifted relative to the other by the tide (but not, as I was getting at, compared to the slack water condition - there is a zone here where both boats are knocked compared to slack water).

I still don't get the yellow zone at the bottom. Surely, this is where it is on the leeward side of BOTH boats, even the knocked one? For the black spotty zones it is on the leeward side of the lifted boat and the windward side of the knocked boat. I now agree that the top yellow zone shows where one boat is being lifted relative to the other, the starboard tack boat for the left hand segment of this zone and the port tack boat in the right hand segment. However, it isn't a big segment and it certainly isn't an area where the effect is very strong. More relevant for square-riggers...

[Dockhead]

Originally Posted by Seaworthy Lass

Light winds travelling from Plymouth to Cherbourg and you DO want to know that you should keep the current closer to your windward bow and that you should tack to achieve this. If you had a Portsmouth sailor on board what do you think he would do the whole way?

And if you are racing it would make a huge difference to you knowing the correct tack to be on at any particular time. The effect may be less in the sectors where the the current is closer to the windward bow, but it does exist.

I do not disagree!

I don't want to take anything away from your excellent work.

The most excellent thing about it is that you have proved that the Effect is not caused by the current's being on the lee bow, news to me and probably everyone else as well.

But I think it's important to recognize that the situations where you get a lift from the Effect with the current on the weather bow will be quite rare in practice -- marginal situations.

Therefore, I am casting my lot with the salty old Channel sailors, who had 50,000 sea miles before you and I were even stains on the sheet, who said:

"Sonny, when ye set out across the Channel, and ye have the misfortune to have to tack across, time your tacks to coincide with the changes of the tide, and tack to keep the tide on your lee bow. Like that, you won't fart around out there all night trying to lay the Needles."

They were exactly right, and the way they described it applies to every conceivable Poole-Needles-Bembridge to Cherbourg crossing.

One great achievement of this thread is that we know that the Effect is not caused by having current on your lee bow. Another great achievement is that we know that those old salts were not talking bullocks -- that flavor of the Lee Bow Effect is real.

[Hoofsmit]

[QUOTE="Seaworthy Lass;
Why use "lee" to describe a universal effect just because it applies for the English Channel? Is it the epicentre of the universe or something like that? [/QUOTE]
No but Greenwich ain't far away

( tongue in cheek ...... Again)

[s/v Moondancer]

So after 581 posts you finally worked out that you put the tide on the bow that allows you stay closest to the rhumb line and closes the tacking angle...great work!

Now Seaworthy Lass can continue with her massive thread proving that she knows more about crossing the Channel than the RYA.

Moderators, like children, should be seen and not heard!

[Seaworthy Lass]

Originally Posted by europaflyer

Angela, I'm glad we finally managed to reconcile our opinions yesterday, but you've really lost me with your last few posts.

Firstly, any diagram calculating whether something is 'on the windward side' is looking at the wind relative to the boat, so must necessarily be looking at apparent wind. Yours doesn't - it looks at true wind.

Valid.

Originally Posted by europaflyer

I can't picture where the yellow sector at the lower part of the diagram comes from. Probably my fault, but I just don't get it... This is a closehauled effect, and in this zone the current is on the leeward side of a boat on either tack... isn't it?

Now that made me take a second look. And a third and a fourth. I have even tried holding it upside down and back to front. Standing on my head did not help either. Did I really say that sector had the current on the windward side?

What was I thinking?
It actually stemmed from a diagram days ago and I didn't look again this time around, just merrily reached for the yellow texta (it did feel so good colouring it in LOL).

The only thing I can say is that thankfully no bets were accepted. Whew! Bedtime here I think. Maybe I can just scrub a few of my posts before I go to bed . All that voluntary hard work behind the scenes must have some rewards.

[europaflyer]

Originally Posted by s/v Moondancer

So after 581 posts you finally worked out that you put the tide on the bow that allows you stay closest to the rhumb line and closes the tacking angle...great work!

Now Seaworthy Lass can continue with her massive thread proving that she knows more about crossing the Channel than the RYA.

Moderators should be seen and not heard!

Bit harsh.

That's the most basic case of the effect. No-one is disputing that. Seaworthy is talking about the really fine points of the effect and I am happy to hear what she has to say.

So where is the effect greatest then? When the current is at 90 degrees to the ground wind? Or true wind? or apparent wind? At which of these points is there the greatest difference between the tacks? Or the greatest difference from slack water? Also, are there cases where a lift can be given with a current from the windward side as me and Seaworthy claim? Or not?

Your trying to trivialise this is silly.