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

[europaflyer]

Originally Posted by Dockhead

Interesting question --

Some Channel sailors believe that the LBE works because it shortens your distance sailed over ground. This is obvious nonsense, and it threw me off -- that's why when we started this thread, my hunch was that there is no such thing.

Now we know that there definitely is such a thing, and lee bowing, at least in the standard Channel situation does keep you closer to the rhumb line.

Interesting coincidence.

And a handy coincidence, but coincidence nonetheless. Too many people explain it this way, which we know isn't the 'real' reason.

Having a consensus feels pretty good.

Now to read through the CTS thread!

[Hoofsmit]

this is leeway

THE BALANCE OF AERODYNAMIC AND HYDRODYNAMIC FORCES

The aerodynamic forces acting on a sail and hydrodynamic-hydrostatic forces acting on a hull

must be

balanced for a sailing yacht. The forces and their relative positions, hence the moments,
are given in Figure 1. Aerodynamic forces are generated by the wind with a true wind speed (VJ
at a true wind angle of (y). However wind forces act on the sail relative to the yacht, that is at
apparent wind angle (p) and at apparent wind speed (V,). Figure 1d is called aerodynamic wind
triangle. It may be observed that speed against wind direction, or speed made good, can be
calculated as V,,=V, Cos(y). This speed is the most important feature in yacht racing. The six
force-moment balance equations for a yacht can be written as :
1. F,=R 4. MpA=Mp
2. FH=FS FORCES 5. MH=MR MOMENTS (1)

3. F,,=FvW 6. M,,=Mm

beyond my scope ! but I do know more current creates more apparent , more leeway which then must be worked into this cts

just in case someone wanted to disprove Lee bow effect !

[Paul Elliott]

Leeway is most certainly being considered here, particularly if you are using your polars and your intimate knowledge of your boat's sailing characteristics. Your polars tell you your speed through the water for a given True Wind (water-ref) Speed and Angle. You then factor in your leeway and this will be used to determine your optimum heading.

Or, simply looking at the "CTS Method" course, you know that you have to adjust your course and any tacking angles to compensate for leeway.

I just don't see how any of this changes the principles we are discussing. The boat doesn't go exactly where it's pointed. It sails at different speeds, and with different amounts of leeway, as you change your point of sail, or as the conditions change. Who is claiming anything different, or where do these facts invalidate any of the conclusions?

[Paul Elliott]

Originally Posted by goboatingnow

i attach a drawing , to illustrate how I see the lee bow effect

this example converts everything into ground referenced wind, because lee bow effect is about wind angle and only wind angle.

you can see that by switching the current from the windward bow to the leeward bow the apparent wind angle increases.

In real life you have a trade off between pinching ( i.e. i.e. less wind angle in order to get the lee bow effect ) and the freeing effect of the lee bow. In many cases especially with poor pointing boats, the effect is negative, because the current effect is usually small in comparison with the ground wind and the boat speed.

note I have specifically ignored the effect of moving the boats orientation in relation to the original ground wind. the change in that angle could be large or small in relation to the subsequent change in apparent wind due to the lee bow effect. its really taken up in the "pinching" trade off. in racing we are often talking about 2 -5 degrees of orientation change

note that the lee bow effect is only for situations where the current is almost on the nose , not elsewhere ( this has been covered by SL)

Sorry just noticed a label error . the current wind win word bow, should be labelled apparent wind windward bow , I've now fixed that

I stand by, in my lambs costume!

dave

One difficulty with your diagram is that even while you have the current-induced wind shifting many degrees as you swing the boat to put the current to one side or the other, your boat-motion-generated wind remains pointing straight down. You can't have one swing without the other swinging as well.

I claim that in every case if 're going to pinch, I can show you a course that will get you there faster by sailing the optimum wind angle (or angles, since tacking may be involved). By definition, pinching is sailing closer to the wind (water-referenced True, or boat-referenced Apparent, it doesn't matter) than your maximum VMG angle. Any other definition of pinching is not really pinching. I claim that this holds true no matter where the current is flowing, or with no current at all.

The only time that pinching is appropriate is when you miscalculated your last tack and the time/speed lost in the tacking maneuver would be worse than the pinching penalty. Or, if an obstacle appears, or the conditions change, but that's not the issue, is it?

[Hoofsmit]

Originally Posted by Paul Elliott

Leeway is most certainly being considered here, particularly if you are using your polars and your intimate knowledge of your boat's sailing characteristics. Your polars tell you your speed through the water for a given True Wind (water-ref) Speed and Angle. You then factor in your leeway and this will be used to determine your optimum heading.

Or, simply looking at the "CTS Method" course, you know that you have to adjust your course and any tacking angles to compensate for leeway.

I just don't see how any of this changes the principles we are discussing. The boat doesn't go exactly where it's pointed. It sails at different speeds, and with different amounts of leeway, as you change your point of sail, or as the conditions change. Who is claiming anything different, or where do these facts invalidate any of the conclusions?

I am not invalidating them, they do not prove or disprove the lee bow effect

you have answered your own question, the word you used was
" compensate"
That's what a human or auto helm does, every tack every wnd change even true wind reading from your instrument which is actually not correct to an equation because of wind sheer across your sail, every movement of your helm is a correction to a cts,
if you feel I am wrong
leave the marina an pop your course to steer and seize your helm sit back and see what happens ---CTS IS A best guess, sorry but if you can not understand that this could be solved by the work done here (which I do not undermine, as much as those that have gone before unless I can prove the variable )
ps , read up a bit on polars , they are a guess because they cannot provide the test condition with no variables, the SAR organisation can not even get a drift of a life rafts leeway to be accurate, what hope have we of proving leebow effect is not true...... it is as most now say a rule of thumb to help us make those compensations
I have nothing left in me for an argument on this topic , but I will post links of some of my last 5 days of research it you wish
I do thank you for your input as I have learnt from it
thanks
Paul M.

[Hoofsmit]

ERE BOY.....
(WHAT)
MAKE SURE YOU DO PUT THAT TIDE ON YER LUERD BOW
OR THOSE WAVES WEL SEND EE TU BISCAY IF EE DUNT



you know i'm right

good bye

[goboatingnow]

Originally Posted by Paul Elliott

One difficulty with your diagram is that even while you have the current-induced wind shifting many degrees as you swing the boat to put the current to one side or the other, your boat-motion-generated wind remains pointing straight down. You can't have one swing without the other swinging as well.

I claim that in every case if 're going to pinch, I can show you a course that will get you there faster by sailing the optimum wind angle (or angles, since tacking may be involved). By definition, pinching is sailing closer to the wind (water-referenced True, or boat-referenced Apparent, it doesn't matter) than your maximum VMG angle. Any other definition of pinching is not really pinching. I claim that this holds true no matter where the current is flowing, or with no current at all.

The only time that pinching is appropriate is when you miscalculated your last tack and the time/speed lost in the tacking maneuver would be worse than the pinching penalty. Or, if an obstacle appears, or the conditions change, but that's not the issue, is it?

Too many pages to read. But boat induced wind is always pointing directly at the bow. Hence my diagram is right in that regard.

Funnily no one has challenged the diagram. And it's seems to be a subset of SLs diagram. And it's seems the consensus is there is an effect at different situations on both windward and leeward bows, though the windward effects are unusual.

furthermore. I never ever said pinching was better, nor did I mention anything about speed. All I mentioned was an improvement in wind angle , ie lift. Whether its exploitable is entirely a different story.

When the effect was described to me it was in very narrowly defined terms. , ie moving the current a degree or two by pinching up , could result in a freeing wind , which technically removed the pinching. All the other terms in relation to the effect of current being used here are news to me.

The only reason pinching comes into it at all, was to define the situation in the most narrowest of situations , ie close hauled , where coming up more would result in entering the pinching region. Outside of that situation you do not need any additional wind angle.

I would dispute your intention in relation to VMG and pinching. Many boats have sail combinations when falling out can result in better VMGs yet are still sailing aerodynamically with tighter VMG angles. I would always define pinching as sailing so that by coming up above close hauled the sails begin to luff.

Dave

[goboatingnow]

Just summarise the non CTS issue I agree with Seaworthy summation


"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.
"

Not trying to derail the current discussion which is very different , just trying to summarise a very long and convoluted thread

( nor do I disagree with the notion there are other particular cases where there is windward lift or leeward headers. But I would argue that any current not fine on the bow is not a " bow effect "

Dave

[Paul Elliott]

Originally Posted by goboatingnow

Too many pages to read. But boat induced wind is always pointing directly at the bow. Hence my diagram is right in that regard. [...]

Sorry, no. In your diagram you have the boat changing course or heading slightly to "put the current on the leeward or windward bow" (paraphrased). Therefore the bow no longer points straight up, and neither should the boat-induced wind vector.

You also show a very large change in current-induced wind, where the only other change is this few degrees of boat heading. That's just not going to happen. Attach numbers to your vectors and do the math.

I've not had the time, and won't for a few more hours, but tonight I will analyze your diagram more closely. I have this feeling that there are still more issues with it.

[goboatingnow]

Paul et al, Ive redrawn the vectors to more closely correspond with reality , in the previous one , I emulated boat direction change by changing the current direction.

IN this case we have a windward mark ( though not directly to windward)

and we have a current on the bow, The helm wish to point "higher " at the mark. ( boat wind , is an analog for heading only in the reveerse direction)

Clearly the apparent wind angle is better( but not by much) after the pinch with the current , then should the pinch be executed without the current

So the effect of manoeuvre is to execute a turn , with less apparent wind effect that had the current not been there in the first place.

None of this invalidates what SL says, in there are many other current directions that cause freeing and heading of the apparent wind, both on the lee and windward side.

What I am merely demonstrating , is that the effect of a pinch is less pronounced with the current , and the manoeuvre just happens to put the current on the lee bow.

Of course the net effect may not result in a gain , or in a tiny effect . but it is clear ( and this point was made by SL) that sailing directly into the current offers the best wind angle, of course progress to the mark , is another thing entirely

I would contend that the diagram shows an " effect" and the current in this ONE case happens to end up on the lee bow, and there is an improvement in the ratio of heading change to apparent wind angle because the current is there.

Its correct that its not specifically a lee bow effect, but it is the moniker that is attached in this specific case of it

Note as an aside, I would argue that True wind is never used in any diagrams that contain ground referenced destinations. True wind never effects a boat. it is a mathematical vector addition thats all. ( An intermediate result of a computation so to speak)

Just to summarise, the lee bow effect, i.e. what started this thread , and the "pinch" does result in the ability to lay closer to a windward mark, precisely because of the existence of the current , and that the effect of a pinch is reduced because of that current ( which happens to end up on the lee side) .

Its more, that a small change in direction in this case, results in a lee bow current , then the lee bow current facilitates the change in direction.


Note , I make no comment of the reasons one would want to "pinch" up to weather in the first place, or whether that conveys any advantage.


BTW, I fully agree with SL and others that state the name is a misnomer, it should be called the "head bow effect" or something. but it was so misnamed because the helm only wants to turn to weather and hence the current ends up on the lee side by the nature of the turn.

The same effect would happen on a loo ward turn, but noone is interested in real life in that and the gain in sail angle is dwarfed by the gain from boat polars


I stand by


dave

[Seaworthy Lass]

Originally Posted by goboatingnow

Just summarise the non CTS issue I agree with Seaworthy summation


"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.
"

Not trying to derail the current discussion which is very different , just trying to summarise a very long and convoluted thread

( nor do I disagree with the notion there are other particular cases where there is windward lift or leeward headers. But I would argue that any current not fine on the bow is not a " bow effect "

Dave

Dave that is plain sneaky popping in a quote of mine from a post I made way back at #2 when I retracted it totally by post #59 LOL.

This was my retraction:

Originally Posted by Seaworthy Lass

My honour is lost LOL.

I fell asleep with a pencil and ruler in my hands last night, so I do at least need an A for effort. My initial thoughts months ago when I burst out laughing when I first read about the lee bow effect were right and I was just really hoping I could wriggle my way out of my comments earlier in this thread, but I fell asleep trying to achieve the impossible.

The simplest answer is actually correct. If the ground wind is steady and the current does not alter, then the true wind is constant. Pinching does nothing but slow you down. I should have heeded my high school maths teacher's advice when she said "Draw a good diagram" before I sprinted off on a different tangent.

It is oh so painful admitting I was pitifully wrong. I am off now with tail between my legs to wipe the egg off my face and patch up my wounds .

I have kept repeating this over and over the last few hundred posts, many of my comments in exchanges with you:

Originally Posted by Seaworthy Lass

Dave......
The 'Lee Bow Effect' is absolutely a myth. It is in fact laughable .

My cone diagram beautifully illustrates why this is so:

[Seaworthy Lass]

Dave, I am sorry, there is so much wrong with your 2 diagrams that it would takes ages to describe. I very unfortunatley have no time today , boat chores call (I am just having a morning cuppa before commencing).

You have mixed up the types of wind to start with. The only type of wind you can affect by pinching is the apparent wind (you cannot change the ground or true wind, no matter what you tried to do, although some sailors would probably sell their souls to be able to do so ).

So you can only alter apparent wind by your boat movement (speed plus leeway).

If you alter your heading, yes of course the apparent wind will alter, both because of a change in your direction and because of a change in boat speed that brings.

If you pinch you instantly slow down. Your VMG instantly drops. It doesn't matter which side of the boat you are presenting to the current.

If the current is on the bow you are ALREADY on the tack lifted by current. Irrespective of which side of the bow the current is on. Pinching will only slow you down.

Paul, if you have the time are you able to discuss all this with Dave. I must start on boat work .

[Paul Elliott]

Dave, family life has gotten in the way of analyzing and working out these diagrams. I hope tomorrow I can make the time.

I do want to bring in some actual polars and leeway calculations, so we can look at VMG and CMG (velocity made good in the direction of a waypoint). Since we're talking about pinching and otherwise sailing non-optimum angles, I really want to explore how these interact. If I just arbitrarily assume some boatspeed then the exercise could be misleading. I'm on the road this week, so I picked up some graph paper, a protractor, compass, and ruler. It should be fun!

In the meantime, do take a look at this diagram (below). It shows a sample polar for one windspeed, as well as the optimum upwind and downwind angles, as well as how VMG falls off as you sail higher or lower.

[Hoofsmit]

Originally Posted by goboatingnow

Too many pages to read. But boat induced wind is always pointing directly at the bow. Hence my diagram is right in that regard.

Funnily no one has challenged the diagram. And it's seems to be a subset of SLs diagram. And it's seems the consensus is there is an effect at different situations on both windward and leeward bows, though the windward effects are unusual.

furthermore. I never ever said pinching was better, nor did I mention anything about speed. All I mentioned was an improvement in wind angle , ie lift. Whether its exploitable is entirely a different story.

When the effect was described to me it was in very narrowly defined terms. , ie moving the current a degree or two by pinching up , could result in a freeing wind , which technically removed the pinching. All the other terms in relation to the effect of current being used here are news to me.

The only reason pinching comes into it at all, was to define the situation in the most narrowest of situations , ie close hauled , where coming up more would result in entering the pinching region. Outside of that situation you do not need any additional wind angle.

I would dispute your intention in relation to VMG and pinching. Many boats have sail combinations when falling out can result in better VMGs yet are still sailing aerodynamically with tighter VMG angles. I would always define pinching as sailing so that by coming up above close hauled the sails begin to luff.

Dave

Now I know you are all going to ignore this because but, read and absorb it please
The effect is caused by the current on the bow ( don,t run away)

Think "apposed and equal force"
Logic states no current no additional force, it is only that we cannot work this out that we have to turn current to a wind vector
the wind is ground based.
think mooring ... sails up ..... all wind vectors give a force equal to the mooring line and we stay still ...... because our wetted area gives a resistance to the current. it is the current that affects the apparent not the apparent wind that effects the current, think out the box and you start looking at the effect which really make the difference

We just mix it all up into apparent wind so we can plot it

It is purely simple when you are thick like me

[Dockhead]

Originally Posted by Paul Elliott

You also show a very large change in current-induced wind, where the only other change is this few degrees of boat heading. That's just not going to happen. Attach numbers to your vectors and do the math.

Exactly. This is a different flavor of Lee Bow Effect than what we have been discussing lately. This is the "if you can pinch to get the current from fine on the weather bow to fine on the lee bow, you'll get a lift" variant.

We've read probably dozens of refutations of this in both external sources, and here. It is based on the persistent erroneous feeling that the keel "feels" the current and is "pushed" upwind by it, if you can just get the current on the lee side.

This effect does not exist, and Dave's drawing actually demonstrates this -- there is no magic and no watershed about the current going from fine on the weather bow to fine on the lee bow -- it's just x degrees more or less like any other x degrees, which can be seen in the math, which will belie any huge difference in wind from a few degrees of heading change taking the current to just the other side of the bow. The wind will change smoothly, one degree at a time.

By the time the current is fine on the bow when we are close hauled, the other, real flavor of Lee Bow Effect will not be working (it needs perpendicular-ish current to ground wind), and to the extent it may be working just a little bit, in fact as we know from SWL's work, it is likely to need the current on the weather bow.

So, sorry Dave -- it don't work at all the way you have in mind