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

[Seaworthy Lass]

Originally Posted by Hoofsmit

seaworthy: did you get this?

my comment of cutting the tablecloth in to 50 non parallel strips then moving them at different velocities is one I stand by, it cannot be used in your cts to steer unless you draw a new vector for each strip and then understand your hull also alters the leeway to each vector, can you understand where we are coming from?

please don't ignore this
or at least prove us wrong on each strip, I can not understand your reference to water moving at one velocity and one direction as your main reference in this senario

Hoof, I did see this and did get it. I agree the strips are different if current is different around the course at any particular moment. This does not invalidate the tablecloth analogy though .

And yes, I agree CTS cannot be used if the current us changing with position rather than time. I have stated that after my initial brain freeze .

[DumnMad]

[DumnMad]

Quote; Seaworthy Lass "Unfortunately some people (eg Hoofsmit) still don't understand this and think that the current is somehow acting differently on the two side of the hull creating some kind of lift when the current is on the lee side of the boat. It does NOT."

An odd comment. See link; Fin - Wikipedia, the free encyclopedia

[Dockhead]

Originally Posted by Seaworthy Lass

Hoof, I did see this and did get it. I agree the strips are different if current is different around the course at any particular moment. This does not invalidate the tablecloth analogy though .

And yes, I agree CTS cannot be used if the current us changing with position rather than time. I have stated that after my initial brain freeze .

You still need a CTS. It's just the problem gains another level of complexity, making it probably totally impossible to do by hand.

This is actually one of the reasons why I abandoned hand-done CTS calculations after our discussions early in the year. It turns out that there are enough variations in the current according to position, even in the Channel, that it significantly influences the results.

It would be really interesting to know how they do it in serious racing. I almost said -- America's Cup in SF Bay, but the boat speeds are so high in relation to the currents that they probably don't need to do it the same normal-speed boats do.

[Dockhead]

Originally Posted by goboatingnow

jeepers talk about taking me out of context.

What I was illustrating is that racing uses ground referenced marks, hence currents do not have an equal effect on boats irrespective of heading, a claim made by dave perry et all and the lee bow myth busters.

we have CTS calculations precisely because the destination is ATTACHED to the ground. If you merely wanted to sale 140 miles on a course of 100 degrees , you have no need of CTS or currents or anything ground referenced,

Jeepers.
Dave

Sorry if I offended Maybe got a little carried away there rhetorically :sheepishgrin:

But the point is the same -- if you're sailing across a changing current, you have to figure out in some way -- standard CTS calculation or something else if you have it -- to derive the line you have to sail across, because you simply can't sail right at your ground-based mark, which is going to be bobbing and weaving from the point of view of the water. This is a pure navigation problem having nothing to do with sailing.

If you find yourself needing to pinch, you have a failure of navigation, without regard whatsoever to how you are doing your navigation, whether regular CTS calculation, gut feel, etc.. It means you are downwind of the line you should have been advancing down from the very beginning of the leg. Just like as if you were sailing without any current at all -- if you need to pinch to lay the mark, you didn't stay long enough on the other tack, and you would have gotten there faster if you had. The current has nothing to do with it.

That's what I mean by separating navigation from sailing -- two different problems. Your goal in navigation is to calculate the ideal path through the water which will bring you right to the mark at the end of the leg. Your goal in sailing is to get down that line as fast as possible, which in the case of an upwind mark, means sailing every minute at an angle to the wind, whichever tack you are on, which is your optimum VMG to windward.

Note I did not say optimum VMG to your mark -- different kettle of fish. Obviously if your ideal line of advance through the water (what would have been your CTS if you could lay it) is not dead upwind, you will not have symmetrical tacks, and one tack will give higher VMG to the mark or rather, to the spot in the water you're aiming at. You have to spend a certain (literally certain -- precise) amount of time on the non-favored tack in order to be able to lay that spot.

This seems really elementary to me.

[Dockhead]

Just starting to think about the limits of the Effect. Very frustrating since I can only do it 5 minutes at a time. Have to get the lines off and sail back in a few minutes.

At first I wondered -- does the wind have to shift so much that the favored tack flops right around to the other side? So that it doesn't work except in very narrow circumstances?

Now the obvious answer -- of course not. You will get some profit from sailing the unfavored tack after a wind shift of even one degree towards that tack. So this is nothing more than the old rule to "sail the lifted tack", in a wind shift. How bloody obvious is that .

Could it really be just that simple? Could it be that it just seems more mysterious because it gets confounded with the question of in what direction you should be sailing? If you get rid of the CTS questions, it just boils down to sailing the lifted tack?

By Jove, I think so. And it also means that Seaworthy would be absolutely correct in that it has nothing whatsoever to do with the lee bow, since a lift will come from even one degree of wind shift, theoretically.

[foolishsailor]

Originally Posted by Hoofsmit

seaworthy: did you get this? my comment of cutting the tablecloth in to 50 non parallel strips then moving them at different velocities is one I stand by, it cannot be used in your cts to steer unless you draw a new vector for each strip and then understand your hull also alters the leeway to each vector, can you understand where we are coming from?

Yes this is correct.

It is really hard for people to get this as you CAN calulate your cts as it is just a sum of all 50vectors and it will drop you off right at your destination, however...and this is the important part..

If you are going against all these variable currents the course calculated by CTS will NOT be the fastest way to get from origin to destination.

CTS is simply the sum of all vectors - what it doesnt do is determine how to minimize the time spent in an adverse tidal stream.

Good example

[foolishsailor]

Originally Posted by Dockhead

I almost said -- America's Cup in SF Bay, but the boat speeds are so high in relation to the currents that they probably don't need to do it the same normal-speed boats do.

Not true actually. The reason Koteski was replaced on Team Oracle was that he made a massive error on an upwind leg where he tacked into the current and the 1.2kt differential in the current between themselves and the Kiwis was enoguh that they lost the race and he lost his job. No matter how fast you boat goes a knot is still a knot. You would be gobsmacked how much a race boat owner will spend to get his boat to go .25kts faster than a peer (think 8000€ for sails each year on a 27' boat) and then on the race course they will make a foolish mistake and miss a wind shift or tidal stream which is a speed gain you get for free... Poor Koteski he has been immortalized - when a sailor chooses the wrong side of a course when racing it is said, "Did you see John do a Koteski last leg? Ouch, that cost him a few places."

[foolishsailor]

Originally Posted by DumnMad

Quote; Seaworthy Lass "Unfortunately some people (eg Hoofsmit) still don't understand this and think that the current is somehow acting differently on the two side of the hull creating some kind of lift when the current is on the lee side of the boat. It does NOT." An odd comment. See link; Fin - Wikipedia, the free encyclopedia

Of course water flowing across a foil provides lift, that is not what we are discussing. The point we sre making is that current doesnt generate lift because the boat, like a floting piece of driftwood, moves with the current.

The flow of water over the foils is generated by the boat bieng propelled through the water by its sails. The current is "invisible" to the hull.

[foolishsailor]

Originally Posted by Dockhead

Just starting to think about the limits of the Effect. Very frustrating since I can only do it 5 minutes at a time. Have to get the lines off and sail back in a few minutes. At first I wondered -- does the wind have to shift so much that the favored tack flops right around to the other side? So that it doesn't work except in very narrow circumstances? Now the obvious answer -- of course not. You will get some profit from sailing the unfavored tack after a wind shift of even one degree towards that tack. So this is nothing more than the old rule to "sail the lifted tack", in a wind shift. How bloody obvious is that . Could it really be just that simple? Could it be that it just seems more mysterious because it gets confounded with the question of in what direction you should be sailing? If you get rid of the CTS questions, it just boils down to sailing the lifted tack? By Jove, I think so. And it also means that Seaworthy would be absolutely correct in that it has nothing whatsoever to do with the lee bow, since a lift will come from even one degree of wind shift, theoretically.

Once you start talking about wind shift you are talking about a different situation.

Edit: oops hit send too soon...

Tacking on a wind shift is just basic strategy.

I thought we were debating the idea that curret creates apparent wind shifts and these shifts can be used advantageously by boats to gain advantage.

To this I say no it cant as i have said before due to the fact that any lift is canceled out by the fact that there will be an equal header on the other tack.

Additionally if there are unequal lengths of time spent on port and starboard tacks the lifted leg will always be the shortest as the header will be found in the higher favorable curent and there fore where the boat would wish to be.

[Dockhead]

Originally Posted by foolishsailor

Yes this is correct.

It is really hard for people to get this as you CAN calulate your cts as it is just a sum of all 50vectors and it will drop you off right at your destination, however...and this is the important part..

If you are going against all these variable currents the course calculated by CTS will NOT be the fastest way to get from origin to destination.

CTS is simply the sum of all vectors - what it doesnt do is determine how to minimize the time spent in an adverse tidal stream.

Good example

Yes.

The only way to do it precisely is with brute force, for which you need a computer.

Since every real life CTS problem has at least some element of this, I now do it exclusively by computer.

[Dockhead]

Originally Posted by foolishsailor

Once you start talking about wind shift you are talking about a different situation.

Edit: oops hit send too soon...

Tacking on a wind shift is just basic strategy.

I thought we were debating the idea that curret creates apparent wind shifts and these shifts can be used advantageously by boats to gain advantage.

To this I say no it cant as i have said before due to the fact that any lift is canceled out by the fact that there will be an equal header on the other tack.

Additionally if there are unequal lengths of time spent on port and starboard tacks the lifted leg will always be the shortest as the header will be found in the higher favorable curent and there fore where the boat would wish to be.

If you look back at the first idealized channel crossing diagram, I think you'll see that far from being headed on the other tack, you are lifted again.

It's really very simple - you've got to sail some, at least, on both tacks, right? When you know the wind will shift due to tide change, avoid sailing either tack unfavorably. Always sail the lifted tack to the extent you've still got time to go on that tack.

[europaflyer]

Originally Posted by Seaworthy Lass

FACTS:
1. If we are tacking with any amount of cross current relative to ground wind then one tack is lifted and one is knocked (simply because the wind induced by current vector alters the direction if the true wind and the tacking angles swing around).
Although this may not lift the tack enough to more closely lay the mark, in many cases it will.
This can be used to great advantage if the current is reversing over a leg of a journey (eg the Channel crossing).
No one is disputing this (I hope LOL).

Agreed - 'the effect' (whatever we choose to call it) is not a myth.

Originally Posted by Seaworthy Lass

I have successfully debunked the myths I think. Europaflyer has finally seen the light that there is no special significance to the lee side, Dockhead is very close to it. FoolishSailor is the only one who understood right from the beginning that the myths are just myths.

Without being too pedantic I hope I can say that I never claimed a special significance to the lee side. I just thought that through your repetition of 'the LBE is a myth' that you were disagreeing with the effect as stated above. My misunderstanding... probably.

'Bow effect' is a good name. However, I do think that where is is not obvious which tack puts the current nearer the bow, using the angle the current makes with the true wind is the easiest way to work it out as per my post #473. This is exactly the same thing as you propose, but pretty much every boat has a wind gadget which gives you true wind direction whereas working out where the tide would be coming from if you were on the other tack is not a trivial calculation.

Note that previously I was using the bearing of the origin of the current as if it were wind (as was everyone else). This is the same concept as putting the current 'nearer the bow' or 'on the lee bow' (or not!). The usual way in the real world is to take the actual bearing of the direction of the current. To restate things in these terms then:

Current between 0 and 180 relative to true wind - starboard tack favoured.
Current between 180 and 360 relative to true wind - port tack favoured.

IE.

1-Find bearing of true wind (easy to do with modern instruments)
2-Find the bearing of the tide from the tidal atlas.
3-Deduct the true wind bearing from tide bearing.
4-Add 360 if negative.
-If the result is between 0 and 180, sail on starboard.
-If between 180 and 360, sail on port.

A hideous overcomplication, but worth stating for completeness. As I think you said first Angela, the important thing is just to sail on the favoured tack, which is what we all should be doing anyway. This is all any of our theory boils down to.

The only case I can think of where this does not hold true is if ground wind is offset to one side of the destination by a small amount, so that the true wind is exactly from the destination. In this case there is no favoured tack, but if you start out on the wrong tack (tide closer to your stern) then you will rapidly end up some way down tide and a favoured tack will emerge. In this case, using either your 'bow theory' or my 'true wind current angle' theory will allow you to set off on the right tack. Or even more simply, just using the tack that gives you the COG closest to destination.

Therefore -
-always sail the favoured tack when possible
-if no favoured tack exists, sail the tack which gives you the most direct COG.

Will provide a diagram of this scenario if needed... but I'm sure you follow what I'm saying without that.

[Dockhead]

Originally Posted by europaflyer

IE.

1-

The only case I can think of where this does not hold true is if ground wind is offset to one side of the destination by a small amount, so that the true wind is exactly from the destination. In this case there is no favoured tack, but if you start out on the wrong tack (tide closer to your stern) then you will rapidly end up some way down tide and a favoured tack will emerge. In this case, using either your 'bow theory' or my 'true wind current angle' theory will allow you to set off on the right tack. Or even more simply, just using the tack that gives you the COG closest to destination.

Therefored -
-always sail the favoured tack when possible
-if no favoured tack exists, sail the tack which gives you the most direct COG.

Will provide a diagram of this scenario if needed... but I'm sure you follow what I'm saying without that.

I think there is an error of stray ground reference here.

If the true wind is exactly from the destination, that by no means means there is no favorable tack. What is the direction of your desired CTS, or desired average heading, derived from your CTS calculation? This may be a quite different direction from bearing to your destination.

Also, if the true wind will shift because of a change of tide, then in any case one tack will be RELATIVELY favored at any given time. So if the current is running westwards, for example, and you're sailing North into a North wind, and your desired average heading happens to be dead upwind during this phase of the tide, you have no favored tack for the moment. But you know the
the current will turn around to eastwards, so you know the wind will veer, where your port tack will be headed. So you certainly had better be doing whatever port tack you need to do, during the westward phase of the tide.

The ONLY time you can sail simply by COG is if the current is constant.

[europaflyer]

Originally Posted by Dockhead

I think there is an error of stray ground reference here.

If the true wind is exactly from the destination, that by no means means there is no favorable tack. What is the direction of your desired CTS, or desired average heading, derived from your CTS calculation? This may be a quite different direction from bearing to your destination.

Also, if the true wind will shift because of a change of tide, then in any case one tack will be RELATIVELY favored at any given time. So if the current is running westwards, for example, and you're sailing North into a North wind, and your desired average heading happens to be dead upwind during this phase of the tide, you have no favored tack for the moment. But you know the
the current will turn around to eastwards, so you know the wind will veer, where your port tack will be headed. So you certainly had better be doing whatever port tack you need to do, during the westward phase of the tide.

The ONLY time you can sail simply by COG is if the current is constant.

OK, so during the west going tide there is no favoured tack as you say. However, you acknowledge that you need to do any port tack during this phase as it will be headed during the east-going tide. My point was just that the port tack in this instance would would give COG closer to destination, so my saying that 'if there is no favoured tack then take the tack that gives best COG' was just a rule of thumb which covers scenarios like this. Once the tide switches to east-going then there will be a clear favoured (starboard) tack and COG becomes irrelevant. I think we are agreeing with each other.