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

[foolishsailor]

That is not what I meant - but is still right I guess.

I was stating that my simple scenario refutes the claim of the effects existence. However I guess my scenario is also a "claim" and not proof either.

As a mathmatician - you should be able to prove this numerically?

I cant see how it is anything more than a summation of vectors of force and the "lee bow effect" claim is effectively stating that changing the direction and magnitude of one of those vectors will change the sum of the remaining such that the Ground Referenced VMG to the mark will increase.

I think you can "sense" the fallacy of this as well.

Regardless, I have several polars and will draft up a drawing to what ever everyone here agrees is the best opportunity to show the effect.

[estarzinger]

Originally Posted by foolishsailor

As a mathmatician - you should be able to prove this numerically?
.

It is certainly provable.

But calculating individual special cases cannot ever prove your claim (that the effect never exists), unless you can also prove that those special cases represent all possible "optima" for the reverse claim.

As you say, it's a vector calculation where an optimum is to be found. I suspect it would be easier to prove the general case as a simple algebraic (eg using variables rather than specific values) calculus problem. I think the only assumption we would have to make to do that is that the polar curve in the relevant upwind range is some sort of smooth 2nd order curve and does not have any funny kinks. I could do this, but am not going to bother, because I think the steady state case is useless and we all agree on the changing state case.

As you point out, one individual case would/could prove the reverse claim (that it does exist at least in some situations). Today, to look for that, a mathematician might simply program a computer to throw random numbers into a vector calculation, and let it run for an hour, and see if it finds such a case. That is not "rigorous", and not a proof of your claim if it failed, but would probably find a counter case if one existed. There is certainly a more elegant way to look for an example, but brute force has mostly replaced elegance since I left school.

[Seaworthy Lass]

Originally Posted by estarzinger

It is certainly provable.

But calculating individual special cases cannot ever prove your claim (that the effect never exists), unless you can also prove that those special cases represent all possible "optima" for the reverse claim.

As you say, it's a vector calculation where an optimum is to be found. I suspect it would be easier to prove the general case as a simple algebraic (eg using variables rather than specific values) calculus problem. I think the only assumption we would have to make to do that is that the polar curve in the relevant upwind range is some sort of smooth 2nd order curve and does not have any funny kinks. I could do this, but am not going to bother, because I think the steady state case is useless and we all agree on the changing state case.

As you point out, one individual case would/could prove the reverse claim (that it does exist at least in some situations). Today, to look for that, a mathematician might simply program a computer to throw random numbers into a vector calculation, and let it run for an hour, and see if it finds such a case. That is not "rigorous", and not a proof of your claim if it failed, but would probably find a counter case if one existed. There is certainly a more elegant way to look for an example, but brute force has mostly replaced elegance since I left school.

Evans, could you please clarify something? Are you saying that if you are close hauled at your optimal heading with optimal sail trim and the current is close to the bow but on the windward side, that you think there is some advantage to altering your heading slightly to get the current on your lee side?

If not, under what circumstances do you see this "lee bow effect" helping you?

[estarzinger]

SL,

I have said repeatedly that I am making no comment at all about the answer. I am commenting on methodology. (Edit:actually more correctly, I have repeatedly said I suspect you are right but that you have not proved it).

You have a thread here intending to prove or disprove something, and are 50 pages in without accomplishing that. I am pointing out what might need to be done to actually prove it.

By the way, the "calculus" approach actually does not require much calculus. You need to set up a complete and accurate equation for sailing time to the mark (including an algebraic polar equation). Then (formally) you take the first derivative of sailing angle. If you are right the current speed and direction will completely disappear from that first derivative. However, what that means in non-calculus terms is that in the complete time to the mark equation, the current variables will be only "added on" and not involved in (multiply to) any term that includes sailing angle. If you can show that you have made your proof.

I suspect that is best done using a ground reference.

There is another way to prove it using your water reference approach. You need to show that if you sail any other than the water referenced vmg angle the distance (thru the water) on the course to steer gets longer. We know by definition the (water referenced) speed to the mark will be slower, so if you show it's also a longer distance you can show any other angle will definitely be slower to the mark.

[TeddyDiver]

IMHO, The consept of 'lee bow efffect' comes from the illusion of being faster...

[Paul Elliott]

Originally Posted by Seaworthy Lass

Evans, could you please clarify something? Are you saying that if you are close hauled at your optimal heading with optimal sail trim and the current is close to the bow but on the windward side, that you think there is some advantage to altering your heading slightly to get the current on your lee side?

If not, under what circumstances do you see this "lee bow effect" helping you?

And is this the Lee-Bow effect we are debating? And this is supposedly to be applied in unchanging (current, wind) conditions?

This has been my understanding of the Lee-Bow effect, and I claim, without proof, that it is false. I would love for someone to provide a test case where they believe that the technique works.

[Seaworthy Lass]

Originally Posted by Paul Elliott

And is this the Lee-Bow effect we are debating? And this is supposedly to be applied in unchanging (current, wind) conditions?

This has been my understanding of the Lee-Bow effect, and I claim, without proof, that it is false. I would love for someone to provide a test case where they believe that the technique works.

This is certainly the "lee bow effect" we are debating with Goboatingnow and Hoofsmit (and possibly with Estarzinger, as he is sitting on the fence).

I too, would love an explanation how this is supposed to work.

[goboatingnow]

Originally Posted by Seaworthy Lass

This is certainly the "lee bow effect" we are debating with Goboatingnow and Hoofsmit (and possibly with Estarzinger, as he is sitting on the fence).

I too, would love an explanation how this is supposed to work.

The effect is rather misnamed. What's clear and You showed it yourself is that there is a wind angle benefit when sailing with the current on the bow, over the situation in no current , hence this allows the helm to head up slightly and take advantage of this slight improvement in wind angle. That may or may not result in the current on the lee bow, but the effect is available when it ends up there as a results of heading up.

Whether this results in any net gain is another debate entirely and those with boat polars and CTS and VMG are missing the point

Dav

[Paul Elliott]

And to clarify a bit further, by "close hauled at your optimal heading with optimal sail trim" I mean sailing the optimum current-adjusted course at your best angle per your polars. This may involve tacking. We will ignore the time lost in making the tacks.

What complicates this is that the optimum current-adjusted course will change if your boatspeed changes. In the real-world this would be an iterative adjustment.

What bugs me about the while Lee-Bow thing is that it sort of implies that the current is somehow changing the hydrodynamics of how your boat is sailing. It doesn't! By changing your course to put the current to port or starboard, all you are doing is changing your course! Nothing more than that. Sail to your polars and pick your course -- that's how you get there the fastest.

[Paul Elliott]

Originally Posted by goboatingnow

The effect is rather misnamed. What's clear and You showed it yourself is that there is a wind angle benefit when sailing with the current on the bow, over the situation in no current , hence this allows the helm to head up slightly and take advantage of this slight improvement in wind angle. That may or may not result in the current on the lee bow, but the effect is available when it ends up there as a results of heading up.

Whether this results in any net gain is another debate entirely and those with boat polars and CTS and VMG are missing the point

Dav

Dave, what I think you are describing is the relationship between Ground Wind, Current, and True Wind. If so, what's the debate?

I will suggest that this simple and well-understood relationship should not be given a name (such as lee-bow effect) which implies nonsense and has confused many people.

[Seaworthy Lass]

Originally Posted by goboatingnow

The effect is rather misnamed. What's clear and You showed it yourself is that there is a wind angle benefit when sailing with the current on the bow, over the situation in no current , hence this allows the helm to head up slightly and take advantage of this slight improvement in wind angle. That may or may not result in the current on the lee bow, but the effect is available when it ends up there as a results of heading up.

Whether this results in any net gain is another debate entirely and those with boat polars and CTS and VMG are missing the point

Dav

Dave, you have completely misunderstood what I showed. That beautifully simple cone diagram shows that of the two tacks the one with the current closer to the bow is the lifted one compared to the ground wind.

This is purely because maximum lift relative to ground wind is produced when the current is between 0 and 90 degrees of the ground wind, on either side of it (the exact angle depends on the relative amounts of ground wind and wind induced by current, it would only be with the current exactly on the bow under very limited conditions).

Because we are trying to sail into the ground wind and the angle between tacks is very roughly 90 degrees, the lifted tack (relative to ground wind) is the one with the current closer to being on the bow when comparing the two tacks.

The effect has absolutely nothing to do with the current hitting either side of the bow. Pinching to put the current on the lee bow will have no effect other than that pinching under any other circumstances would.

The cone diagram again:

[estarzinger]

SL,

I am "sitting on the fence" for two reasons:

1. I don't know if you were involved in the DDWFTTW debate? The question was asked if it was possible to sail dead down wind faster than the true wind speed (with zero current). Most people's intuition lead them to immediately say this was totally impossible. I was one of the few who said I suspected it was possible (Not because I was smarter, but because I happened to know and have discussed it with some people who were). And in fact it is not only possible but has now been done and videoed (you can google it). However that experience and debate leads me to be careful about what is "obvious". A lot of smart people said something was "obviously impossible" when it was not.

2. Given my education, i want a true and correct proof before saying something is proven. I see no reason to say it has been proven when in fact it has not (in this thread at least).

[forsailbyowner]

Ok i didnt read the whole thread and havent attempted the math, but I tacked up through the skyway bridge taking the southwest channel into an 20kt east wind the other night and made some observations. I arranged the sails in their best close hauled position with the boom centered and the jib right off the spreaders creating a fairly even slot, I found the best course forward in this trim was obtained by adjusting the autopilot to where the helm is in a centered position. I wouldnt look at the course or heading at all. I experimented and if I had the helm to weather it would slow and immediately as the helm went to the opposite direction my course would worsen in relation to the destination. I had hours to play with this and over and over it proved true. No idea how the math for this would work,

[goboatingnow]

Originally Posted by Seaworthy Lass

Dave, you have completely misunderstood what I showed. That beautifully simple cone diagram shows that of the two tacks the one with the current closer to the bow is the lifted one compared to the ground wind.

This is purely because maximum lift relative to ground wind is produced when the current is between 0 and 90 degrees of the ground wind, on either side of it (the exact angle depends on the relative amounts of ground wind and wind induced by current, it would only be with the current exactly on the bow under very limited conditions).

Because we are trying to sail into the ground wind and the angle between tacks is very roughly 90 degrees, the lifted tack (relative to ground wind) is the one with the current closer to being on the bow when comparing the two tacks.

The effect has absolutely nothing to do with the current hitting either side of the bow. Pinching to put the current on the lee bow will have no effect other than that pinching under any other circumstances would.

The cone diagram again:

you cannot complete you conclusions without continuing the example to include apparent wind.

Futhermore, with the greatest respect, it is not a beautifully simple diagram, it actually obfuscates the issue

I have supplied a vector diagram that illustrates the issue, that is that sailing in a ground on the bow , in effect causes a wind that produces an increases in apparent wind angle over a situation where there is no current. The net effect of that is it allows a helm to head up slightly over the situation where there is no current on the bow. Whether thats a lee bow effect Id don't know as none has actually defined the effect in any concise and consistent way


Also tacks , CTS, VMG, etc need to be removed from the debate , its has nothing to do with anything related to the "effect". There are other effects that have a bearing on tacks etc, this isn't it. This debates is utterly confused by tacks, and cts and other extraneous issues that relates to other tide situations


dave

[estarzinger]

Originally Posted by goboatingnow

sailing in a current on the bow , in effect causes a wind that produces an increases in apparent wind angle over a situation where there is no current. The net effect of that is it allows a helm to head up slightly over the situation where there is no current on the bow.

Dave, I believe we all agree with you on this. Current will effect the wind experienced on the boat (vs no current) both in speed and direction. And allow you to sail different courses than you could with no current and one rack will be lifted vs no current. We all agree with that.

It is why we also all agree that in variable current there is an opportunity to sail the lifted tack.

It just happens that is not what this thread is mostly about.

What this thread is mostly about is the debate between two tactics . . . In a constant unchanging current . . . . One proposed tactic is to always sail the water referenced optimum vmg angle no matter the current . . . . and the other proposed tactic is to sail a higher/slower mode on the tack with the current on the lee bow when the current is pushing you upwind and a slower/faster mode with the current on the windward bow.

Some of the thread is about a particular case. . . . when your water referenced vmg angle puts the current dead on the bow is it better to sail a bit higher/slower mode to get the current on your lee bow.

The predominate opinion (both here and in the racing community) is that it is best to sail the optimum water referenced vmg angle. But despite a lot of arm waving and diagrams, no-one has "proven" this (in a rigorous fashion).