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

[Paul Elliott]

Originally Posted by goboatingnow

No he is motoring at 5kts against the current, not 10kts ( he never mentions that ) so rerun your logic above

dave

In that case he would be wrong. Are you sure about the motoring speed?

Do you and I agree that if he is motoring 10kts directly into the 5kt current that the Apparent Wind is 10kts (etc)?

And why don't you show me your solution for the two cases I proposed? We can check our results. My process is a simple solution using the vectors from my example, but I can show you the steps this evening.

[Dockhead]

Originally Posted by goboatingnow

I referenced all vectors to ground. As you cannot draw vectors referenced to different reference points.

Firstly

I have shown the wind generated by current pushing the boat through the air as a ground referenced vector opposite to the current direction.

Secondly

Forget True wind as mariners call it. There is only a set of ground winds and boat movement winds

Now lets restate.

a boat sails in apparent wind that's it nothing else. It does not experience true wind ground wind my wind or your wind. It experiences apparent wind , the sailing angle of that wind relative to the boat is the determining factor which is related to the direction of ground wind and boat heading.

Thirdly

Dock head continuously mixes up an issue.

Current can be represented by a " ground wind vector " and that is what I said when I mentioned current wind.

So when sailing in a current you have ground wind that represents the sum of current and earth ( ground wind ) this wind is fixed in direction for the duration of that current

You will notice if my diagram that even though the boat heading vector changed the current wind and ground wind do not change they remain constant on the vector diagram.

What changes when you change heading is the apparent wind. What I show is that the current wind cause an increase in apparent wind angle against the situation where no current exists.

This improvement continues to exist when the helm comes up to weather over the situation where he cones up to weather without the current . This is maximised when current is on the bow. The effect of current wind also continues round the compass

So yes the effect should be called bow current effect , rather then the lee bow effect , but the effect is still evident in the particular case of the current ending up fine on the lee bow, hence the moniker

It has nothing to do CTS. It has to do with apparent wind angles , apparent wind is made up of all ground wind effects , ( current as represented by a wind in opposition ) , boating heading vector as expressed by vector wind in opposition to boat heading

Just forget the STW true wind. It plays no part in anything. it's merely an intermediate vector sum of " certain " vectors. The boat never ever experiences it.

You cannot look at the bow effect of current unless you resolve everything apparent , because its a sailing angle issue.


Dave

Sorry, I'm just getting back up to speed having been buried in work the last couple of days.

There is a lot of argument in here about the frame of reference. In fact, of course, there is no right or wrong frame of reference. If you make no mistakes in computing the vectors, and don't mix up the sequence of any of them, you can describe any problem with any frame of reference. We could reference everything to the core of the earth, or the sun, if we like, adding a set of vectors for every new reference point which is moving in relation to the other entities that we are describing.

So of course -- Dave's drawing everything in ground references is fine -- if he doesn't get confused about anything, and doesn't make any mistakes.

But why add the extra layer of complexity of ground references?! It makes the problem vastly more complicated, and makes understanding vastly more difficult. Our boats sail in the interface between air and water -- in the sheer between them. True wind -- far from being something your boat "never experiences" -- is on the contrary what it sails in. All you add to get apparent wind is boat speed (not heading, nothing). If you express the problem with a water, rather than ground reference -- and Dave, as an engineer, I am sure will agree that it can be so expressed -- then all this business about current direction disappears, and your boat sails exactly the same way as it sails is slack water. The current is then not influencing sailing in the least; it is just sweeping your boat together with the water it is sailing in, in some direction in relation to land.

Separating these two situations -- wind moving in relation to water; water moving in relation to land -- so extremely simplifies the problem.

All this business about current wind optimizing apparent wind at certain angles to the current is just a crazy mishmash -- I agree that if you make no mistakes, you can express it this way, but I doubt that many people could do it. And besides, it fundamentally confuses the question of how to sail the boat (all determined by the wind versus water interface with one set of vectors), and in what direction to sail the boat in order to lay your mark (all determined by the water versus land interface, with a different set of vectors). These questions can be completely separated, with great profit for understanding the whole situation, and for practically laying your mark faster. If you don't separate them, I think confusion is almost guaranteed.

You can imagine this any way you like -- we have three media:

1. Land is still (ground-referenced) -- wind and water are moving. Very complex because the wind/water interface is not a direct function. You have to deal with all three media in order to solve the water/air interface.

2. Water is still (water-reference) -- land and wind are moving. Much simpler to calculate and easier to visualize. The two key interfaces (land/water, air/water) are direct functions which can be calculated and dealt with separately.

3. Air is still (air-referenced) -- we haven't tried this. It would be ok for dealing with the air/water interface -- better than the ground-referenced point of view! But water/ground will be a complex, indirect function.

I don't think that all three ways of looking at this are equal in their usefulness, not at all.


As to Ground Wind, it is irrelevant in a steady current. I think Dave has this backwards. Apparent wind, Dave correctly said, is what moves the boat. True Wind is just one function away from Apparent Wind -- just boat speed. Ground Wind is two functions away from Apparent Wind, making a much more complicated and unintuitive relationship. Why in the world would you think about it while sailing except to predict what the True Wind will be when the tide changes? It is not an accident, N.B.! That your instruments don't generally give you Ground Wind, and if they do, they give it in meteorological form referenced to compass rose, not in sailing form, referenced to the bow. This is NOT an accident!


As to pinching: Dave has mentioned that there are cases where short-term pinching can improve VMG to windward. This may be so -- let's leave this aside. The question is whether there is some case where a higher angle to the wind than your optimum VMG to windward angle to the wind can get you there faster, in some cases where the current is "aligned" in some particular way. The answer is that you absolutely cannot, and if you think you can, then you have made a mistake in your visualization and/or calculation. If you find yourself in a situation where it happens like that, then you have not been sailing (or tacking towards) the right direction, and you would have gotten there faster if you had been tacking towards the right direction in the first place. This is absolutely obvious if you frame the problem in a water-referenced way. I think we can all agree that the problem can be framed in three different ways which are equally valid mathematically. I will bet a case of Mumms that a profitable pinching strategy cannot be shown mathematically in a water-referenced world (water-referenced just to save the brain damage from trying to do the calculations with a ground reference).

My thesis is this:

The fastest way to lay an upwind mark (any mark which cannot be laid in one tack) across a non-changing, geographically uniform current is to calculate a CTS or desired average heading, and tack up that line through the water at your optimum VMG to windward angle to the wind. If we assume no error in helming or estimation of boat speed, then it makes no difference whatsoever which tack we are on when, as long as we do precisely the right amount of aggregate distance on both tacks.

In a non-changing current, the ground-water interface is a very simple calculation. A certain boat speed and current angle will give us a precise heading (or line of advance) which will carry us to the mark. Once we have done this calculation, then we can absolutely forget the ground, ground wind, and all ground references. Now have only two media to deal with -- the water-air interface. This calculation is now simplicity itself, as the boat sails as if the tide is slack, with no weird "bow effects" resulting from working bass-ackwards through a third medium -- the ground.

A case of Mumms to the first person who takes up the challenge by also putting up a case of Mumms, and who demonstrates mathematically that this is wrong. Which of course cannot be done, since this thesis is mathematically correct.

[Seaworthy Lass]

Originally Posted by Dockhead

My thesis is this:

The fastest way to lay an upwind mark (any mark which cannot be laid in one tack) across a non-changing, geographically uniform current is to calculate a CTS or desired average heading, and tack up that line through the water at your optimum VMG to windward angle to the wind. If we assume no error in helming or estimation of boat speed, then it makes no difference whatsoever which tack we are on when, as long as we do precisely the right amount of aggregate distance on both tacks.

In a non-changing current, the ground-water interface is a very simple calculation. A certain boat speed and current angle will give us a precise heading (or line of advance) which will carry us to the mark. Once we have done this calculation, then we can absolutely forget the ground, ground wind, and all ground references. Now have only two media to deal with -- the water-air interface. This calculation is now simplicity itself, as the boat sails as if the tide is slack, with no weird "bow effects" resulting from working bass-ackwards through a third medium -- the ground.

A case of Mumms to the first person who takes up the challenge by also putting up a case of Mumms, and who demonstrates mathematically that this is wrong. Which of course cannot be done, since this thesis is mathematically correct.

My only disagreement with this is that if the amount of cross current is sufficient, you may be able to lay the CTS without tacking at all .
Does that win me at least a bottle of Mumms? I will not be greedy and ask for a case .

[Dockhead]

Originally Posted by Seaworthy Lass

My only disagreement with this is that if the amount of cross current is sufficient, you may be able to lay the CTS without tacking at all .
Does that win me at least a bottle of Mumms? I will not be greedy and ask for a case .

My darling, I can find a bottle of Mumms for you any time

But no, if you can lay the CTS without tacking, then this is not an "upwind mark" for purposes of this problem. By definition of the problem, you can't lay it. Just like if you could lay the mark but for the current -- that is also irrelevant -- that is an upwind mark for you. Think in water-referenced terms and it is vastly easier to conceptualize.


By the way and totally off topic, I wonder how many land people realize that the phrase "to get laid" is actually a corruption of the phrase "to lay her" (who-whom confusion), which comes from nautical terminology?

[Seaworthy Lass]

Originally Posted by Dockhead

My darling, I can find a bottle of Mumms for you any time

But no, if you can lay the CTS without tacking, then this is not an "upwind mark" for purposes of this problem. By definition of the problem, you can't lay it. Just like if you could lay the mark but for the current -- that is also irrelevant -- that is an upwind mark for you. Think in water-referenced terms and it is vastly easier to conceptualize.


By the way and totally off topic, I wonder how many land people realize that the phrase "to get laid" is actually a corruption of the phrase "to lay her" (who-whom confusion), which comes from nautical terminology?

Excuse me Sir, I am discussing a situation where your destination is DIRECTLY upwind of your departure point. You will not have to tack up the CTS if there is sufficient current to lay the mark .

PS I am very partial to champagne (nothing less than vintage for seaworthy lassies though ).

[Dockhead]

Originally Posted by Seaworthy Lass

Excuse me Sir, I am discussing a situation where your destination is DIRECTLY upwind of your departure point. You will not have to tack up the CTS if there is sufficient current to lay the mark .

PS I am very partial to champagne (nothing less than vintage for seaworthy lassies though ).

The champagne is yours in any case, but your scenario is not included in the problem I posed. "Directly upwind" from ground perspective is certainly not going to be directly upwind in water perspective in your scenario -- think water.

The problem must needs involve tacking.

And it does not at all need to be directly upwind (water perspective) -- just enough upwind that you can't lay it.

And actually I think it's a really good illustration of the power of the water perspective --

With a non-changing, geographically uniform current, if you work it from a water perspective it's totally straightforward, almost exactly like sailing at slack tide. If you try to work it from a ground perspective, you will get brain damage, and probably the wrong result, even if, theoretically, you can do it from a ground perspective.

All of this is an extension of our CTS discussion. Imagine trying to work a CTS from a ground perspective. The process of calculating a CTS is nothing other than translation of ground coordinates to water in order to find the straight line through the water which will get you to your mark most directly. Imagine trying to compute the "S" curve -- and then follow it! yikes! -- to get an efficient passage across changing currents. It's really the same incredibly awkward frame of reference for tacking across moving water.

[Seaworthy Lass]

Originally Posted by goboatingnow

Seaworthy you have just shown the existence of a current generated wind referenced to ground

Dave, where on earth have I done that?
There is no current generated wind referenced to ground. If you fixed yourself to the ground while you were in water (eg hung on to an underwater post) there would be no wind generated by current, as you are ground based. It is only if you let go of the ground and are now water based that you would experience current generated wind (that is if you didn't sink LOL).

Originally Posted by goboatingnow

Do this excercise

Take a ground wind direction , take a current set and drift vector , assume a given boat speed and direction

Now please compute the wind vector the crew actually feel and the one the boat actually sails in

Please draw this for me and we can proceed step by step from there.

OK, diagram is shown below:
Diagram 1 GROUND WIND:
There is a 12 knot ground wind from the north. There is westerly 5 knot current. A 5 knot wind is induced by the current from the west if you are floating in the water.

Diagram 2 TRUE WIND:
The true wind (sum of ground wind plus wind induced by movement in current) is 13 knots from 337 degrees. If your sails were down (and there was no windage) this is exactly the wind you would feel. You would have absolutely no idea how much of it was coming from ground wind and how much from current. Your log would read zero. Unless you had a reference point on land or a GPS/chartplotter to look at, you would have no idea you were moving. You would just feel the true wind.

Diagram 3 APPARENT WIND:
Now you put up sails and head off at 27 degree heading at 6 knots (log speed).
The apparent wind (sum of true wind plus wind induced by your boat speed plus leeway) is 17 knots from 352 degrees.
This is the wind you now feel. This is what the sails are reacting to and the boat is sailing to.

Your course over ground I have not worked out, but would be somewhere NW.

(Dockhead, this is a classic situation where your mark is directly into wind relative to the departure point ie bearing of B from A is 360), but you would not need to tack up your CTS to get there . You would call this mark upwind would you not? The mark may not be upwind CTS, but it is upwind relative to A. We are only arguing about nomenclature now, but I think I win on a technicality )

Originally Posted by goboatingnow

( by the way do you accept The fact that current on the bow improves apparent wind angle )
( said in a nice way )

No Dave, sorry, this is incorrect.

If you are tacking with cross current, then the lifted tack is the one with the current closer to being on the bow.

Do not try and read anything else into it.
It does NOT mean the lift is greatest when the current is on the bow (look at my cone diagram, maximum change in true wind angle will occur with the current anywhere from 0 to 90 degrees to the ground wind depending on the proportion of current amount to ground wind amount (nothing to do with where your bow is pointing). It the amount of current is significantly smaller than the amount of wind, then the effect is maximum at close to 90 degrees to the ground wind (and probably about 45 degrees off your lee bow depending on how high your boat can point).

If you are tacking the current can be coming anywhere from 0 to 180 degrees relative to the true wind. As you are trying hard to sail into wind, then it is coming from your lee side most of the time, but not necessarily.

The key is that of the two tacks, the one most lifted is the one with the current more on the bow and that is simply because you are trying to head into wind. If you were going downwind, it is the other way around (current more on the stern is the lifted of the two tacks. It is NOT the action of having the current on the bow that lifts the tack. It would do the same thing if the boat was round in shape.

[Seaworthy Lass]

Originally Posted by Dockhead

My thesis is this:

The fastest way to lay an upwind mark (any mark which cannot be laid in one tack) across a non-changing, geographically uniform current is to calculate a CTS or desired average heading, and tack up that line through the water at your optimum VMG to windward angle to the wind.
...
A case of Mumms to the first person who takes up the challenge by also putting up a case of Mumms, and who demonstrates mathematically that this is wrong. Which of course cannot be done, since this thesis is mathematically correct.

Originally Posted by Dockhead

The champagne is yours in any case, but your scenario is not included in the problem I posed. "Directly upwind" from ground perspective is certainly not going to be directly upwind in water perspective in your scenario -- think water.

The problem must needs involve tacking.

And it does not at all need to be directly upwind (water perspective) -- just enough upwind that you can't lay it.

And actually I think it's a really good illustration of the power of the water perspective --

With a non-changing, geographically uniform current, if you work it from a water perspective it's totally straightforward, almost exactly like sailing at slack tide. If you try to work it from a ground perspective, you will get brain damage, and probably the wrong result, even if, theoretically, you can do it from a ground perspective.

All of this is an extension of our CTS discussion. Imagine trying to work a CTS from a ground perspective. The process of calculating a CTS is nothing other than translation of ground coordinates to water in order to find the straight line through the water which will get you to your mark most directly. Imagine trying to compute the "S" curve -- and then follow it! yikes! -- to get an efficient passage across changing currents. It's really the same incredibly awkward frame of reference for tacking across moving water.

I understand what you are saying and agree fully and have never disagreed with this. I just have you on a minor technicality . You stated in your thesis that you had an upwind mark. You did not say the mark was directly up the CTS.

I think I have shown an exception to your thesis . I claim a bottle of bubbly for at least finding an exception (while still agreeing with the basis of what you posted ).

[Dockhead]

LOL, you are persistent! . Come get your Champagne already and leave me alone!

But there is not even a minor technicality. I specifically defined "upwind mark" as "one which cannot be laid in one tack". I don't see how it could be more clear. If you can lay it in one tack, it does not fall within the problem set.

[Seaworthy Lass]

Originally Posted by Dockhead

LOL, you are persistent! . Come get your Champagne already and leave me alone!

I can be so very annoyingly persistent LOL.
Afraid the champagne will have to be couriered . Just hauled out and working hard.

I at least fully agree with you that if the CTS to get to a mark in the quickest possible time is upwind, then all you do is tack up the CTS. No argument from me there.

Onto other things. I do not think we disagree either with our concepts of ground, true and apparent wind, or if we do then I have possibly worded myself badly. Could you please take a look at my response to Dave (GBN) and check if that is a reasonable way of explaining things?

I am struggling to make Dave understand (as I can see you and Paul are, as well as several members on the other thread). If Dave thinks this way, then I am sure lots of other members do as well and it would be good to be able to get the concept across correctly.

[I.Grind]

I'm with SL on this one. In fact I believe I said the same thing a zillion posts ago. It has even happened to me recently. In a situation where winds are light and current is strong, it is entirely possible to fetch a mark that is dead upwind on one tack.

Example: wind, due North very light, current flowing East 2kts, distance to mark, 2 NM. Boat is a RP 1D48 sails very close hauled, easily out points the rest of the fleet, let's say 30 degrees off the wind. After starting on starboard CTS would be 330. At a speed of approx. 4 kts after 30 minutes more or less you will fetch the mark, having been swept 1 nm East by the current. Real life really happened. Exact beatings modified to simplify explanation but not far off.

You'll have to draw your own diagrams, as I am in bed, on my iPad, in a blackout.

[Dockhead]

Originally Posted by Seaworthy Lass

Excuse me Sir, I am discussing a situation where your destination is DIRECTLY upwind of your departure point. You will not have to tack up the CTS if there is sufficient current to lay the mark .

PS I am very partial to champagne (nothing less than vintage for seaworthy lassies though ).

So does it fit the given thesis? Word for word :"you can't lay it"

I did NOT write "you couldn't lay it but for the current". A very different thesis. I also did not write "your destination is directly upwind." I wrote the thesis carefully and precisely, and your scenario does not fit the it. Come on, this is a silly distraction.

[Dockhead]

Originally Posted by I.Grind

I'm with SL on this one. In fact I believe I said the same thing a zillion posts ago. It has even happened to me recently. In a situation where winds are light and current is strong, it is entirely possible to fetch a mark that is dead upwind on one tack.

Example: wind, due North very light, current flowing East 2kts, distance to mark, 2 NM. Boat is a RP 1D48 sails very close hauled, easily out points the rest of the fleet, let's say 30 degrees off the wind. After starting on starboard CTS would be 330. At a speed of approx. 4 kts after 30 minutes more or less you will fetch the mark, having been swept 1 nm East by the current. Real life really happened. Exact beatings modified to simplify explanation but not far off.

You'll have to draw your own diagrams, as I am in bed, on my iPad, in a blackout.

No one disagrees. In fact, there are situations where you can lay your mark without even sailing - just drift to it. This is all completely irrelevant.

[Hoofsmit]

I just need to know the variable vectors that are being used,
can you tell me your leeway application on say each 5 degree heading to windward and since we are using true as water referenced
"The apparent wind (sum of true wind plus wind induced by your boat speed plus leeway)"
As in windage of hull and lateral movement from sails etc etc are you just applying your boats polars?
just going through a few things ! will post a few diagrams and findings as soon as is proofed ; )

[Paul Elliott]

Hoofsmit, if you want to figure leeway, just use 5 degrees on all points of sail, other than (say) within 20 degrees of dead downwind). Use zero for the that.

The only significant difference you will see is that your course through the water will be different than your heading (leeway) and the Apparent Wind / True Wind calculations will have to account for the heading offset. I believe that accurately-done polars will take leeway into account and give you boat speed as a function of actual True Wind.

Ultimately, it's only going to make a fairly small difference in your tacking angles.