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

[Dockhead]

Originally Posted by Seaworthy Lass

Ah, such a simple thing I haven't stressed then .
My theory is: There is tack that is advantageous when tacking in a situation with variable cross current. Following the tack that gives you a heading closest to the calculated CTS (and tacking on this criteria) will give you the shortest time taken for the journey. This tack is also with the current on the lee bow I think, but I have not had the time to think about if this applies universally (it certainly does for all situations I can think of, just try it out).

I thought you disagreed with this (or maybe we have progressed and you have an open mind regarding this LOL ).

My dear, that's not a theory!! That's just a hunch about the result! You need to state how and why it works, in order to say you have a theory!

You give us just a hint of where you might be going with this, and only a hint, by saying: "Following the tack that gives you a heading closest to the calculated CTS will give you the shortest time for the journey." But I can't imagine how this is going to work -- you yourself said, correcting me, something like, "true wind is always and everywhere the same at a given time in the tide cycle, so you're not going to get any lift from wind speed resulting from when you choose to tack." A simple and elegant disproof of one of the things Lee Bowers say, and what I had, idiotically, taken as read.

Why does not the exact same argument apply to wind angle? Wherever you end up being due to the way you have tacked, you've still got the same point in the water to get towards, by tacking towards, on the same average constant heading. Why in the world would it make any difference when you choose to tack? The wind angle changes, but you feel it the same everywhere, you haven't gotten any advantage by tacking so as to be uptide or downtide. The average of all the wind angle changes is still going to be the same, no matter where you are.

We've agreed on this in the case of non-changing current; why doesn't the same principle apply for a changing current? Over to you!


* My mind is open. I have an inkling of a couple ways how it might be possible. I doubt it, but it's conceivable. But I'm not going to take your fun away from you You were the one who decided to play the white pieces, i.e. the Lee Bowers side.

[Paul Elliott]

Originally Posted by Seaworthy Lass

The polars are extremely important for showing that pinching does not help. Aside from that they are irrelevant, as maximum performance occurs at their peak and that is what we should aim for. Sails perfectly trimmed close hauled so that you are at the maximum point of the polar is the only configuration Dockhead and I are discussing.



The reason we need to consider changing current (with a component of the current perpendicular to the ground wind) is because it is the only one where it makes a difference what tack you are on. There is otherwise absolutely no advantage to one tack over the other.

OK, so we're not worrying about the fact that the optimum AWA varies with windspeed(?) I suppose in most cases this will not change the favored tack, but I can imagine some strange situations where it could. Probably not worth adding to the confusion though.

Changing direction of the component of current perpendicular to ground wind alters true wind direction and this is why there is a tack that is advantageous.

True. I guess the issue I have with the lee-bow effect, is that there is no magic "effect". The physical realities are simply Ground Wind, and current vector. If we analyze this using water-referenced True Wind and our boat's polars, then the whole thing is simply a Course To Steer exercise. There is no magic lee-bow effect.

[Dockhead]

I was joking about "might be a racer", of course. I did do a season of dinghy sailing in my misspent youth, on 420's, but didn't do any tactics, so I know zero about them.

Impatiently waiting for Seaworthy's exposition of her theory, I have been digging around in the 'net. I think we've agreed that we've reduce the Lee Bow problem to a question of how to deal with shifts in the true wind.

There is a tactic called "sailing the lifted tack", which I think might offer a clue.

If we can figure out how that works, maybe it will advance our understanding.

As far as I understand, in absolutely non-moving water, so we can ignore these things, you can gain an advantage sailing on the tack which is lifted by a wind shift -- thus allowing you to sail closer to the course line towards your mark than you could before, saving you miles.

Does that work? If you were to stay on the other tack, you would be sailing further from the course line than before the wind shift, is that right? That seems reasonable to me.

It seems that would work because your tacking strategy is going to be different, depending on where your mark is, in relation to the wind. If the mark is someplace not dead upwind, but someplace you could almost sail to on one tack, then you will favor one tack -- you will sail most of the way on one tack, and just a short distance on the other.

Maybe this is a clue?

[Paul Elliott]

Originally Posted by Dockhead

[...] Why in the world would it make any difference when you choose to tack? The wind angle changes, but you feel it the same everywhere, you haven't gotten any advantage by tacking so as to be uptide or downtide. The average of all the wind angle changes is still going to be the same, no matter where you are.[...]

Of course your tacking decisions do matter, especially as you begin to close on your mark. Tacking slows you down, and you would rather not have to throw in a final short-tack if it could be avoided. If you were watching the America's Cup final races, this becomes painfully obvious. The boat that was making the best speed and further upwind may have been the furthest from the mark, ultimately needing an extra tack and costing them the race.

But I think this is not the issue we are discussing.

[Hoofsmit]

Do you agree with these definitions of lee bow and wind direction , we seem to be getting muddled with direction and wind speed. Ground wind.....? Sorry don' t subscribe to such a thing . Tidal wind in theory yes , but when I'm on my mooring with a 5 knot spring tide wiz zing by I do not register 5 knots of wind, if I am motoring into a 5 knot tide but in effect staying still over ground I do, so what's going on?

[Hoofsmit]

Just to
add the term" into the tide" Refers to the direction or tidal stream not rise or fall of the tide...... At least in Cornwall

[Seaworthy Lass]

Originally Posted by Hoofsmit

Do you agree with these definitions of lee bow and wind direction , we seem to be getting muddled with direction and wind speed. Ground wind.....? Sorry don' t subscribe to such a thing . Tidal wind in theory yes , but when I'm on my mooring with a 5 knot spring tide wiz zing by I do not register 5 knots of wind, if I am motoring into a 5 knot tide but in effect staying still over ground I do, so what's going on?

You don't subscribe to ground wind? I will try and sign you on LOL.

If you are sitting at a mooring and there is no ground wind, only current, of course you will feel no wind. You are essentially fixed to the ground (I say essentially, as there is still a bit of movement possible due to current changes). If ground wind is zero and you are not moving relative to the ground, you will feel no wind. If you feel a wind while sitting on a mooring with steady current, that is ground wind.

If you are motoring to exactly counteract the current, then you would not feel the wind either if there was no ground wind. If you are feeling a wind in these circumstances (and it may be felt from any direction), that is due to the ground wind also .

If you let go of the mooring and just drifted and the ground wind was zero, you would feel a wind coming from the opposite direction to the current. If you felt no wind then the ground wind would be cancelling it out.

Is that enough for you to consider subscribing to ground wind?

----------------

I don't agree with the definitions given. This is my take:

Ground wind is wind referenced to land (dry or wet, ie on shore or seabed).

True wind is referenced to water. It us the sum of ground wind and the wind induced by current (and leeway).

Apparent wind is the wind you feel. On the water it is the sum of true wind and the effect of your speed. On land it is the sum of ground wind and the effect of your speed. (When you are in a car and stationary and stick your hand out of the window it is ground wind you feel; when the car is moving it is the effect of the car's speed plus ground wind that you feel on your hand when it is out of the window).

Induced wind? I had always thought that this was the wind induced by eating too many beans .

[osprey877]

Conveyor belt analogy works for jogging or car racing, but not for sailing.
If the current and wind direction are not in exactly the same direction then when you add the current vector to the true wind vetctor and boat speed, the conveyor belt will increase the apparent wind speed more, or decrease it less, on one tack compared to the other. That is the lee bow effect

[Paul Elliott]

Originally Posted by osprey877

Conveyor belt analogy works for jogging or car racing, but not for sailing.
If the current and wind direction are not in exactly the same direction then when you add the current vector to the true wind vector and boat speed, the conveyor belt will increase the apparent wind speed more, or decrease it less, on one tack compared to the other. That is the lee bow effect

Ahh, but it doesn't. You are sailing in a wind that is the vector sum of the ground wind, and the current-derived wind. This is what we are calling the True Wind. The True Wind (and thus the Apparent Wind you measure on the deck of your boat) will be the same on either tack.

The current affects your course over ground.

[Seaworthy Lass]

Originally Posted by osprey877

Conveyor belt analogy works for jogging or car racing, but not for sailing.
If the current and wind direction are not in exactly the same direction then when you add the current vector to the true wind vetctor and boat speed, the conveyor belt will increase the apparent wind speed more, or decrease it less, on one tack compared to the other. That is the lee bow effect

The conveyor belt analogy doesn't work, as a conveyor belt only works back and forth. Current can move in any direction. Sliding a tablecloth around on a table or a rug around on the floor is a much better analogy to current over a seabed.

So you are saying the lee bow effect is just an error which is induced when considering the current a conveyor belt rather than a tablecloth LOL?

If not, could you please draw a diagram to explain what you mean?

Edited to add: I have just read Paul's post and agree with what he has said.

[Seaworthy Lass]

Originally Posted by Dockhead

My dear, that's not a theory!! That's just a hunch about the result! You need to state how and why it works, in order to say you have a theory!

It is not a hunch that there is an advantageous tack. It is because you know a certain shift in the true wind will occur further along, enabling you to point differently when it occurs. This governs your selection of the initial tack.

The key is understanding first that a predictable change in true wind direction occurs as the current changes.

If that doesn't make sense or is insufficient then I will put my thinking cap on to come up with a better, clearer explanation.

Originally Posted by Dockhead

Why does not the exact same argument apply to wind angle? Wherever you end up being due to the way you have tacked, you've still got the same point in the water to get towards, by tacking towards, on the same average constant heading. Why in the world would it make any difference when you choose to tack?

It makes no difference if the true wind is not altering.
It makes a huge difference if the true wind is altering. Get it wrong and you will be way back at the end of the fleet. Just ask any racer . I do not think we are debating that when a known change will occur for the true wind due to a change in ground wind, that there is an advantageous tack to be on. I feel a bit like I am reinventing the wheel having to come up with mathematical proof for this.

The true wind is the sum of ground wind and the effect of current. It makes no difference if the change in true wind is due to a change in the ground wind or the change in current. Therefore, if there is a predictable change in current coming, there is a known advantageous tack to be on .

Originally Posted by Dockhead

The wind angle changes, but you feel it the same everywhere, you haven't gotten any advantage by tacking so as to be uptide or downtide. The average of all the wind angle changes is still going to be the same, no matter where you are.
We've agreed on this in the case of non-changing current; why doesn't the same principle apply for a changing current? Over to you!

Simply because changing current will cause a change in the true wind . With no change in true wind, there is no advantageous tack. That is the basis of all this.

Originally Posted by Dockhead

You were the one who decided to play the white pieces, i.e. the Lee Bowers side.

I would not call myself an exclusive "lee bower" LOL, although in a debate I know you need to pick sides . It applies to a very limited set of conditions (in other situations I am a "windward bower" or "no bower"). The situation where I am possibly a lee bower is very specific - I think it is when variable cross current exactly cancelling out, but this is more of a hunch at the moment. I just need a bit of quiet time when I don't have a million and one distractions to sit with pen and paper to show why the lee bow effect applies in this specific set of conditions.

The fact that conditions are very specific really means we should ditch this term, as some people consider it a universal truth when sailing with current. This is certainly not the case.

[Hoofsmit]

Seaworthy lass:
Thanks for your explanation , I think the RYA book of navigation obviously uses different terminology to this thread, I'm still getting my head round it!
What I still don't get this we still have not definitely confirmed what everybody thinks lee bowing is. My first photo from one of my books explains purely the tidal affect others explain it as a wind on you sails advantage whether by pointing angle or wind velocity. I know the op tried to cover this in his first post, but I think it would be great to know which ever the posts cover.
Waiting with excitement for some real calcs and diagrams now ; )

[Hoofsmit]

Just to clarify what I understand as lee bow effect.

Looking at my first photo, to me this scenario is when the term is used it is simply a rule of thumb ( in my dinghy racing years with a mental CTS to the next mark)

In this scenario , the wave action of wind 'against' tide ( stream) will be on my leeward bow thus pushing me into wind thus less rudder thus more speed , on top of this my foils are now 'catching ' tide thus pushing me nearer my rumb line.

I wait for more correction

[Hoofsmit]

In the above scenario , you can now use some maths or trig and your boats polars to work out if pinching is more productive with a particular tide to wind ratio that turning on a loosing tack

[Dockhead]

OK, here's one possible formulation of the Lee Bow Effect idea. It's probably incomplete and oversimplified, but here goes.

Whenever we are trying to lay a goal which is dead upwind, and assuming our boat as the same optimum VMG to windward angle on both tacks (and if not, you'd better find a rigger ), then both tacks will produce the same VMG to windward. If your tacking angle is 90 degrees including leeway (I wish!), then your VMG to windward is about 70.7% of boat speed, calculated geometrically.

However, if the wind shifts so that your goal is no longer dead upwind, both tacks are not equal in terms of possible VMG to your goal. You'll have a higher VMG to your goal on that tack, the courseline of which is closer to the vector to your goal. You still have to make the "unfavored" tack, but the length of this one will be much less.

For example, if you are sailing to a goal which bears 000, and the True Wind is blowing from 022, assuming the water is not moving, you can lay the goal in one long starboard tack on a heading (corrected for leeway) of 337, and one short tack on a heading of 067, or any combination of long and short tacks in the same aggregate proportion. Your average speed to your goal will not be equal to the VMG to your goal during your starboard tack, because you will knock down the average a bit during the unfavored tack, but your average speed will be higher than you could have achieved sailing to a goal dead upwind.

This is pretty basic stuff, I guess, and real racers will probably be laughing at us cruisers trying to figure this stuff out.

OK, so superficially, at least, it would seem that you ought to be able to harvest this effect in a changing current situation by sailing the favored tack on each tide. That would look something like this:



In fact, if your lee bow is in the current, you are on the favored tack.

Could that be all there is to it?

This doesn't consider the fact that we are being swept back and forth by the tide, and that our mark is spot in the water since we're sailing a constant bearing according to our calculated CTS.

On the other hand, the drawing is still valid, I think.