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

[Seaworthy Lass]

Originally Posted by Paul Elliott

If there is zero ground wind, then the True wind will be coming from the direction the current is moving. But your boat is *in* this current, so unless you are looking at landmarks, or the GPS, you can't even tell that you are in a current. All you know is that there is some wind, coming from some direction. I'm not even sure I understand what you mean by this:

Your ability to point? Relative to the wind? To the ground??? Can you clarify?

Hang on, I will draw a diagram. A picture tells a thousand words .

And you have it the wrong way around:
If there is zero ground wind, then the True wind will be coming from the opposite direction to which the current is moving.

[Dockhead]

Originally Posted by Seaworthy Lass

OK forget about ground wind. The point I was making was that true wind depends on current (amount and direction). We have agreed on that.

What about this statement:


(I don't disagree with anything you said in the post you made after the one I quoted above, but this is a separate issue)

Ground Wind becomes relevant when the current changes, not our hypothetical AFAIK.

The way you shape your tactics to respond to changes in the True Wind resulting from the change of tide are exactly the same as what you would do with any other predicted change in the True Wind.

So if the Ground Wind is blowing steady, and so you're going to have 6 hours of 18 knots and then you know you're going to have 6 hours at 26 knots (actually doesn't quite work that way, as the current varies smoothly, not "on" and "off"), you would deal with that in your tactics exactly as if you were sailing in a stationary body of water and knew from the weather forecast that the wind would increase after six hours.

[Seaworthy Lass]

Originally Posted by Dockhead

I hate to quote my own posts, but I'm adding to it here:

Another corollary of this is the following:

Pinching in a moving body of water will never give you any kind of lift.

If you think you need to be pinching, you have gotten to the problem in a backwards way and made a mistake along the way.

To win the race, you need maximum average VMG to mark over the leg.

You will get maximum VMG by sailing optimally according to your polars.

You have to set up your tactics according to your Course to Steer, however. So if the Bearing to Mark is dead upwind, you are not actually sailing dead upwind on your Course to Steer, and vice versa, and the tactics will be different. If you confuse these, you will lose to anyone who doesn't. But the optimal tactics will never involve pinching.

This is EXACTLY what my first reaction was (and for the same reasons). I actually laughed when I read about the mythical lee bow effect.

I have thought it through more carefully this morning and I changed my mind (a woman's prerogative). Now I just have the task of trying to change yours .

I am drawing diagrams to illustrate my question and I will photograph and attach in a few minutes.

[DeepFrz]

My position is this; without diagrams this discussion makes no sense what so ever.

[Dockhead]

Originally Posted by Seaworthy Lass

This is EXACTLY what my first reaction was (and for the same reasons). I actually laughed when I read about the mythical lee bow effect.

I have thought it through more carefully this morning and I changed my mind (a woman's prerogative). Now I just have the task of trying to change yours .

I am drawing diagrams to illustrate my question and I will photograph and attach in a few minutes.

Great; it will be great fun. What I have is just a hypothesis; blow it up if you can.

Just for the sake of everyone's sanity, please don't calculate tactics referenced to ground, and please don't forget your Course to Steer We know you know something about CTS!

[Dockhead]

Are we going to use pinching as the criterion? Maybe formulate the challenge like this:

Is there any case where sailing in a moving body of water will lead to pinching able to give you a lift, just because the water is moving?

I say -- NFW. If you get your course right accounting for the motion of the body of water, you sail as usual -- tacking and sailing for optimum VMG to windward for a windward CTS, for example.

Let the fun begin. What are the stakes?

[Seaworthy Lass]

Originally Posted by Dockhead

Let the fun begin. What are the stakes?

Honour LOL?

[Hoofsmit]

Ok .. I get the increase in wind on your sail ( true , ground , apparent , call it what you want !) that fact is that we are talking about A to B quicker by lee bowing , go race a clinker dingy in a tidal estuary for a few seasons, than go disprove lee bowing works. Sod the maths : )

[Paul Elliott]

Originally Posted by Seaworthy Lass

Hang on, I will draw a diagram. A picture tells a thousand words .

And you have it the wrong way around:
If there is zero ground wind, then the True wind will be coming from the opposite direction to which the current is moving.

Ground Wind = 0
Current = 5 kts, to the north (flowing from south to north)
-----------
Ground Wind = 5 kts, from the north [edit: <--- WRONG!]

And I think that's what I originally said.

[what I meant to type was this:]
True Wind = 5 kts, from the north

[Dockhead]

Originally Posted by Hoofsmit

Ok .. I get the increase in wind on your sail ( true , ground , apparent , call it what you want !) that fact is that we are talking about A to B quicker by lee bowing , go race a clinker dingy in a tidal estuary for a few seasons, than go disprove lee bowing works. Sod the maths : )

I am not disputing that it seems to work.

I am guessing that it seems to work because you haven't properly worked out your course to steer to your mark.

We will only know for sure with maths!

[Seaworthy Lass]

Originally Posted by Paul Elliott

Ground Wind = 0
Current = 5 kts, to the north (flowing from south to north)
-----------
Ground Wind = 5 kts, from the north

And I think that's what I originally said.

Sorry, misinterpreted what you were saying. I think you will find my statement was correct too.

Just trying to find the easiest way to illustrate what I mean and it is taking time. I am not my quickest at this time LOL.

[Hoofsmit]

Sorry , but I will leave you to it , to me I would just prefer to try it .... If you are getting headed up ... great ..... If not tack , I work in a job that no one can make sense of why sometimes things work and some don't , I would rather work things out in reality rather than theory . Our fore fathers sailed when the world was flat but still sailed from A to B. Wish you all well in your workings ; )

[Seaworthy Lass]

I haven't dozed off .

Just struggling to illustrate it. Maybe the fact that I am struggling means that piching will not work LOL. It is past midnight and my eyelids are drooping (I am not my brightest at this time of night), but I don't want my honour to droop either, so I am persisting for now. Sleep may get the better of me though .

[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 still enjoyed the exercise though . One interesting thing pottering with vectors was looking at the "lift"achieved with pinching just a little. This must be the effect that people describe. The problem is that it slows you down and in total actually hinders you. Maybe the reason why it gives someone an advantage is that it is done while the current is decreasing and everyone else on the opposite tack have just misjudged their final tacks given the decreasing current. Can any sailors who have experienced an advantage pinching at this stage, report if the quicker time in rounding the mark is noticed more when the current is getting stronger or weaker?

If anyone is still interested in one scrap of what I think, I will discuss the "lee bow effect" which has nothing to do with pinching and exists tacking into current which will be varying. Perhaps I can redeem myself a little there.

[Dockhead]

OK, I have been thinking about this, and can offer some arguments.

Thesis: sailing dynamics cannot possibly be affected by any constant current. (We're talking about constant, not changing currents so far because it's simpler. I think actually that everything we have been talking about is the same with changing currents, but we'll get to that later if we need to.)

Here's one vivid way to visualize it:

A current is relative motion of water and land. We obviously don't care about the earth's spinning on its axis, or passing around the sun, or the universe expanding, now do we? Did anyone claim we get a lift from sailing towards the direction of the sun? Of course not.

OK, so once we are clear that we're talking about relative motion, then we can conceptualize this motion in either a geocentric or hydrocentric way, right? That is, we can conceptualize the land being still and the water moving, or we conceptualize the water being still and the land moving, although nothing is actually still, of course -- the "stilness" of the land is purely conceptualization, as the earth is spinning, etc.

So let's try thinking hydrocentrically for a change, which as I argued above is going to make more sense mathematically and probably conceptually for this problem. That is because for purposes of how our boats sail, the water IS still. We move in water; everything we do except running aground and laying marks is relative to the water, and it is not so hard to imagine the shoals moving towards us and the marks moving around out there. In particular -- crucially, for this discussion -- when we sail, using the wind, we are sailing in the relative motion between wind and water. We simply cannot understand the sailing part if we conceptualize both water and wind moving at the same time -- we get an idiotic confusion. This confusion disappears when we make the water hold still in our minds.

OK, so now we have rounded some mark, and are heading towards the windward mark, and the tide is running. What does this look like?

The windward mark is moving relative to us, and so in order to work out how to lay it, we can't just head straight towards it, or tack towards it, we have to calculate the expected vectors of the mark's motion, so we know which way to go. That's an ordinary Course to Steer calculation. Without this, we don't even know where we're going, and we're screwed before we start.

Now that we have a Course to Steer, we know where we're going. That is to say -- we have Point "B" in the water, which we have to lay in order to lay the windward mark. We must have some point in the water to sail towards, because we can't sail towards a point fixed to the land -- we're sailing through water, after all. The Course to Steer calculation does the translation from a land coordinate to a water coordinate, if you like.

OK, so Point "B" in the water is that place where the windward mark will be when we get there (by the way, this definitely works with a changing current as well as a constant one).

Now!!! Delete the windward mark from your mind. It is no longer relevant. We are at Point A in the water -- in water which we conceptualize as being still. We have Point B in the water. We have wind, which is constant in speed and direction in relation to the water (true speed and direction, which is what we sail in).

How do you get from Point A to Point B? Sail as usual -- it's a straight line in our hydrocentric world. All tacks relative to this straight line work just like any tacks anywhere. That is to say -- if we can't lay Point B on one tack, then the fastest way to get there will be by maintaining the highest VMG to windward at all times and on both tacks which means, for God's sake, no pinching, what a perversion.

So we sail, ignoring the mark itself, which we have deleted from our minds. If our Course to Steer calculation was perfect, then the mark (which is moving relative to the water, which we conceptualize to be still!) will arrive at Point B simultaneously with us. In reality, the CTS calculation will not be perfect, so we will have to correct, but that does not detract from the point.
The motion of the seabed relative to our boat cannot affect sailing dynamics. It is utterly and categorically impossible. Sailing dynamics are purely and solely determined by the relative motion between air and water.

This is not the only way to conceptualize the problem, but I suggest it is the easiest. You cannot conceptualize sailing dynamics at all if you can't conceptualize the relative motion between air and water. You can't conceptualize this -- and can't do any math with regard to it -- without conceptualizing one or the other of the media to be holding still. So it can't be the land! I think we just can't work the problem conceptualizing the land to be still.