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

[Freedom45]

Originally Posted by Dockhead

Crossing the Channel at springs in a slow enough boat, it is actually possible to never arrive We proved it mathematically in the other thread

Run out the sweeps!

[Dockhead]

Originally Posted by Hoofsmit

This is my point, they are ground referenced

If you use the tablecloth as suggested by some we are mixing it up, we then start to ignore that our hull is in the water at an angle to the current, we have lift because of that angle, I just am not intellectual enough to prove it

This is what makes dock heads post I referred to so wrong

If you had wheels on the seabed, then you would get a lift from having the current hitting your keel at an angle.

However, you do not. So the current on your keel is not like water flowing from a tap onto your hand (your hand is attached to the ground through you).

Your keel doesn't feel the current. Your boat sails in water, not over ground. It is just carried along in the current and does not feel it in any way other than through changes in the wind speed/direction created by the current.

If you really doubt this, then try this simple experiment: When the tide is really ripping sometimes at springs, take the sails down and let all way come off. When you have zero boat speed, trying turning the wheel. Does it do anything? Of course not -- because the current doesn't push against your boat, neither your rudder nor your keel. That's because your boat is not attached to the seabed in any way -- there is nothing to push against.

Your boat sails in the interface between water and air. Your boat has no idea what the seabed below it is doing. It feels the seabed no more than it feels the earth rotating on its axis, or rotating around the sun.

That's why my post was so right. You have to deal with the current separately, with your CTS calculation, before you start thinking about sailing.

[foolishsailor]

Originally Posted by Dockhead

Your boat sails in the interface between water and air. Your boat has no idea what the seabed below it is doing. It feels the seabed no more than it feels the earth rotating on its axis, or rotating around the sun.

+1000

When you say things like this I think I may have a "man-crush" on you.

edit: and, maybe I like this post so much as it helps to illustrates my position that Ground Referenced Wind has no relevance to sailors...

[Dockhead]

Originally Posted by foolishsailor

+1000

When you say things like this I think I may have a "man-crush" on you.

Oh sh$t. Now I'm going to have to beat the carp out of you so no one thinks we're gay!

[europaflyer]

Originally Posted by Paul Elliott

But in your diagram, in both cases the TWS(water) velocity changes with current, and in both cases the boat speed will therefore change, thus changing the Apparernt Wind. For that matter, when the wind speed changes, so does our boat's optimum angle change (per the polars), also resulting in a different Apparent Wind speed /angle.

You are right in that I have previously only been considering situations where the boat speed remains constant. Having a dynamic boat speed adds an extra layer of complexity to an already highly complex problem.

It is worth noting that these angular differences, while we can have fun chewing the fat over them, will in reality be quite small in most circumstances. The windspeed I used on my diagram is roughly equal to the current speed and boatspeed, which is a pretty unusual situation! In the real world, we would be splitting hairs.

If we can agree on three points then I think I will be happy...

1) That the action of current affects the true wind, and in so doing can potentially affect the favoured tack
2) That this favoured tack will change at slack water, requiring the boat to tack to remain on the favoured tack.
3) That calling this the 'lee bow effect' is damn confusing, but potentially useful in the real world

[foolishsailor]

Nah...

We'll just have to finish those bottles of Mt. Gay and ginger Beer and walk Shooters Hill arm in arm...

Wait...did I say Gay and Ginger in one sentence?

oh ****!!!

[foolishsailor]

Originally Posted by europaflyer

1) That the action of current affects the true wind, and in so doing can potentially affect the favoured tack
2) That this favoured tack will change at slack water, requiring the boat to tack to remain on the favoured tack.
3) That calling this the 'lee bow effect' is damn confusing, but potentially useful in the real world

My wife calls me stubborn so apologies in advance...

1) That the action of current affects the true wind, and in so doing can potentially affect the favoured tack

In a situation where there is current, the current itself dictates your decisions not the apparent wind it generates.

2) That this favored tack will change at slack water, requiring the boat to tack to remain on the favored tack.

You are now talking including ΔT (change in Time) into the equation and that complexity removes the simplicity and deserves more discussion

3) That calling this the 'lee bow effect' is damn confusing, but potentially useful in the real world

No, it isn't useful as it doesn't exist in any of the forms discussed. At best some of the misconceptions of its various discussed forms are covered by other discussed principals.

This is why I would like you to take an example you think demonstrates this and draw it out from origin to destination using the images I provided.

[Paul Elliott]

Originally Posted by europaflyer

[...]
If we can agree on three points then I think I will be happy...

1) That the action of current affects the true wind, and in so doing can potentially affect the favoured tack
2) That this favoured tack will change at slack water, requiring the boat to tack to remain on the favoured tack.
3) That calling this the 'lee bow effect' is damn confusing, but potentially useful in the real world

Well, if you're Happy I can't be Happy too, so I guess I'll have to be Grumpy or Sneezy.

But I do completely agree with your points 1 and 3. And I appreciate your use of True Wind (water-referenced as God intended!)

Point 2 only holds in some cases. It depends on the wind, the current, and where you're trying to go.

[edit: Point 3 -- Useful? Perhaps once we agree on the definition.]

[europaflyer]

Originally Posted by foolishsailor

My wife calls me stubborn so apologies in advance...

1) That the action of current affects the true wind, and in so doing can potentially affect the favoured tack

In a situation where there is current, the current itself dictates your decisions not the apparent wind it generates.

2) That this favored tack will change at slack water, requiring the boat to tack to remain on the favored tack.

You are now talking including ΔT (change in Time) into the equation and that complexity removes the simplicity and deserves more discussion

3) That calling this the 'lee bow effect' is damn confusing, but potentially useful in the real world

No, it isn't useful as it doesn't exist in any of the forms discussed. At best some of the misconceptions of its various discussed forms are covered by other discussed principals.

This is why I would like you to take an example you think demonstrates this and draw it out from origin to destination using the images I provided.

How do apparent wind shifts not affect you decisions?!

This whole thing related to what you are pleased to call delta-t, as in a case where the current is constant there would be no changes in true wind, which is what this is all about.

The image you provide has a current which is different at different points in the stream, which makes for a very, very complex problem indeed as the time spent in the different current areas now comes in to play. It is now, by my reckoning, a seven variable problem, and a vector analysis of this type is totally beyond the scope of understanding what is really quite a simple effect.

What is really all that is important in the real world, as I think Seaworthy first pointed out, is just staying on the favoured tack. This is all this whole thing boils down to. Currents changing direction every six hours is one of the things which can potentially alter the favoured tack, so having to tack over at slack water to stay on favoured tack is not an uncommon problem. That's all we're talking about here.

Originally Posted by Paul Elliott

Well, if you're Happy I can't be Happy too, so I guess I'll have to be Grumpy or Sneezy.

But I do completely agree with your points 1 and 3. And I appreciate your use of True Wind (water-referenced as God intended!)

Point 2 only holds in some cases. It depends on the wind, the current, and where you're trying to go.

[edit: Point 3 -- Useful? Perhaps once we agree on the definition.]

You're right, I didn't make my meaning clear in point 2 and it isn't entirely correct. I mean that given that the current has affected the favoured tack (point 1) then a current reversal will alter the favoured tack (point 2). My fault.

Point 3 is because the most important case, where the effect is greatest, is the classic case of crossing a channel to a destination directly across it, directly upwind (or thereabouts). In this case, 'lee bow effect' is perfectly acceptable terminology, and in cases where it is possible to get a lift from a current not on the lee bow the effect is tiny, and unimportant compared to the many other factors affecting strategic decisions.

[goboatingnow]

Dave, the diagram shows a lift or knock resulting from the current direction, as it shows the angle of the true wind altering when the current veers around the ground wind ie veers the wind induced by current around the ground wind (true wind is just the sum of ground wind and wind due to current).

If the angle of the true wind alters while its amount stays the same (we are discussing constant amounts of ground speed and current here), then you have a lift or knock.

As you have true wind and your sails are up and trimmed close hauled, your boat is moving, so of course there is a boat speed vector, resulting in an apparent wind vector.

As the amounts of true wind and current and boat speed are identical on both tacks then the amount of apparent wind is identical, just its angle veers around showing you that you are lifted or knocke

Yes but know of this deals with the lee bow effect. I can see the advantage to placing a current fine on the lee bow, I can clearly see the advantage of having the current broad on the bow, as I will be lifted to the mark, ( that is assuming I want to be lifted)

and yes of course if you tack ( and have equal tacks) you cancel the effect out. But few races have equal tacks. furthermore lew bowing I thought was always a racing term, i.e. a race is a ground referenced course. i.e. movement by the current is cancelled, but the apparent wind freeing is not cancelled.

What I do understand is that a current fine on the lee bow, would produce in effect a easing of the wind angle, which shows the effect of pinching to put the current over to the lee side then produces the freeing of apparent wind. In most cases the effect would be very small

Hence I can rationalise the lee bow effect , which is an apparent wind shift , as opposed to the boat being "lifted" to the mark by the current.

I don't think the effect is mythical.

dave

[goboatingnow]

To illustrate what I understand as the lee bow effect. take this extreme case, You have a 10 knot ground wind say at 50 degrees of the windward bow, , with no current,

The apparent wind, will be forward of that wind,

Now add in a 10 know current fine on the lee bow, that in effect causes a secondary ground wind , ( opposite direction at current speed) , that current related ground wind, adds to the existing ground wind , so as to move the ground wind aft, this results in a freeing of the wind angle to sail in.

Hence the effect of pinching so as to place the current on the lee side, would actually result in a freeing of the wind, removing the pinching and allowing the boat to point higher, then the normal ground wind would suggest.

the opposite is a windward bow effect , would cause the ground wind generated by current to cause a number of effects , depending on the angle. but it wouldn't necessary cause a header.

none of this has anything to do with CTS, and Im not sure how it even entered the situated, nor is it anything to do with if or not if the boat is lifted by the current towards mark, thats nothing to do with lee bow effect


The key is to ignore TRUE wind as mariners describe it, its a hypothetical calculation , you never experience it or sail in it. true wind merely became water referenced wind, because mariners for centuries had no way to determine ground speed, hence water speed was used. But the fact is what most mariners actually do with water referenced wind, is to use it as if it was ground wind.

The most common howler I see its people saying " you sail in the True, i.e. water referenced wind) , Thats of course nonsense, because you sail in apparent wind, thats the only real wind. You never actually can experience TRUE wind as mariners define it, as the water is always moving and that movement causes an apparent wind , however small. if you stop that movement, i.e. by anchoring you are experiencing ground wind( which the rest of the non marine world calls TRUE wind)

( this leaves aside the polar boys etc, who found a use for a STW)

dave

[Hoofsmit]

Originally Posted by Paul Elliott

Well, if you're Happy I can't be Happy too, so I guess I'll have to be Grumpy or Sneezy.

But I do completely agree with your points 1 and 3. And I appreciate your use of True Wind (water-referenced as God intended!)

Point 2 only holds in some cases. It depends on the wind, the current, and where you're trying to go.

[edit: Point 3 -- Useful? Perhaps once we agree on the definition.]

good luck I am fed up trying


I have figured we are going no where now because some people will not look outside the box and question there own theories


the Cts thread is a classic of this .... there are many ways of getting from A to B
but the big picture is hitting a variable tolerance which only him above knows. ( the last 1/12 of a passage should never be when you start looking if you are on track to hit your destination , If you are that dogmatic to stick to your previous calculations I would not be your crew)

I have learnt in my job ( of thirty years) and to some extent in my sailing, that the academics stand by there own theories to protect there egos
often to the detriment of others or further learning

The facts of this thread prove that I should stay open minded and except that the things I learn through experience serve me better than following an academics point of view but if I can learn something I will maybe charge my ways but only when I have done it enough times to then call it experience

Some have no respect for tradition and there aim in life is to disprove sound principles, I am not one of these as long as it works within my tolerences

[Hoofsmit]

just for those that doubt and enjoy there maths
2.6 Fluid-dynamic effects of keel shapeThe first step to make when designing something is to understand what are its main purposes. For a keel, it is what we have done many times in this work, particularly in section 2.5. The second step is to examine how the changes in the characteristics of the object affect its behavior. This is the aim of this section. Basically we will examine the effects of the parameters of section 2.1.2 on the fluid-dynamic behavior of the keel.

2D effects. First of all we want to consider the 2D effects, i.e. the effects due to the shape of the sections of the keel.


Figure 2.21: Section parameters
Here is a list of the changeable parameters: for each of them there are specified the effects of an increase of the parameter itself. Everything is referred to figure 2.21.
Thickness/chord ratio (t/c)The stall angle, the pressure drag and the volume for the ballast increase.Maximum thickness position (m)The profile drag without angle of attack and the L/D ratio decrease, the ballast is nearer to the stern.Radius of leading edge (r)The L/D ratio, the profile drag without angle of attack and the stall angle increase.All the effects mentioned in the list are more or less related to viscosity. The code we are going to use is inviscid, so no one of these will be shown by our calculations.

The profiles we are going to use are of the NACA 00XY series. They are commonly used either for keels or for rudders, and a huge quantity of experimental data on them is available from aeronautical studies. This series includes symmetric profiles, with the position of the maximum thickness set at 30% of the chord; they are defined by analytical functions. The `XY' digits represent the percentage thickness/ chord ratio of the profile, thus the NACA 0012 profile has t/c=12%. In section A.1 it is shown how these profiles have been inserted in the geometry of the keel.

3D effects. We have already seen in section 2.2.3 how the 3D parameters, especially the aspect ratio, act in changing the value of the induced drag, which is also strictly related to the lift (equation 2.21). That is why now we want to show their influence on the lift curve slope C¢L of equation 2.29.

The aspect ratio is the most important parameter in this case too. Knowing the lift curve slope c¢L¥ of the 2D profile, that is an infinite aspect ratio wing, it is possible to estimate C¢L with:
C¢L=dCL
da
= c¢L¥
1+c¢L¥
pAR
.(2.29)The 2D theory gives c¢L¥=2 p, and the experimental data confirm this [1]. Equation 2.29 is valid for big aspect ratios. When the aspect ratio is very small the lift-curve is not linear but concave. On the other hand with small angles of attack the curve can still be considered linear. This is exactly the case of the keel we consider, with small aspect ratio at small angles of attack: equation 2.29 is not valid, but the important fact is, anyway, that the smaller is the AR, the smaller is the lift curve slope. Figure 2.22 shows the experimental data on aerofoils of the same section (Göttingen 389) but of different AR.


Figure 2.22: Experimental lift curves for different aspect ratios


for those that only look in two dimensions read this whole article then come back here and talk some more re the lee bow effect .... it not just about wind or cts or vmg on water ... our hulls are in the water and that water is not a table cloth if varies in viscosity direct and rate

[Seaworthy Lass]

Originally Posted by goboatingnow

Yes but know of this deals with the lee bow effect. I can see the advantage to placing a current fine on the lee bow, I can clearly see the advantage of having the current broad on the bow, as I will be lifted to the mark, ( that is assuming I want to be lifted)

Dave, there is no such thing specifically as the "lee bow affect", at least no more so than there is a "windward bow effect". It is truly erroneous to in add the word "lee" when it can be either side.

The tack with the current broader on the bow of the two will be KNOCKED more not lifted more. Check out my cone diagram below and just pick any current direction and look at the true wind (the red line that joins up with the end of the current vector) and see which tack is lifted more of the two.

Originally Posted by goboatingnow

and yes of course if you tack ( and have equal tacks) you cancel the effect out. But few races have equal tacks. furthermore lew bowing I thought was always a racing term, i.e. a race is a ground referenced course. i.e. movement by the current is cancelled, but the apparent wind freeing is not cancelled.

What I do understand is that a current fine on the lee bow, would produce in effect a easing of the wind angle, which shows the effect of pinching to put the current over to the lee side then produces the freeing of apparent wind. In most cases the effect would be very small

Hence I can rationalise the lee bow effect , which is an apparent wind shift , as opposed to the boat being "lifted" to the mark by the current.

I don't think the effect is mythical.

dave

This effect is entirely a myth. This thread has clearly shown that.
Truly Dave. It does not exist. Please don't make the same mistake I made early in this thread. There is no specific advantage to having the current on the lee side of the boat.

[Hoofsmit]

here we go again...... I will pop back in when you lot stop discusing wind

just a quick one for the table clothers

if the table cloth is split into 50 non parallel sections and each section is moves at a different rate , how does your cts allow for this real illustration of a area like the Fowey in cornwall uk