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

[Seaworthy Lass]

The bit about pinching being detrimental is correct regardless of whether or not the current is changing, but are you saying there isn't a preferred tack to be on when the current is changing . I don't think that is right.

[Dockhead]

I think the above is a robust argument based on the same principles as our Course to Steer discussion last year, on which everyone agreed in the end (now how often does that happen on CF! ). The fundamental principle of both arguments is that the shortest distance between any two points is a straight line, and when sailing, that straight line must be through the water. For the Lee Bow discussion we have to add to this -- the fastest way to Point B in the water (and Point B can only be a point in the water, as we cannot sail towards a point fixed to land, through moving water, by pointing at it) is by sailing so as to make maximum VMG towards that point. It means making maximum VMG to windward if we can't lay that point (N.B. point in the water!) on one tack.

So here's another hypothesis, actually a guess so far:

I am guessing that the Lee Bow Effect might really seem to work in some cases in a geocentrically conceptualized world, as it might in some cases minimize the damage resulting from not having properly done Course to Steer towards the windward mark.

This can be easily investigated, but I haven't gotten that far yet. Maybe someone would like to take this up.

For example, if at the start of our leg, the windward mark is dead upwind, we can't lay it by tacking straight towards it. Because we will be swept off that course by the tide (expressed geocentrically) or the mark will move out of position (expressed hydrocentrically) while we're tacking. If we keep the lee bow in the current, it might (need to think about this) keep us closer to the right course. In fact (!), pinching here might even be beneficial, compared to just tacking in the wrong direction (!!!). But will not be nearly as effective as tacking efficiently in the correct direction according to a correctly worked out CTS.

That's a hypothesis, or a guess. I will be interested to see if anyone can build on that.

[Dockhead]

Originally Posted by Seaworthy Lass

The bit about pinching being detrimental is correct regardless of whether or not the current is changing, but are you saying there isn't a preferred tack to be on when the current is changing . I don't think that is right.

I haven't gotten to changing currents yet, but I'm probably now ready to go there.

I think probably this is just the same as in our Course to Steer problems. It doesn't matter (leaving aside for a moment the changes in true wind resulting from changes in the current) how the windward mark moves around -- let it be swept 30 miles up and 30 miles down before we get to it -- the only thing that matters is that we arrive at it over the shortest possible path through the water.

I think that's a robust proposition.

Following that: We get to the point in the water which is at the end of that line by sailing so as to maximize VMG, that is maximum VMG to windward if we can't lay it in one tack.

So which tack we are at what point cannot matter if we assume the wind doesn't change. That is mathematically definite if we make the same VMG to windward on both tacks -- the ONLY thing which matters is VMG to windward.

But the wind does change with the current. So our tactics need to consider the predicted change in the true wind just like any other predicted change in the true wind. So in fact we might want to tack uptide on the first leg, rather than downtide, so that we will have less, rather than more, true wind on the last tack, if the wind be so strong that we would be forced to reef and sail less efficiently, or the opposite in lighter wind. But this is a different problem and should be left aside.

Leaving out affects of changing wind, no -- I can say with robust confidence -- it does not matter which tack you are on when.

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

The problem is that it slows you down and in total actually hinders you. Maybe the reason why it gives someone an advantage is that it is done while the current is decreasing and everyone else on the opposite tack have just misjudged their final tacks given the decreasing current. Can any sailors who have experienced an advantage pinching at this stage, report if the quicker time in rounding the mark is noticed more when the current is getting stronger or weaker?

Originally Posted by Dockhead

So here's another hypothesis, actually a guess so far:

I am guessing that the Lee Bow Effect might really seem to work in some cases in a geocentrically conceptualized world, as it might in some cases minimize the damage resulting from not having properly done Course to Steer towards the windward mark.......

That's a hypothesis, or a guess. I will be interested to see if anyone can build on that.

It is the judgement of when to turn that I think is flawed and this will be accentuated if the current is altering (which is usually is in practice). I don't know that you would call this CTS, as your CTS is limited by your pointing ability when close hauled.

Also if the current is variable there is a preferred tack to be on. That will be affecting part of the fleet as well.

Just tossing ideas back and forth.

[Seaworthy Lass]

Originally Posted by Dockhead

I haven't gotten to changing currents yet, but I'm probably now ready to go there.

I think probably this is just the same as in our Course to Steer problems. It doesn't matter (leaving aside for a moment the changes in true wind resulting from changes in the current) how the windward mark moves around -- let it be swept 30 miles up and 30 miles down before we get to it -- the only thing that matters is that we arrive at it over the shortest possible path through the water.

I think that's a robust proposition.

Following that: We get to the point in the water which is at the end of that line by sailing so as to maximize VMG, that is maximum VMG to windward if we can't lay it in one tack.

So which tack we are at what point cannot matter if we assume the wind doesn't change. That is mathematically definite if we make the same VMG to windward on both tacks -- the ONLY thing which matters is VMG to windward.

But the wind does change with the current. So our tactics need to consider the predicted change in the true wind just like any other predicted change in the true wind. So in fact we might want to tack uptide on the first leg, rather than downtide, so that we will have less, rather than more, true wind on the last tack, if the wind be so strong that we would be forced to reef and sail less efficiently, or the opposite in lighter wind. But this is a different problem and should be left aside.

Leaving out affects of changing wind, no -- I can say with robust confidence -- it does not matter which tack you are on when.

Ah ha! There is the crunch! The true wind IS altering when the current is variable. So there is a favourable tack to be on, just as there is if ground wind is changing.

And the favourable tack to be on is the one with the current on your lee bow. That is the "true" lee bow effect (but pinching to achieve this does not work unless you have misjudged when to make your final tack).

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

Ah ha! There is the crunch! The true wind IS altering when the current is variable. So there is a favourable tack to be on, just as there is if ground wind is changing.

And the favourable tack to be on is the one with the current on your lee bow. That is the "true" lee bow effect (but pinching to achieve this does not work unless you have misjudged when to make your final tack).

I can actually draw a diagram to illustrate that, but we are sailing at the moment and I don't have a steady platform, so I will do it after dinner.

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

I can actually draw a diagram to illustrate that, but we are sailing at the moment and I don't have a steady platform, so I will do it after dinner.

I think another way of thinking about it is that having the current on your lee side gives you a heading that it closest to the CTS (that you cannot achieve as you need to tack). Travelling as close as possible to the correct CTS is favourable. Do not tack until the current changes direction (unless it is the final tack, you need then to work out when to make this).

[Dockhead]

Originally Posted by Seaworthy Lass

It is the judgement of when to turn that I think is flawed and this will be accentuated if the current is altering (which is usually is in practice). I don't know that you would call this CTS, as your CTS is limited by your pointing ability when close hauled.

Also if the current is variable there is a preferred tack to be on. That will be affecting part of the fleet as well.

Just tossing ideas back and forth.

Great idea-tossing -- really helps the thought processes

Why would you care how the sea bed is moving under you, when it turns? It's irrelevant to your sailing.

When you're tacking, you can't follow a constant heading, because you can't sail the constant heading calculated by your CTS calculated.

But the principle is the same -- you tack around a line of advance which is represented by your CTS.

CTS doesn't actually give you a line -- it gives you a point in the water. You need to tack towards this point in the water, obviously not towards the mark, which is moving around out there.

[Dockhead]

Originally Posted by Seaworthy Lass

I think another way of thinking about it is that having the current on your lee side gives you a heading that it closest to the CTS (that you cannot achieve as you need to tack). Travelling as close as possible to the correct CTS is favourable. Do not tack until the current changes direction (unless it is the final tack, you need then to work out when to make this).

Think for a minute about the water being still. Why would it matter whether the seabed is moving to leeward or windward under you? It doesn't matter at all, leaving aside the wind.

[Dockhead]

Originally Posted by Seaworthy Lass

Ah ha! There is the crunch! The true wind IS altering when the current is variable. So there is a favourable tack to be on, just as there is if ground wind is changing.

And the favourable tack to be on is the one with the current on your lee bow. That is the "true" lee bow effect (but pinching to achieve this does not work unless you have misjudged when to make your final tack).

Yes, I agree, absolutely if you need to be on one tack rather than the other from point of view of changing wind, then this is valid.

If you need more wind, then lee bow. If you need less wind, windward bow. This effect is real. But this is the only real part of it -- the rest is pure confusion.

[Seaworthy Lass]

Originally Posted by Dockhead

Think for a minute about the water being still. Why would it matter whether the seabed is moving to leeward or windward under you? It doesn't matter at all.

The water is still, but the true wind isn't. That is the key.

(That made me stop and think last night when I was rattling on about pinching, but I was too pig headed to admit it straight away LOL).

[Seaworthy Lass]

Originally Posted by Dockhead

Yes, I agree, absolutely if you need to be on one tack rather than the other from point of view of changing wind, then this is valid.

If you need more wind, then lee bow. If you need less wind, windward bow. This effect is real. But this is the only real part of it -- the rest is pure confusion.

But you sail to true wind. And true wind is the sum of ground wind and wind due to current (and leeway, but that complicates this too much at the moment).

The boat does not know if the true wind is changing due to a change in ground wind or current. But it IS changing, so there is a favourable tack to be on, just as there is if ground wind is changing.

[Dockhead]

Originally Posted by Seaworthy Lass

The water is still, but the true wind isn't. That is the key.

(That made me stop and think last night when I was rattling on about pinching, but I was too pig headed to admit it straight away LOL).

Yes, the wind changes, absolutely. But it's like any other predicted change in the wind and can be dealt with like that if we peel away the other nonsense.

But pinching is a key part of the proposition -- Lee Bow Effect proponents argue that even pinching will give you a lift in certain cases. I think we've demolished that now.

[Dockhead]

Originally Posted by Dockhead

Great idea-tossing -- really helps the thought processes

Why would you care how the sea bed is moving under you, when it turns? It's irrelevant to your sailing.

When you're tacking, you can't follow a constant heading, because you can't sail the constant heading calculated by your CTS calculated.

But the principle is the same -- you tack around a line of advance which is represented by your CTS.

CTS doesn't actually give you a line -- it gives you a point in the water. You need to tack towards this point in the water, obviously not towards the mark, which is moving around out there.

Sorry, a correction --

"Line of advance" is the wrong term -- ground-referenced, geocentric.

To speak more precisely: to sail a correct CTS while tacking, we tack around an average heading corresponding to our CTS.

[Seaworthy Lass]

Thought I shouldn't be lazy so I have come down to illustrate what I mean.

I have drawn a water referenced diagram. So the CTS is just from A (departure point) to B (destination).

The current is flowing from E to W initially, then W to E. The ground wind is constant and marked. The average wind due to current is marked. The direction will change when the current reverses.

At A there are two tacking options. Boat 1 takes the option with the current on the lee bow, Boat 2 goes on the opposite tack. I have marked on the distance travelled after x time.

The current then reverses. Boat 1 then tacks to keep the current on the lee bow. This tack actually gets him to B, but that is immaterial, this scenario would continue no matter how far away B was. Boat 2 has 2 options when the current reverses, continue on the same tack, or tack, but I ran out of paper and only showed one. The second option would not help. It is too late having made the mistake initially.

The story gets more complicated if the current is actually oblique enough that when it reverses you will eventually be able to sail to the CTS. Not sure what you do then. Just head to B along the CTS before the current alters if you can make B, but what if you can't? Need to think about that.

(Internet is getting pathetic, took ages to load this).

Edited to add: i think discussing lee bow is just confusing and probably irrelevant. Call it the tack that enables you to head closest to the CTS.