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

[Paul Elliott]

Re: leeway and resistance to being just blown downwind.

Originally Posted by Hoofsmit

so how do you calculate for that resistance? as in the length of your vector?

I'm not sure how you calculate the resistance, as it is a combination of drag and lift. Leeway is the important factor though, and you can roughly measure that by seeing how far your your wake angle is off your centerline. There are also some fancy 2-axis water-speedometers that will directly measure this angle. There are some rules of thumb used to calculate leeway, with the coefficients different for each hull design, and these estimate leeway fairly accurately using measured heel angle and boatspeed.

Or just call it five degrees, and most of us won't be too far off. Obviously leeway will depend on sail trim, which shows up in the heel angle.

[Paul Elliott]

Originally Posted by goboatingnow

That may be your interpretation. But CTS has nothing to do with tacking [...]

It's not my interpretation, it's an IN THIS DISCUSSION DEFINED term for a method used to calculate an end-to-end course. As I've said, I didn't define it, and I wish it had been called something else to avoid this kind of confusion.

Why don't we call it "The CTS Method Course", so we can restore the term "CTS" to it's original meaning of "the course the helmsman should be steering"? OK, that's a lousy suggestion, but I hope my point is understood.

[Paul Elliott]

One more bit of CTS Method history: The original derivation assumed motorboats running at a fixed speed through the water. All this sailboat "tacking" stuff was left as an exercise for the student, being intuitively obvious and all that.

[goboatingnow]

Originally Posted by Paul Elliott

It's not my interpretation, it's an IN THIS DISCUSSION DEFINED term for a method used to calculate an end-to-end course. As I've said, I didn't define it, and I wish it had been called something else to avoid this kind of confusion.

Why don't we call it "The CTS Method Course", so we can restore the term "CTS" to it's original meaning of "the course the helmsman should be steering"? OK, that's a lousy suggestion, but I hope my point is understood.

Yes that's fine , but none of this is relevant to the diagram.

Dave

[Paul Elliott]

Originally Posted by goboatingnow

Yes that's fine , but none of this is relevant to the diagram.

Dave

Then why did you make such a big deal about it?

Love and kisses,

[Seaworthy Lass]

Originally Posted by goboatingnow

That may be your interpretation. But CTS has nothing to do with tacking , CTS is the commanded course to steer , the navigator in computing CTS, will take into account ability for the vessel to be able to proceed in any given direction and will modify his CTS calculations accordingly

In a multi hour current cts scenario , it's makes no sense to tack around your CTS , as the wind direction may result in undoing all the benefits of say a multi hour CTS. If one computes a CTS that one cannot achieve or one finds that a previously CTS is now un achievable , then one recomputes a new CTS using the appropriate data at hand.

But CTS has nothing to do with that diagram it's just wrong IMHO
Dave

Dave, if you know you have to tack there is no 'new' CTS to calculate. You are simply close hauled. Your exact heading will then vary according to your boat's characteristics and the strength of the wind, not some calculation you make on a new CTS. If conditions are constant you simply tack up the CTS, doing your very best .

[Seaworthy Lass]

Originally Posted by goboatingnow

I don't understand that diagram , since there is no apparent wind vector how can a windward tide cause You to be lifted. Doesn't make sense the term " lifted " or headed refers to effect of changes in cog due to changes in ground wind relative the a ground referenced destination , ie a racing mark.

Dave

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 knocked.

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

Hi FS
Well, I would fail miserably as a tactician LOL. I am at the stage where my head is spinning looking at this problem, but I think I would do the exact opposite to what you suggest and I know you are an experienced racer. Doubt a sticky is ever going to be on that trophy if you follow my strategies given we are poles apart (I am at least getting the hang of the lingo, so there is some hope).
.......
How much faster is it actually doing what you suggest? I am still puzzling how your technique works.

Edited to add: scrap all this, it is rubbish.

I woke up thinking about your example. The reason why my proposed strategy is wrong (and it must be wrong, I know that you are an experienced sailor in these waters) is that I have focussed on where the conditions are advantageous regarding a lift or knock in the true wind.

The other way of looking at it is where are the conditions advantageous for a current you are trying to avoid (it is heading you the whole way). That is on the opposite side of the course.

But how do we know which of the two factors is greater without knowing the boat speed? It think there are some boats that my strategy would work for and some that yours would. You would need to know boat speed and make your calculation and decision based on that.

[Seaworthy Lass]

Originally Posted by europaflyer

Hi Angela,
I will try to use your nomenclature as used on the diagram in this post. Note that what you call true wind I have previously called apparent or 'water referenced true' wind, which is a potential source of confusion.

Yes, I did pick that up and I am OK with it. I know your terminology was initially different to mine.

Originally Posted by europaflyer

Your diagram shows both winds - ground wind (what I call true wind) as the thick black central line, and the true wind (what I call apparent wind) as the red lines.

You have changed your definition of apparent wind now?

Apparent wind has the wind due to boat speed (and leeway) vectors added onto true wind. I have never heard the true wind being referred to as the apparent wind by anyone before (they may both be the same when your boat is not moving through the water, but that that does not change things).

Originally Posted by europaflyer

It clearly shows the effect that the current has on altering the true wind from the ground wind. This is the effect I am talking about - the effect on the ground wind by the current to create a new true wind. It is a great little diagram, and I hope you don't mind my copying it below to save people the trouble.

I don't mind. I think it is neat too. I will draw more come diagrams later when I have time just to show how the appearance of the cone changes as the relative strength of ground wind and current alter). It is cool stuff .

Originally Posted by europaflyer

It shows when the current lines up with the apparent wind (which in this case, with no boat speed, is equal to the true wind) there is nil effect on the direction of the wind (360 and 180 deg. apparent).

That is irrelevant. Your apparent wind will alter according to the boat speed and that depends on the boat's characteristics and how well you have trimmed your sails. This will NOT alter how the effect is centred.

As you yourself have stated before the 'effect' has nothing to do with the boat (and apparent wind has lots to do with the boat), so why are you wanting to look at apparent wind?
And why on earth do you think it would be centred on apparent wind (a boat dependent parameter)?

It is bad enough me referencing it to the bow of the boat, but that is a better choice if you need a boat referenced rule. I am not saying the effect is maximum when the bow is exactly into the current (I have stressed this is not necessarily the case), but when your bow is trying to point into the ground wind as it does when you are on a tack then the lift occurs in the sector where you are pointing more towards the current than on the other tack. That is just plain vector addition.

We are not discussing where the effect is maximum ie exactly on the bow or a bit to windward or a bit to leeward - it can be any of these depending on the amounts of ground wind relative to current and how your tacking angle fits into this scheme. It is actually irrelevant as you are just trying to sail close hauled, you will not drop down from being close hauled simply because you think the effect is a bit greater there (and it certainly wouldn't work pinching LOL, your speed would drop off too much).

The affect is definitely centred on the ground wind though. If you actually went and drew it out based on apparent wind then you would see the effect is definitely not centred on the apparent wind. Try it out by drawing diagrams. Make sure you keep the amounts of ground wind and current steady as I have in my diagram.

Originally Posted by europaflyer

The two red lines which are most different angle wise from the ground wind are, you can see, at exactly 90 degrees and 270 degrees to the current.

No, no, no .

Have a careful look at my diagram again. The maximum shift in the angle of the true wind does NOT occur at 90 and 270. It occurs at a tangent to the circle (this is why I said before that the maximum position was very complicated to compute and depended on the relative amounts of ground wind and current). It would only be close to 90 and 270 if the ground speed was infinitely larger than the current.

The maximum effect of the current on true wind angle is shifted closer to 0 degrees on both sides. It is not on your bow, but it is closer to your bow than your stern on the other tack. It will be on one side or the other of your bow depending on your actual possible tacking angle and depending on the relative amounts of ground and true wind.

It is further away on the other tack (the knocked one), so using your bow is just a convenient reference point.

The cone diagram is just extremely useful for understanding how changing current directions affect the true wind. It should really not be termed an 'effect', but if it has to be called something it would be a 'bow effect' not a 'lee bow effect' when tacking and a 'stern effect' when gybing. It has absolutely nothing universally to do with the lee side of the boat, so including it in the term is plain ridiculous in my opinion.

This is why there is no such thing as a 'lee bow effect'. You are falling into the trap of extending the 'pinching lee bow effect' myth to cover other things. It does not.

Originally Posted by europaflyer

A great feature of the diagram which you probably didn't even realise you included, which shows conclusively that what matters here is the angle of the current relative to true wind (in zero boat speed scenario) or apparent wind (if there is boat speed) and NOT the ground wind. The point at 90/270 deg. to ground wind is at a slightly lower angle. A subtle but important distinction.

LOL. So if it is at a lower angle how does it make it the maximum?
You can't say 90 and 270 is where the maximum change to the angle if the true occurs, but then turn around and say that it is actually a bit lower than elsewhere.
That is not a definition of maximum that I have ever heard of .

The maximum change in the angle of the true wind is shift is dramatically different to 90 and 270 when the current amount gets close to the ground wind amount.

Even if the current is well off the bow in these extreme case to achieve the mamimum effect, it is still closer to your bow compared to the other tack.

In these extreme cases the true wind eventually alters enough that you no longer need to be close hauled to get to the mark and then all his changes anyway. You need to then just go for the mark and none of this is relevant .

Originally Posted by europaflyer

The diagram shows that when the current is between 0 and 180 degrees to the true wind then it creates a favourable port tack, and when it is between 180 and 360 degrees it creates a favourable starboard tack.

Sure, but that is irrelevant.
What matters is that whether the current is between 0 and 180 or 180 and 360 gives you no information about whether the current is on the lee or windward side of the boat on the most lifted tack.

Originally Posted by europaflyer

At exactly 360 and 180 degrees there is nil effect, and the ground wind is the same as the true wind. Every six hours, the current would flip 180 degrees and in so doing alter the favourable tack. This is all the 'lee bow effect' is.

It is important to note that the lifted tack is not necessarily the one closest to your CTS (the one by definition that is 'favourable'). It depends on where the mark is. It may not be exactly into wind. ie the lifted tack is not necessarily the one you want to take first (discussing only non changing conditions along the leg here).

There is no "lee bow effect". This is not how the term was coined initially. You are trying to impart some significance to the current being on the lee side of the boat. It is significant only in very specific circumstances. In others it is the windward side that is lifted more .

And the effect is a 'stern effect' when you are gybing, so a 'bow effect' only applies in limited conditions (tacking).

As Paul said, it better to just drop any reference to any 'bow effect'.
As FoolishSailor said it boils down to looking at vectors.
I agree wholeheartedly with both these statements.

[Dockhead]

Originally Posted by goboatingnow

That may be your interpretation. But CTS has nothing to do with tacking , CTS is the commanded course to steer , the navigator in computing CTS, will take into account ability for the vessel to be able to proceed in any given direction and will modify his CTS calculations accordingly

In a multi hour current cts scenario , it's makes no sense to tack around your CTS , as the wind direction may result in undoing all the benefits of say a multi hour CTS. If one computes a CTS that one cannot achieve or one finds that a previously CTS is now un achievable , then one recomputes a new CTS using the appropriate data at hand.

OK, point well taken, but how would you call the line through the water you are striving to advance along with your tacking? It's a crucial concept. It is calculated with an ordinary CTS calculation and it would be your CTS if you could lay it. Without this line, you can't tack efficiently through changing currents, I'm sure you will agree.

[Dockhead]

Originally Posted by europaflyer

An interesting post, a long essay really and exactly the sort of thing I end up writing. Unfortunately, Dockhead is trying to dispute that current alters sailing dynamics by somehow pushing the keel around. The point he refuted is not one that anyone who understands the LBE for what it is - alteration of apparent wind by current - would have made in the first place. It is (with respect) the product of his own misunderstanding.

No, no, no! Where do you see the keel being pushed around?

The point is for a moment to lose the land, which is extremely confusing, adding another layer of vectors.

The point is that once you have done your CTS calculation -- and you must do it first -- you have a water-referenced point to sail to.

Then you calculate the True Wind shifts which will occur as a result of the tide changes, and suddenly you have what is almost a normal sailing question.

That was the point of all that.

[europaflyer]

Originally Posted by Seaworthy Lass

That is irrelevant. Your apparent wind will alter according to the boat speed and that depends on the boat's characteristics and how well you have trimmed your sails. This will NOT alter how the effect is centred.

As you yourself have stated before the 'effect' has nothing to do with the boat (and apparent wind has lots to do with the boat), so why are you wanting to look at apparent wind?
And why on earth do you think it would be centred on apparent wind (a boat dependent parameter)?

The affect is definitely centred on the ground wind though. If you actually went and drew it out based on apparent wind then you would see the effect is definitely not centred on the apparent wind. Try it out by drawing diagrams. Make sure you keep the amounts of ground wind and current steady as I have in my diagram.

Here is a diagram showing four scenarios. The upper boats on both diagrams are in a no-current situation, and have exactly the same apparent wind angle. In the left hand diagram, a current is added at 180 to the ground wind. It produces a change in apparent wind angle for the lower boat. On the right hand diagram, a current is added at 180 degrees to the apparent wind, with no effect on its direction. The effect relates to the alteration of apparent wind angle by the current. What matters, then, is the angle of the current to the apparent wind.

Originally Posted by Seaworthy Lass

No, no, no .

Have a careful look at my diagram again. The maximum shift in the angle of the true wind does NOT occur at 90 and 270. It occurs at a tangent to the circle (this is why I said before that the maximum position was very complicated to compute and depended on the relative amounts of ground wind and current). It would only be close to 90 and 270 if the ground speed was infinitely larger than the current.

It does occur at a tangent (ie. 90 or 270 degrees) to the circle. We agree. The 'circle' is your current vectors. Having the true wind (equal to apparent wind as your diagram includes no boat speed) at a tangent to it proves my point about the effect being based on the angle of the current to the apparent wind.

[Hoofsmit]

Originally Posted by Dockhead

No, no, no! Where do you see the keel being pushed around?

The point is for a moment to lose the land, which is extremely confusing, adding another layer of vectors.

The point is that once you have done your CTS calculation -- and you must do it first -- you have a water-referenced point to sail to.

Then you calculate the True Wind shifts which will occur as a result of the tide changes, and suddenly you have what is almost a normal sailing question.

That was the point of all that.

This goes right back to my original posts
you are disregarding other factors that any sailor in a strong tidal area will tell you can not be looked at in this way
Your tidal vector will often be longer than your heading /speed vector thus simple affect of drawing a line with no length leads you to fall off the table before you get you knife and fork in place

[Hoofsmit]

paul

lift? resistance ? are we starting to think outside the box ?

its time we started looking under the table ( sorry my humour again)

[Hoofsmit]

Originally Posted by europaflyer

Here is a diagram showing four scenarios. The upper boats on both diagrams are in a no-current situation, and have exactly the same apparent wind angle. In the left hand diagram, a current is added at 180 to the ground wind. It produces a change in apparent wind angle for the lower boat. On the right hand diagram, a current is added at 180 degrees to the apparent wind, with no effect on its direction. The effect relates to the alteration of apparent wind angle by the current. What matters, then, is the angle of the current to the apparent wind.



It does occur at a tangent (ie. 90 or 270 degrees) to the circle. We agree. The 'circle' is your current vectors. Having the true wind (equal to apparent wind as your diagram includes no boat speed) at a tangent to it proves my point about the effect being based on the angle of the current to the apparent wind.

you need to break down you apparent wind into current /tide and leeway else its just a cts calc.

the current wind must have a resistance .... it can not be true if it does not..... this is why we have to think out of the box and get off the table cloth, the tide is often stronger than we can make head way