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

[europaflyer]

Originally Posted by goboatingnow

forget tacking, forget slack water , the lee bow effect is just what it says, i.e. switching the current from fine on the windward bow to fine on the lee bow ( no tack involved) cause a freeing of the apparent wind.

Whether that freeing is useful, is another thing entirely


dave

Agreed. However, once the current is coming from the other direction you need to tack to keep it on the lee bow. Savvy?

[Seaworthy Lass]

Originally Posted by goboatingnow

forget tacking, forget slack water , the lee bow effect is just what it says, i.e. switching the current from fine on the windward bow to fine on the lee bow ( no tack involved) cause a freeing of the apparent wind.

Whether that freeing is useful, is another thing entirely

dave

Dave there is no freeing of the apparent wind. You are not 'lifted' at all, you have just headed up. The 'effect' is non existent! .

I am drawing another cone diagram to show this more clearly.

[Hoofsmit]

Originally Posted by europaflyer

Agreed. However, once the current is coming from the other direction you need to tack to keep it on the lee bow. Savvy?

no no no ....... this is lee bowing the tide

the lee bowing effect is to do with the current being on the lee side of your hull and foils this provides lift as in a aerocraft wing

until you lot look at the current vectors of forward movement against current vectors you will argue until the cows come home

[europaflyer]

Originally Posted by Hoofsmit

no no no ....... this is lee bowing the tide

the lee bowing effect is to do with the current being on the lee side of your hull and foils this provides lift as in a aerocraft wing

until you lot look at the current vectors of forward movement again current vectors you will argue until the cows come home

No it's not. This is the most common misconception of what the lee bow effect is and we got past this ages ago with the 'pinching' debate. It's nothing to do with hydrodynamics, and you will get no more force from the keel than you would for slack water.

It's just a wind-shift effect caused by the boat sitting in a moving body of water.

[foolishsailor]

Originally Posted by goboatingnow

You are mixing up effects with benefits. SeaWorthies cone, clearly shows that certain currents fine on the windward bow when switched to currents fine on the leeward bow, causing a freeing of the ground wind, and hence a freeing of the apparent wind. hence The boat can point higher…. Thats the end of the lee bow effect. Whether the "advantage" can be utilised to achieve anything is purely based on the course and what is desired. Clearly if one is attempting to make a mark to windward and finds that it is just going to be missed, then realising there is a current fine ( and thats important), then by pinching up to place the current on the lee bow, then the boat is freed ( and no longer pinches) and lays the mark in the same tack. Discussions about tacking onto the other tack miss the point entirely I don't need a diagram . SLs has drawn a fine one. Dave

No, im not mixing effects.

You do need a diagram as what you ate saying is clearly proven false by the diagrams posted previously.

Unlike Angela, who has the patience of Job, I dont wish to get into a word battle with you over this.

Either you are too uninterested, lazy, or cant provide a diagram representation of how your position is true and mine is false.

The same holds for Europaflyer.

I dont mean this aggresively - but I also dont have the time or desire to discuss this with words for 20 pages.

If you cant draw it - then you cant prove it.

[goboatingnow]

Originally Posted by europaflyer

Agreed. However, once the current is coming from the other direction you need to tack to keep it on the lee bow. Savvy?

why as you've agreed, the apparent wind is freed , so why tack

dave

[europaflyer]

Originally Posted by foolishsailor

No, im not mixing effects.

You do need a diagram as what you ate saying is clearly proven false by the diagrams posted previously.

Unlike Angela, who has the patience of Job, I dont wish to get into a word battle with you over this.

Either you are too uninterested, lazy, or cant provide a diagram representation of how your position is true and mine is false.

The same holds for Europaflyer.

I dont mean this aggresively - but I also dont have the time or desire to discuss this with words for 20 pages.

If you cant draw it - then you cant prove it.

I have provided diagrams. They did prove it in my mind, and I think to some others as well.

Like you, I mean this in the nicest possible way, but I have not heard a coherent argument against the effect as I have presented it, nor have I seen any diagrams which go against what I have posted. Rather, just a lot of confused thinking which I can often not begin to understand.

I post below my original diagram, this time with the terms changed to what we have agreed in this thread which I am happy with. It is the classic case of the so-called 'lee bow effect', and shows the boat with the current on the lee bow pointing far closer to the ground wind than the other boat. Without wanting to sound patronising, can you just state as clearly as possible what it is you dispute about this, because so far I really don't understand what people are disagreeing with me about.

[europaflyer]

Originally Posted by goboatingnow

why as you've agreed, the apparent wind is freed , so why tack

dave

The apparent wind is freed while the current is on the bow. Six hours later, the situation is reversed and you need to be on the other tack to get the same effect.

[foolishsailor]

That diagram proves and instantaneous effect.

Extend that drawing to show how a boat uses that effect from origin to destination to beat a boat that doesn't.

Preferably use my diagram with a grid - so that others can see your calculations and question them using the grid.

The drawing simply shows apparent wind minus current effect - it doesn't prove anything.

Again, I hope this medium doesn't make my challenge into more than a discussion over beers - which is how I mean it.

[Hoofsmit]

you all poo poo my hand in the tap scenario because you don't understand the resistant affects

I will try to speak your language ...... floating down stream with no wind is no reasoning there are no hull dynamics involved

resist that drift by 1knt of wind and you move either with it against or across it, why, because the wind is ground referenced ..... my hand is ground referenced and that movement is over the seabed

[Seaworthy Lass]

Originally Posted by goboatingnow

forget tacking, forget slack water , the lee bow effect is just what it says, i.e. switching the current from fine on the windward bow to fine on the lee bow ( no tack involved) cause a freeing of the apparent wind.

Whether that freeing is useful, is another thing entirely
dave

Dave, I have just thought of another way of looking at it.
Consider these 5 points carefully and tell me which if any you disagree with:

1. The ground wind does not depend on what angle your boat is pointing or in fact whether or not there is a boat LOL does it?

2. The wind induced by current does not depend on which way your boat is pointing either (or again it would be the same if there was no boat at all).

3. The true wind is just the addition of the ground wind and wind induced by current. It is totally independent of the boat or where it is pointing.

4. You sail to the true wind not to the direction of the ground wind or current.

5. If you try and pinch when you are close hauled, it will slow you down. If not you were not close hauled to start with. Pinching will never get you to the mark more quickly.

These five sentences added together are really powerful, far more so than any diagram. Really read them and ponder on this. The 'Lee Bow Effect' is absolutely a myth. It is in fact laughable .

[Hoofsmit]

When the angle of leeway is large, the local flow on the suction side of a yacht keel can form an angle ai with the far-field flow direction of even 40°. The situation is shown in figure 2.28. Clearly, the higher the angle ai, the stronger the tip vortex.


Figure 2.28: Cross-flow on a keel

[europaflyer]

Originally Posted by foolishsailor

That diagram proves and instantaneous effect.

Extend that drawing to show how a boat uses that effect from origin to destination to beat a boat that doesn't.

Preferably use my diagram with a grid - so that others can see your calculations and question them using the grid.

The drawing simply shows apparent wind minus current effect - it doesn't prove anything.

Again, I hope this medium doesn't make my challenge into more than a discussion over beers - which is how I mean it.

OK. Here is a simple little diagram, of a simplistic 'race to the far side' sort of race (OK, not realistic, but the diagram for a race to a destination with multiple tides would be hideously complex and I haven't got the time!).

On the left, no current. Both boats travel the same distance through the water in the time period which the distance vectors show, and both reach the far shore at the same time, as you would expect.

On the right, a current is added. The initial reaction of most people is that this will not affect which boat gets to the far side first, as it is at 90 degrees to the shore. A perfectly sensible idea. However, my previous diagram shows that one boat (lee bow) is pointing closer to the ground wind in this situation. It's distance vector gets it to the far shore and beyond in the time it took the other (zero tide) boats just to reach the far shore. In the same time, the other boat doesn't even reach the shore.

This is all I am talking about. It is called an 'effect' because the initial reaction of most people is that adding the current will not alter which boat gets there first. Because of the shift in true wind, it does.

Extending this to a cross Channel race, with dozens of different tidal vectors to add, and perhaps a destination which is not upwind or one which is not directly across the channel, is far more complex. I thought I would use this most simple case first. Is this what you were meaning?

ps. You know things are getting serious when we have to remind each other how nice we're being in each post.

edit: note that the vectors on the right don't show course over ground, rather course through the water. Doesn't matter from the point of view of getting to the far side, as the tide is at 90 degrees to the shore and the effect on both boats is equal.

[Seaworthy Lass]

Originally Posted by europaflyer

Oh dear, oh dear.

Please please do this little exercise for me :

Draw in a ground wind vector

Then draw in two boats on the two tacks (at whatever angle you consider reasonable, the angle you choose is irrelevant).

Then add current in any direction you like (except at 0 and 180 from the ground wind.

Then add the current induced wind vector and show how the true wind has veered with the current added.

You have nearly 360 degrees of current directions to chose from. I challenge you to find one single one where the lifted tack is not the one with the current more on the bow.

Rather than stating 'oh, dear' just find one single example to back up what you are saying please and we can end this once and for all.

[europaflyer]

Originally Posted by Seaworthy Lass

Please please do this little exercise for me :

Draw in a ground wind vector

Then draw in two boats on the two tacks (at whatever angle you consider reasonable, the angle you choose is irrelevant).

Then add current in any direction you like (except at 0 and 180 from the ground wind.

Then add the current induced wind vector and show how the true wind has veered with the current added.

You have nearly 360 degrees of current directions to chose from. I challenge you to find one single one where the lifted tack is not the one with the current more on the bow.

Rather than stating 'oh, dear' just find one single example to back up what you are saying please and we can end this once and for all.

Challenge accepted!

I posted this one a while back and I forget if you ever responded. I've changed the wind names to what we agreed.

Current aft of the beam lifting the boat... or what?