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

[Hoofsmit]

Originally Posted by Seaworthy Lass

Something really weird going on. I did not write post 542, Dockhead did. It has my name as poster.

Had the same in the terminology thread...... I think it's when it's been copied and pasted

[Seaworthy Lass]

Originally Posted by foolishsailor

I will, but not tonight.

Time for a glass of wine.

The fact that Angela is not chiming in means she is doing some serious work on these questions - and I am excited and a bit fearful of what she is going to produce.

I am still working to dismiss lee bow. Dockhead has still not been converted LOL and I think it is worth a little more effort before we go further, to not only lay this "lee" term to rest, but give it a proper burial .

Dockhead, I think it would be a great idea if you started another thread on tacking with current, and transferred all the relevant posts over, leaving this thread just on the lee bow effect. If you have no time, but think this is a good idea, would you like me to do it for you sometime tonight?

[Dockhead]

Originally Posted by Seaworthy Lass

I am still working to dismiss lee bow. Dockhead has still not been converted LOL and I think it is worth a little more effort before we go further, to not only lay this "lee" term to rest, but give it a proper burial .

Dockhead, I think it would be a great idea if you started another thread on tacking with current, and transferred all the relevant posts over, leaving this thread just on the lee bow effect. If you have no time, but think this is a good idea, would you like me to do it for you sometime tonight?

I'm not really arguing -- I am convinced that you have shown something original -- that the Effect is not necessarily attached to the current being on the lee side of the boat . That's a really valuable and enlightening thing

I'm just not convinced that it makes no difference, or that the Effect works 50% of the time with the current on the weather side. I have a strong hunch that the current will be on the weather side of the boat only in the margins. Just chasing that idea a little.

As to what we call it -- there is a really good reason, in any case, to call it something besides LBE. That's because there is such total confusion about what it means. So from that point of view, better to start over again.
'
On the other hand, it is a big claim to redefine terminology, and I think it goes against decent modesty to attempt to do so unless you really have a hell of a case. That is because by redefining terminology you are as if claiming to throw in the trash all the thought which went before. I think that is clearly not justified in this case, because the Effect is just a specific application of a well known sailing principle. We have just sorted out a mass of confusion and created a lot of light around the subject -- fantastic. Maybe a YM article in it, if someone can find the time. But I don't think we should get carried away with ourselves.

[europaflyer]

Originally Posted by Dockhead

I'm not really arguing -- I am convinced that you have shown something original -- that the Effect is not necessarily attached to the current being on the lee side of the boat . That's a really valuable and enlightening thing

I'm just not convinced that it makes no difference, or that the Effect works 50% of the time with the current on the weather side. I have a strong hunch that the current will be on the weather side of the boat only in the margins. Just chasing that idea a little.

Agreed. Like Angela, I managed to produce an example of getting a lift from the weather side in my post #545 which I copy below, but it was quite tricky - it is really marginal as you say. I found what I think she found as well - that it is easier to demonstrate this with a boat that doesn't point well. Which I suppose makes sense. Adding the speed vector to give apparent wind is, I think, important for completeness, and I tried to add this to the diagram.

If we have learned anything from this this thread, though, it is that 'lee bow effect' is a really, really unhelpful name from the point of view of understanding the effect. 'Current wind shift effect' perhaps?

[Dockhead]

Originally Posted by Seaworthy Lass

Now if it works for the current being 30 degrees off the ground wind (and it works even better for angles less than this) then that is already 60 degrees of current it works for, and that is without taking into account current 180 degrees off this. I will work out those angles in a moment, working out a percentage of angles it works for in these conditions.

Did you mean 30 degrees off ground wind, like you wrote, or 30 degrees off the perpendicular? Huge difference -- 30 degrees off ground wind will be already at the thin margins of the Effect.

It's somewhat arbitrary, but I would throw out that the "meat" of the effect will occur within 45 degrees of the perpendicular. Beyond 45 degrees, rapidly diminishing returns.

It's complicated (damnedly) because this is not the only variable -- you also have relative velocities of wind and current.

Still, I'm guessing that in the great majority of cases no one is going to bother with this if the wind and current are aligned closer than 45 degrees. If I'm right about the numbers, then a 90 degree tacking boat will never get the Effect in any usable degree with the wind on the weather side.


As to my boat -- with old Dacron sails and loaded to the gills with tools, ground tackle, and other carp, my standard tacking angle over ground in decent but not ideal conditions is about 100 degrees, which I achieve at 36 degrees to the apparent wind. If I really work on sail trim, and in really good conditions (18-20 knots of non-gusty apparent wind), this comes down to 95 or so. When I get my laminate sails up and have a house-cleaning, I hope to get that down a bit.

[Hoofsmit]

Originally Posted by Dockhead

I'm not really arguing -- I am convinced that you have shown something original -- that the Effect is not necessarily attached to the current being on the lee side of the boat . That's a really valuable and enlightening thing I'm just not convinced that it makes no difference, or that the Effect works 50% of the time with the current on the weather side. I have a strong hunch that the current will be on the weather side of the boat only in the margins. Just chasing that idea a little. As to what we call it -- there is a really good reason, in any case, to call it something besides LBE. That's because there is such total confusion about what it means. So from that point of view, better to start over again. ' On the other hand, it is a big claim to redefine terminology, and I think it goes against decent modesty to attempt to do so unless you really have a hell of a case. That is because by redefining terminology you are as if claiming to throw in the trash all the thought which went before. I think that is clearly not justified in this case, because the Effect is just a specific application of a well known sailing principle. We have just sorted out a mass of confusion and created a lot of light around the subject -- fantastic. Maybe a YM article in it, if someone can find the time. But I don't think we should get carried away with ourselves.

This so refreshing to hear, thankyou

[DumnMad]

Originally Posted by foolishsailor

Of course water flowing across a foil provides lift, that is not what we are discussing. The point we sre making is that current doesnt generate lift because the boat, like a floting piece of driftwood, moves with the current.

The flow of water over the foils is generated by the boat bieng propelled through the water by its sails. The current is "invisible" to the hull.

I agree the current itself doesn't generate lift but being held against the current by wind or motor does mean there will be hydraulic lift, and this lift will alter with the heading. Easier to see if motoring only. When sailing, the angle the hull goes through the water and hull shape are affected by the change in apparent wind speed and the hydraulic affect will change with significant lift occurring if there is supercritical flow one side and only subcritical flow on the other side. I don't believe you can leave hydraulics out of the equations.

[europaflyer]

Originally Posted by DumnMad

I agree the current itself doesn't generate lift but being held against the current by wind or motor does mean there will be hydraulic lift, and this lift will alter with the heading. Easier to see if motoring only. When sailing, the angle the hull goes through the water and hull shape are affected by the change in apparent wind speed and the hydraulic affect will change with significant lift occurring if there is supercritical flow one side and only subcritical flow on the other side. I don't believe you can leave hydraulics out of the equations.

Speed through the water does not change, nor does the angle of attack of the keel. These are the factors that matter. This isn't a hydrodynamic effect, that was dealt with many many pages ago...

[DumnMad]

Speed through the water may not change but water speed on one side of the hull is different from water speed on the other side. This changes with heading.

[Seaworthy Lass]

Originally Posted by Dockhead

I'm not really arguing -- I am convinced that you have shown something original -- that the Effect is not necessarily attached to the current being on the lee side of the boat . That's a really valuable and enlightening thing

I'm just not convinced that it makes no difference, or that the Effect works 50% of the time with the current on the weather side. I have a strong hunch that the current will be on the weather side of the boat only in the margins. Just chasing that idea a little.

It is not exactly 50% of the time. You will note that I have always attached the word "roughly" to it, as it is very complicated. The % will depend on the angle possible between tacks, which depends on the boat's characteristics, the amount of true wind. It also depends heavily on the relative amounts of wind induced my current and ground wind.

I am not talking off the top of my head here. Just look at the original cone diagram and imagine it stretched up (or even down). For any cone diagram, work out what angle of current it works for (just draw on the tacking angle and do this by trial and error as the maths are complicated). Then continue this line down and mark off the other spot where the current can possibly hit it. Measure this off 180 and double it to allow for the other side. It will always be higher than for first current as it would only be equal if the lines were parallel (ie the ground wind was infinity big). The squatter the cone the larger the angle with the current coming from the rear.

Originally Posted by Dockhead

As to what we call it -- there is a really good reason, in any case, to call it something besides LBE. That's because there is such total confusion about what it means. So from that point of view, better to start over again.

On the other hand, it is a big claim to redefine terminology, and I think it goes against decent modesty to attempt to do so unless you really have a hell of a case. That is because by redefining terminology you are as if claiming to throw in the trash all the thought which went before. I think that is clearly not justified in this case, because the Effect is just a specific application of a well known sailing principle. We have just sorted out a mass of confusion and created a lot of light around the subject -- fantastic. Maybe a YM article in it, if someone can find the time. But I don't think we should get carried away with ourselves.

I have a hell of a case LOL. That is why I am jumping up and down about it .

Give me any ground wind amount and any current amount and the angle possible between tacks relative to the ground wind (it is a ground referenced diagram) and I will give you an approximate percentage of current directions it works for.

You can make it as average or as extreme as you like. In fact give me a few lots if you like, it is quite quick for me to work out.

[Dockhead]

Originally Posted by Seaworthy Lass

It is not exactly 50% of the time. You will note that I have always attached the word "roughly" to it, as it is very complicated. The % will depend on the angle possible between tacks, which depends on the boat's characteristics, the amount of true wind. It also depends heavily on the relative amounts of wind induced my current and ground wind.

I am not talking off the top of my head here. Just look at the original cone diagram and imagine it stretched up (or even down). For any cone diagram, work out what angle of current it works for (just draw on the tacking angle and do this by trial and error as the maths are complicated). Then continue this line down and mark off the other spot where the current can possibly hit it. Measure this off 180 and double it to allow for the other side. It will always be higher than for first current as it would only be equal if the lines were parallel (ie the ground wind was infinity big). The squatter the cone the larger the angle with the current coming from the rear.



I have a hell of a case LOL. That is why I am jumping up and down about it .

Give me any ground wind amount and any current amount and the angle possible between tacks relative to the ground wind (it is a ground referenced diagram) and I will give you an approximate percentage of current directions it works for.

You can make it as average or as extreme as you like. In fact give me a few lots if you like, it is quite quick for me to work out.

OK, cool. Let's take a 90 degree tacking boat (for simplicity's sake; can expand to 100 later if we want). Let's take current range from 0 slack tide going up to 4 peak tide. Let's take 6 knots boat speed (Effect will be greater with less boat speed).

Let's take ground wind speed range from 15 to 20 knots.

Do we agree that a skew from perpendicular of more than 45 degrees between ground wind and current axis is already "in the margins"?

So, within 45 degrees or less of skew, are there any cases, playing around with the other parameters, of the Effect working with the current on the weather side of the bow? If there are some cases, what percent, roughly?

I would be very surprised if it is more than 15%.

It will be somewhat greater with a 100 degree tacking angle, but why don't we look at this first.

[Seaworthy Lass]

Originally Posted by Dockhead

OK, cool. Let's take a 90 degree tacking boat (for simplicity's sake; can expand to 100 later if we want). Let's take current range from 0 slack tide going up to 4 peak tide. Let's take 6 knots boat speed (Effect will be greater with less boat speed).

Let's take ground wind speed range from 15 to 20 knots.

Do we agree that a skew from perpendicular of more than 45 degrees between ground wind and current axis is already "in the margins"?

So, within 45 degrees or less of skew, are there any cases, playing around with the other parameters, of the Effect working with the current on the weather side of the bow? If there are some cases, what percent, roughly?

I would be very surprised if it is more than 15%.

It will be somewhat greater with a 100 degree tacking angle, but why don't we look at this first.

Are you a betting man?

[Dockhead]

Originally Posted by Seaworthy Lass

Are you a betting man?

Sure, if I can name the stakes!

[Hoofsmit]

Originally Posted by DumnMad

I agree the current itself doesn't generate lift but being held against the current by wind or motor does mean there will be hydraulic lift, and this lift will alter with the heading. Easier to see if motoring only. When sailing, the angle the hull goes through the water and hull shape are affected by the change in apparent wind speed and the hydraulic affect will change with significant lift occurring if there is supercritical flow one side and only subcritical flow on the other side. I don't believe you can leave hydraulics out of the equations.

I agree but was shouted down
Some will not look at this because the format that gives them a course to steer has a element of this worked into the equation
You have to move away from the basis of this equation of vectors which is based on
water is like a table cloth on a table which can be moved as well as the boat can move on the table cloth
Unfortunately the elements of variable resistance and hydraulics is totally ignored.
To question this CTS equation is to ask a sailor to alter his technical ability to navigate.
They are concreted in this theory the biggest problem they are backed up by an analogy that states the boat does not know what the ground is under it and only reacts to the water and wind

The speed and velocity of the tablecloth ( tide/ current) is worked into the equation as a tidal vector , unfortunately this is not seen as a constant curve but segments related to time plotted

The clever bit is that the wind is then water referenced which makes life very confusing to ask them to go against the principles they use

[DumnMad]

Thanks Hoofsmit. The hydraulic force perpendicular to current flow can be massive. I've seen heavy walled 600 dia steel piles break perpendicular to the flow due to eddie effects.
Changing course to account for current must have an affect on lift.