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

[Dockhead]

About a year ago, we had a great discussion on here which went right to the bottom of the question of how to cross moving bodies of water -- the principle of constant heading crossings and calculating course-to-steer. Hats off to major brains Seaworthy Lass and Andrew Troup who did amazing work, getting all the way into the math, creating mathematical proofs, and Seaworthy even came up with an original technique for how to do it without a computer!

Why don't we turn the same kind of attention to the Lee Bow Effect, which has puzzled me since I started sailing in the English Channel? There is no really clear consensus on whether it is ridiculous nonsense, or whether it really works.

There is a fair amount of public information arguing that it is a myth. See for example:

Destination One Design - Preparation

https://groups.google.com/forum/#!to...ng/TFWS6xe55KQ

June | 2008 | Campbell Sailing

Now, we need to define exactly what we're talking about, since there are several layers to this onion called the lee bow effect, and several possibly different phenomena.

1. Sailors tacking upwind across the Channel sometimes tell you that you should keep the lee bow into the tide. They say that like that you shorten your passage and arrive faster, because the tide takes you less far off the rhumb line so your passage is shorter.

I submit that this is obvious nonsense, for the same reasons we got into when discussing Course To Steer across moving water. Your boat sails through water, not on the seabed, which it doesn't know, so the distance over ground sailed is irrelevant. Your boat only feels the distance sailed through water, and if that distance is identical, then it makes no difference whether you tack with or against the tide.

2. Some racers say you can get a lift from the tide pushing against your bow from the lee side, holding you against the wind and allowing you to sail closer to the wind.

I think this is also nonsense, and I agree with the arguments in the last article linked above.

3. If your boat could profitably use a bit more wind, then you can get a lift by tacking so that the tide increases apparent wind during that part of the passage you really need it.

I think this one is true. Of course the apparent (actually, true) wind is increased when the current and wind are in opposition, and is reduced when they are together.


There are probably some other variants of the Lee Bow Effect.

Discuss!

Paging Seaworthy and Andrew!

[Seaworthy Lass]

I dismissed the effect when I had a quick look at it initially, but I will have a second look and put my thinking cap on.

Give me some time . We are in a wonderful location so I have just come back from a four hour cross country hike. CF members NornaBiron sailed into our anchorage yesterday and we are busy socialising otherwise. This cruising lifestyle can be terribly hectic at times and this gets in the way of doing any maths .

[Paul Elliott]

Here's someone who says there is no magic Lee-Bow effect: http://www.j105.org/docs/noleebow.pdf
I agree. You have the wind, the current, and a place you want to get to. These numbers and your polars will tell you the best course to sail.

[Dockhead]

Here is one statement of the Lee Bow Effect which I found in my reading:

"Lee-Bow Effect: This brings up the famous lee-bow effect. If the current is coming at you at an angle that is very close to the course you are sailing and if, by pinching just a little bit, you can get your lee bow into the flow of the current, the movement of the water is going to push against the hull, the keel, and the rudder, and it is goign to drive you up to windward even though you are going slower over the bottom. If you are on the other tack the current is going to be hitting you broadside and pushing you down. If you can get the lee-bow effect to push you to windward, I feel you also increase the wind pressure on the sails. If I am on the tack that goes across the current I feel I am losing speed and distance to the mark. That is why, unless there is an obvious way to get out of the current entirely, or at least to a slower flow, I think you should always make your longest tack to the next mark sailing in the lee-bow position. And I would do this even if it meant pinching a bit to do so."

June | 2008 | Campbell Sailing

And here is how the author debunks it:

"This seemed appealing enough to my young, malleable, Naples sabot sailing around current-less basins in Southern California brain. I was willing to believe anything out of the Wizard of Zenda’s mouth, or written on the pages in front of me. The idea that a boat sailing alone with its bow slightly above the angle of adverse current would gain distance to windward makes sense, relative a stationary object like a shoreline or a mark anchored in the water. This idea is sufficiently flawed when you either of two things: add another boat to the situation, or remove the mark from the racecourse. At the risk of being flicked into oblivion as I tug on Superman’s cape, the lee-bow effect does not exist when racing against other boats.If two boats are sailing upwind on opposite tacks. According to Melges, the boat with its lee-bow facing the current will gain distance. to windward, and the boat with its windward hip facing the current will lose distance to leeward. In the following diagram. The boat gaining the mythical advantage is in red, as the current crossed the picture right to left. The critical issue is that two boats sailing together upwind are affected exactly equally by the current! Imagine that the two boats are dead in the water. There is no movement whatsoever up the racecourse or towards each other, but there are 2 knots of current running beneath them from right to left across the racecourse. As Mr Melges points out, the red boat is traveling over a distance to windward which happens to be to his left. Likewise, the green boat is traveling distance to leeward. However, there is no advantage to being the red boat or the green boat because both boats are moving at exactly the same speed in exactly the same direction, that of the current. Simply because the red boat is moving to windward, does not mean that the boat is gaining, it only means (in the demonstrated scenario) that it is gaining distance to the left. The green boat is likewise not losing anything, in fact it is gaining the same distance as the red boat toward the left."

Ibid

[goboatingnow]

Originally Posted by Dockhead

About a year ago, we had a great discussion on here which went right to the bottom of the question of how to cross moving bodies of water -- the principle of constant heading crossings and calculating course-to-steer. Hats off to major brains Seaworthy Lass and Andrew Troup who did amazing work, getting all the way into the math, creating mathematical proofs, and Seaworthy even came up with an original technique for how to do it without a computer!

Why don't we turn the same kind of attention to the Lee Bow Effect, which has puzzled me since I started sailing in the English Channel? There is no really clear consensus on whether it is ridiculous nonsense, or whether it really works.

There is a fair amount of public information arguing that it is a myth. See for example:

Destination One Design - Preparation

https://groups.google.com/forum/#!to...ng/TFWS6xe55KQ

June | 2008 | Campbell Sailing

Now, we need to define exactly what we're talking about, since there are several layers to this onion called the lee bow effect, and several possibly different phenomena.

1. Sailors tacking upwind across the Channel sometimes tell you that you should keep the lee bow into the tide. They say that like that you shorten your passage and arrive faster, because the tide takes you less far off the rhumb line so your passage is shorter.

I submit that this is obvious nonsense, for the same reasons we got into when discussing Course To Steer across moving water. Your boat sails through water, not on the seabed, which it doesn't know, so the distance over ground sailed is irrelevant. Your boat only feels the distance sailed through water, and if that distance is identical, then it makes no difference whether you tack with or against the tide.

2. Some racers say you can get a lift from the tide pushing against your bow from the lee side, holding you against the wind and allowing you to sail closer to the wind.

I think this is also nonsense, and I agree with the arguments in the last article linked above.

3. If your boat could profitably use a bit more wind, then you can get a lift by tacking so that the tide increases apparent wind during that part of the passage you really need it.

I think this one is true. Of course the apparent (actually, true) wind is increased when the current and wind are in opposition, and is reduced when they are together.

There are probably some other variants of the Lee Bow Effect.

Discuss!

Paging Seaworthy and Andrew!

Yes you really need to define what you mean by " lee bowing " some of the situations do work.

Dave

[Seaworthy Lass]

I love a challenge . This is my conclusion:

All the articles seem to dismiss the lee bow effect as a myth. This was certainly my thought at first glance. But the effect is real. The reason is not, however, due to "pushing the boat to windward". All the discussions debunking the myth correctly use the analogy of a boat on a "moving rug" or "tablecloth" or "conveyor belt". If all you consider is the effect of the current on the hull, then you arrive at the conclusion that pinching will simply slow you down.

However, I think the effect comes from the change in apparent wind. The basic question is "is there a preferred tack to be on with current?". The answer is yes, with the current on your lee bow. Current moves the boat through the air and creates an apparent wind. When tacking, with the current on your lee bow you get a lift. With the current windward to your bow you are knocked.

What happens when the current is EXACTLY on the bow? Will pinching a little to get it on your lee side help? It may. It depends on the strength of the current relative to the wind speed.

[Dockhead]

Originally Posted by Seaworthy Lass

I love a challenge . This is my conclusion:

All the articles seem to dismiss the lee bow effect as a myth. This was certainly my thought at first glance. But the effect is real. The reason is not, however, due to "pushing the boat to windward". All the discussions debunking the myth correctly use the analogy of a boat on a "moving rug" or "tablecloth" or "conveyor belt". If all you consider is the effect of the current on the hull, then you arrive at the conclusion that pinching will simply slow you down.

However, I think the effect comes from the change in apparent wind. The basic question is "is there a preferred tack to be on with current?". The answer is yes, with the current on your lee bow. Current moves the boat through the air and creates an apparent wind. When tacking, with the current on your lee bow you get a lift. With the current windward to your bow you are knocked.

What happens when the current is EXACTLY on the bow? Will pinching a little to get it on your lee side help? It may. It depends on the strength of the current relative to the wind speed.

I think we're talking about #2 here.

It seems to me that the moving conveyer belt analogy works here. The current doesn't make any difference, and nothing "pushes" you just because you've put the boat's head slightly across the current.

You've got just two media -- the water, and the wind, true wind in relation to the water. The sheer between these media is what we sail in. It might be better to think of the land and the racing marks as moving targets, with the water being stationary (which it is, as far as our boats know, their keels, rudders, etc.). "Lee-bowing" of this sort certainly doesn't work as a way to change sailing dynamics, as far as I can tell.

[Seaworthy Lass]

Originally Posted by Dockhead

I think we're talking about #2 here.

It seems to me that the moving conveyer belt analogy works here. The current doesn't make any difference, and nothing "pushes" you just because you've put the boat's head slightly across the current.

You've got just two media -- the water, and the wind, true wind in relation to the water. The sheer between these media is what we sail in. It might be better to think of the land and the racing marks as moving targets, with the water being stationary (which it is, as far as our boats know, their keels, rudders, etc.). "Lee-bowing" of this sort certainly doesn't work as a way to change sailing dynamics, as far as I can tell.

Think about the zero wind situation.
Can you still make progress?

[transmitterdan]

I sometimes try to debunk ideas by looking at what happens "in the limit". By that I mean that if at the extreme a thing is false then probably it is false under less than extreme conditions.

What could you do if there was no true wind and the current was 10 knots? Could you sail the boat on a course other than down current? Now figure out what courses you could steer and what speeds you could make from polars. Is it reasonable then that you could reach some destination other than dead down current?

[Seaworthy Lass]

Originally Posted by transmitterdan

I sometimes try to debunk ideas by looking at what happens "in the limit". By that I mean that if at the extreme a thing is false then probably it is false under less than extreme conditions.

What could you do if there was no true wind and the current was 10 knots? Could you sail the boat on a course other than down current? Now figure out what courses you could steer and what speeds you could make from polars. Is it reasonable then that you could reach some destination other than dead down current?

I agree with this logic, see my post before yours.

[transmitterdan]

Originally Posted by Seaworthy Lass

I agree with this logic, see my post before yours.

Probably we were typing at the same time

[transmitterdan]

Anybody want to do the polars for their boat? Is there any direction you can sail that corresponds to the purported "lee bow effect"?

[Dockhead]

Originally Posted by transmitterdan

I sometimes try to debunk ideas by looking at what happens "in the limit". By that I mean that if at the extreme a thing is false then probably it is false under less than extreme conditions.

What could you do if there was no true wind and the current was 10 knots? Could you sail the boat on a course other than down current? Now figure out what courses you could steer and what speeds you could make from polars. Is it reasonable then that you could reach some destination other than dead down current?

Reductio ad absurdam -- a powerful logical technique

[Seaworthy Lass]

Dockhead, don't laugh, it is a technique that does work.

I think the fact that you can make progress from A to B if there is zero wind and the current cancels out during the duration of the journey, means that the current is affecting your sailing ability. It makes a difference if you choose to have the current on your windward or leeward side while the apparent wind is heading you. This must also carry on to the situation where you have wind.

[Dockhead]

Originally Posted by Seaworthy Lass

Dockhead, don't laugh, it is a technique that does work.

I think the fact that you can make progress from A to B if there is zero wind and the current cancels out during the duration of the journey, means that the current is affecting your sailing ability. It makes a difference if you choose to have the current on your windward or leeward side while the apparent wind is heading you. This must also carry on to the situation where you have wind.

I was not joking at all. It is of course a technique that works. I have a degree in philosophy

It is equally valid in math, and is sometimes called "one of the mathemetician's finest weapons" Reductio ad Absurdum -- from Wolfram MathWorld

See also: http://en.wikipedia.org/wiki/Proof_by_contradiction