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

[goboatingnow]

Originally Posted by Paul Elliott

Dave, family life has gotten in the way of analyzing and working out these diagrams. I hope tomorrow I can make the time.

I do want to bring in some actual polars and leeway calculations, so we can look at VMG and CMG (velocity made good in the direction of a waypoint). Since we're talking about pinching and otherwise sailing non-optimum angles, I really want to explore how these interact. If I just arbitrarily assume some boatspeed then the exercise could be misleading. I'm on the road this week, so I picked up some graph paper, a protractor, compass, and ruler. It should be fun!

In the meantime, do take a look at this diagram (below). It shows a sample polar for one windspeed, as well as the optimum upwind and downwind angles, as well as how VMG falls off as you sail higher or lower.

Different issue, never argued about VMG.

Dave

[Dockhead]

Originally Posted by Hoofsmit

Now I know you are all going to ignore this because but, read and absorb it please
The effect is caused by the current on the bow ( don,t run away)

Think "apposed and equal force"
Logic states no current no additional force, it is only that we cannot work this out that we have to turn current to a wind vector
the wind is ground based.
think mooring ... sails up ..... all wind vectors give a force equal to the mooring line and we stay still ...... because our wetted area gives a resistance to the current. it is the current that affects the apparent not the apparent wind that effects the current, think out the box and you start looking at the effect which really make the difference

We just mix it all up into apparent wind so we can plot it

It is purely simple when you are thick like me

You get an "A" for effort, or full marks as you guys say over there. Keep poking away at it and you can understand anything. You are not thick.

I'll give you some hints:

When you are on a mooring, your boat is tied to the ground. Thus your boat is not floating in the current, and you have an interaction between current and your boat which creates forces. As you correctly say, if your boat is just lying to the mooring, the force on the mooring pennant will equal force created by the resistance of the hull to the current.

And -- if you get on board and turn the wheel, the boat will veer, because the current is acting on the rudder, since the water is moving in relation to the boat, since the boat is tied to the ground. Furthermore, your log will show Speed through the Water (note: GPS will show 0 SOG).

With me so far?

Now -- get your knife out, and cut the mooring pennant. Your boat will instantly float away, and in a few seconds your log will show 0 Speed through Water (although SOG has appeared on the GPS). Now turn the wheel. Nothing! No reaction!

That is because water is no longer moving over the hull, there is no force exerted by the moving water (which is now no longer moving in relation to the boat). So neither keel nor rudder is doing anything at all.

OK, NOW can you see why the current exerts no force on the keel or rudder? The ONLY way you feel a current in a boat is through the change in the wind. The forces exerted by the water are purely a function of the relative motion between water and boat. You do get relative motion between water and boat is there is as current and your boat is tied to the ground through a mooring. But floating free, there are no forces exerted by the water except the ordinary forces from sailing, which are exactly the same whether or not you are sailing in any current.

Hope this helps.

[goboatingnow]

Originally Posted by Seaworthy Lass

Dave, I am sorry, there is so much wrong with your 2 diagrams that it would takes ages to describe. I very unfortunatley have no time today , boat chores call (I am just having a morning cuppa before commencing).

You have mixed up the types of wind to start with. The only type of wind you can affect by pinching is the apparent wind (you cannot change the ground or true wind, no matter what you tried to do, although some sailors would probably sell their souls to be able to do so ).

So you can only alter apparent wind by your boat movement (speed plus leeway).

If you alter your heading, yes of course the apparent wind will alter, both because of a change in your direction and because of a change in boat speed that brings.

If you pinch you instantly slow down. Your VMG instantly drops. It doesn't matter which side of the boat you are presenting to the current.

If the current is on the bow you are ALREADY on the tack lifted by current. Irrespective of which side of the bow the current is on. Pinching will only slow you down.

Paul, if you have the time are you able to discuss all this with Dave. I must start on boat work .

I have not mixed up any winds , in my last diagram , all winds are ground referenced. I specifically computed the apparent wind vector and showed that as a ratio of heading change there is a net benefit with current on the lee bow and without current


Arguments about VMG are completely an aside. It's an " effect" it's not called the lee bow benefit.

Dave

[goboatingnow]

Originally Posted by Hoofsmit

Now I know you are all going to ignore this because but, read and absorb it please
The effect is caused by the current on the bow ( don,t run away)

Think "apposed and equal force"
Logic states no current no additional force, it is only that we cannot work this out that we have to turn current to a wind vector
the wind is ground based.
think mooring ... sails up ..... all wind vectors give a force equal to the mooring line and we stay still ...... because our wetted area gives a resistance to the current. it is the current that affects the apparent not the apparent wind that effects the current, think out the box and you start looking at the effect which really make the difference

We just mix it all up into apparent wind so we can plot it

It is purely simple when you are thick like me

Current moves a boat with reference to the ground hence it's produces a ground referenced wind.

Dave

[goboatingnow]

Originally Posted by Dockhead

Exactly. This is a different flavor of Lee Bow Effect than what we have been discussing lately. This is the "if you can pinch to get the current from fine on the weather bow to fine on the lee bow, you'll get a lift" variant.

We've read probably dozens of refutations of this in both external sources, and here. It is based on the persistent erroneous feeling that the keel "feels" the current and is "pushed" upwind by it, if you can just get the current on the lee side.

This effect does not exist, and Dave's drawing actually demonstrates this -- there is no magic and no watershed about the current going from fine on the weather bow to fine on the lee bow -- it's just x degrees more or less like any other x degrees, which can be seen in the math, which will belie any huge difference in wind from a few degrees of heading change taking the current to just the other side of the bow. The wind will change smoothly, one degree at a time.

By the time the current is fine on the bow when we are close hauled, the other, real flavor of Lee Bow Effect will not be working (it needs perpendicular-ish current to ground wind), and to the extent it may be working just a little bit, in fact as we know from SWL's work, it is likely to need the current on the weather bow.

So, sorry Dave -- it don't work at all the way you have in mind

My diagram resolves all winds , ground wind , current wind , which are fixed in relation to the earth. , the only wind that's changes is the wind made by the progress of the boat and that changes by heading.

Bring in VMG has nothing to do with it. For the sake of illustration I assumed there is no reduction in doped as a result of pinching, because a reduction in speed actually helps the case ( apparent wind swings aft on slowing down )


The diagram shows that the effect of pinching is reduced if you have a current dead on the bow , and You come up slightly to windward. The effect is reduced in relation to pinching without the current being there before.

The " effect" has nothing to do with , NOR is there necessarily any real benefit ( since we can not determine why a helm would want to pinch in the first place)

(A) VMG ( that's talking about " benefit" not " effect " )
(B) boat polars
(C) true wind
( d) the size of the vectors
(E) CTS

I've read through the thread repeatedly and you need to extract the kernel out of vast tracts of incorrect debate about terminology , lift effects on keels etc.

What it shows is which current of the nose , the effect on apparent wind angle change as a result of coming up to weather , is to reduce it as a proportion of heading angle change , compared to the same situation where no current exists.


People are confusing this with all sorts of other current issues , such as CTS , channel crossings etc.

The original " effect" refers to pinching , yet no one has resolved the vectors to apparent wind and without that you cannot conclude anything in relation to the " pinching " part.


Describing the effects on a 360 basis is great and I'm not arguing. But it's not a " fine on the bow " effect that's therefore being discussed , nor does the fact that " effects" occur on the windward side , invalidate the use of the albeit misnamed moniker. Taking about currents more then 10-15 degrees off the bow is another " effect" entirely

Dave

[goboatingnow]

Originally Posted by Seaworthy Lass

Dave that is plain sneaky popping in a quote of mine from a post I made way back at #2 when I retracted it totally by post #59 LOL.

This was my retraction:

I have kept repeating this over and over the last few hundred posts, many of my comments in exchanges with you:

My cone diagram beautifully illustrates why this is so:

I'm afraid your diagram illustrates many things , but simply restating its a myth with no examination of apparent wind , means You can't conclude that , pinching or any sailing angle only refers to apparent wind , true cannot be used to dispel the " myth"

Dave

[goboatingnow]

Just to restate. You cannot not resolve any issue concern the current on the bow without evaluating all the winds right to the apparent wind.


Secondly people are fundamentally not understanding the original as described " pinching lee bow effect " this has to do purely with a gain in apparent wind angle ratios over the situation with no current . That's all.

Whether any of this is a "benefit" is virtually impossible to resolve because to do that we would have to analyse a whole race section section , taking into account different boat polars , race tactics , progression to a mark etc.

It's an effect that's all.

Dave

[goboatingnow]

Originally Posted by Paul Elliott
Dave, family life has gotten in the way of analyzing and working out these diagrams. I hope tomorrow I can make the time.

I do want to bring in some actual polars and leeway calculations, so we can look at VMG and CMG (velocity made good in the direction of a waypoint). Since we're talking about pinching and otherwise sailing non-optimum angles, I really want to explore how these interact. If I just arbitrarily assume some boatspeed then the exercise could be misleading. I'm on the road this week, so I picked up some graph paper, a protractor, compass, and ruler. It should be fun!

In the meantime, do take a look at this diagram (below). It shows a sample polar for one windspeed, as well as the optimum upwind and downwind angles, as well as how VMG falls off as you sail higher or lower.

That's great but , it's nothing to do with a " supposed lee bow effect and pinching " to mangle up a moniker. None has described it as a " lee bow benefit ". Converting any improvement ( and its small) in the ratio of heading change to apparent wind angle and therefore trying this to a benefit on say a race course is an extremely complex process ,

Just to restate what it is I discussed

( these are not real numbers , merely to show the relative effect on apparent angles )

In effect what's demonstrated , is a heading change of 5 degrees results in a apparent wind angle change that's is less with the current "out in front" , then if the current wasnt there at all. Hence the helm sees a smaller falloff in boat speed ( from polars ) then would otherwise occur.

Arguing LBE isn't there by talking about VMG , etc misses the whole point.

In fact take "pinching out of it entirely " , say the boat is reasonably close hauled , and You head up , the existence of the current improves the loss of apparent angle over the situation in no current . That's all that's happening. VMG and hence progress to whatever the helm is trying to do is completely another topic.


The effect is misnamed , but in the one situation I describe the current happens to end up on the lee side and that has been the use of the term LBE.


Dave

[Dockhead]

Originally Posted by goboatingnow

The diagram shows that the effect of pinching is reduced if you have a current dead on the bow , and You come up slightly to windward. The effect is reduced in relation to pinching without the current being there before.

When you say "the effect of pinching is reduced if you . . . come up slightly to windward" -- what exactly do you mean?

What effect occurs in a few degrees from dead on the bow to slightly to windward, which doesn't occur in the same number of degrees in any other direction?

Your diagram doesn't imply any kind of abrupt transition at the bow or any other place -- just vectors.

To put it another way -- if you get a lift from having the current a few degrees to leeward of the bow, this implies that you get a knock from having the current a few degrees to weather. Where does this knock come from? I cannot imagine what difference it makes, a few degrees plus or minus around the bow.

[goboatingnow]

Originally Posted by Dockhead

When you say "the effect of pinching is reduced if you . . . come up slightly to windward" -- what exactly do you mean?

What effect occurs in a few degrees from dead on the bow to slightly to windward, which doesn't occur in the same number of degrees in any other direction?

Your diagram doesn't imply any kind of abrupt transition at the bow or any other place -- just vectors.

To put it another way -- if you get a lift from having the current a few degrees to leeward of the bow, this implies that you get a knock from having the current a few degrees to weather. Where does this knock come from? I cannot imagine what difference it makes, a few degrees plus or minus around the bow.

As SL p demonstrated a while ago. Having the current dead ahead maximises wind angle ( intuitively because the resulting current ground wind is increasing the apparent wind angle.

And yes you are correct , the benefit is improvement in apparent wind angle reduction as a result of a given heading change ( well all clear on the effect right ) can happen so that the current does not always end up on the lee side. Doesn't invalidate the single situation where it does and gets called LBE.

Ps lets dump the term " knock " there is no nautical usage of the term. it's lift or headed. Wrong use of terminology has bedevilled this thread.


The reverse corollary, is that on bearing away ( which happens to bring the current to the bow) , the ratio of improvement in apparent wind angle ( against heading change angle) is not as great as the situation , where the current did not exist. But as I stated before , turning to loo ward , the effect is utterly overwhelmed by the improvement in boat polars. The " effect " reverses but is lost in other changes

Dave

[Dockhead]

Originally Posted by goboatingnow

As SL p demonstrated a while ago. Having the current dead ahead maximises wind angle ( intuitively because the resulting current ground wind is increasing the apparent wind angle.

And yes you are correct , the benefit is improvement in apparent wind angle reduction as a result of a given heading change ( well all clear on the effect right ) can happen so that the current does not always end up on the lee side. Doesn't invalidate the single situation where it does and gets called LBE.

Ps lets dump the term " knock " there is no nautical usage of the term. it's lift or headed. Wrong use of terminology has bedevilled this thread.


The reverse corollary, is that on bearing away ( which happens to bring the current to the bow) , the ratio of improvement in apparent wind angle ( against heading change angle) is not as great as the situation , where the current did not exist. But as I stated before , turning to loo ward , the effect is utterly overwhelmed by the improvement in boat polars. The " effect " reverses but is lost in other changes

Dave

OK, so if there is no abrupt, magical transition at the bow, when the current goes from slightly on the windward side to slightly on the lee side, then how can it ever be worth the lost VMG to windward of pinching?

You have not said anything that gives me any basis to change my mind, that the "fine on the lee bow" version of the Effect is a myth based on the idea that the current is pushing the keel up.

So far, the only flavor of Lee Bow Effect that seems to have any validity is the one which concerns tacking strategy in a changing current, which is really just a specific application of the "sail the lifted tack" rule.

The "fine on the lee bow" variant of the Lee Bow Effect theory cannot work without an abrupt transition due to non-existent hydrodynamic effects. No "effect" can be found in any of your diagrams, which merely show the smooth changing of vectors as variables are changed. There is no effect in that; just regular sailing.

Unless of course I'm still missing something, but I really don't think I am.

[Dockhead]

As to "knock", by the way --

I've got nothing at all against "header" and "headed", of course.

But "knock" is proper racing terminology, at least in Yankeeland. You would have an argument, of course, that this is not "English" , a language which I perhaps should stop claiming to speak

[goboatingnow]

Originally Posted by Dockhead

OK, so if there is no abrupt, magical transition at the bow, when the current goes from slightly on the windward side to slightly on the lee side, then how can it ever be worth the lost VMG to windward of pinching?

You have not said anything that gives me any basis to change my mind, that the "fine on the lee bow" version of the Effect is a myth based on the idea that the current is pushing the keel up.

So far, the only flavor of Lee Bow Effect that seems to have any validity is the one which concerns tacking strategy in a changing current, which is really just a specific application of the "sail the lifted tack" rule.

The "fine on the lee bow" variant of the Lee Bow Effect theory cannot work without an abrupt transition due to non-existent hydrodynamic effects. No "effect" can be found in any of your diagrams, which merely show the smooth changing of vectors as variables are changed. There is no effect in that; just regular sailing.

Unless of course I'm still missing something, but I really don't think I am.

Ok let me explain

(1) LBE in my mind has never had anything to do with pushing the keel to weather , it has to do with the improvement in wind angles ratios as a result of a particular current

(2) the effect is that , the reduction in apparent wind angle lost that occurs for a given windward heading change over the same apparent wind angle loss due to windward heading change without any current

(3) we all agree that that effect occurs elsewhere on the compass , and does not " require" the current to end up on the lee side, nor is there any abrupt change as the current crosses the bow ( which would be nonsense if it did )

(4) however the effect DOES occur in the situation where the current " ends" up fine on the lee bow. That alone removes the debunking of the myth.

(5) The effect does occur , the fact that it occurs elsewhere on other headings does not invalidate the fact that it occurs in that specific case

(6) pinching is not germane to the case , the effect occurs even when not pinching

(7) the total effect and ability to use the effect to gain advantage is not being determined here, it's not called the lee bow advantage


Dave

[goboatingnow]

Originally Posted by Dockhead

As to "knock", by the way --

I've got nothing at all against "header" and "headed", of course.

But "knock" is proper racing terminology, at least in Yankeeland. You would have an argument, of course, that this is not "English" , a language which I perhaps should stop claiming to speak

Dammed colonials , look at the damage that fool Cornwallis did !!

I have heard the term " knocked " off course, but inn relation to a wind shift , it's always called a header

Dave

[Dockhead]

Originally Posted by goboatingnow

Dammed colonials , look at the damage that fool Cornwallis did !!

I have heard the term " knocked " off course, but inn relation to a wind shift , it's always called a header

Dave

LOL

In our perverted parlance , a "knock" is the opposite of a "lift". It's any change in the environment which worsens your situation. When speaking specifically about wind shifts, yes, we sometimes say "header". So for us it would normal to say something like "The wind threw us a header just as we passed the point; it turned out to be quite a nasty knock for us."

For example, see here: Lifts and Knocks