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LEE BOW EFFECT

Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Technique
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Topic: LEE BOW EFFECT
Posted By: didlydon
Subject: LEE BOW EFFECT
Date Posted: 11 Oct 11 at 1:58pm
It's probably been discussed here before, but someone in my club mentioned the Lee Bow Effect after racing on a light breeze day when the tide was running strongly...... I just nodded & thought I understood, but to be honest didn't really.... So can anyone explain it to me please? Thanx.Geek

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Vareo 365




Replies:
Posted By: rb_stretch
Date Posted: 11 Oct 11 at 2:05pm
My understanding is that when beating against the tide, if the tide is coming at your bow from the lee side then it is effectively pushing you into the wind. This is good because firstly it counteracts leeway and secondly gives you stronger apparent wind as you get pushed into the wind. Sometimes the difference between stuffing and footing is the lee bow effect, which therefore makes it pay to stuff.

Obviously if tide is on the windward side upwind then you are fighting both wind and tide, so can sail anyway near as high or fast.



Posted By: didlydon
Date Posted: 11 Oct 11 at 2:13pm
Stuffing & footing Pinch............ Please explain as well.......

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Vareo 365



Posted By: Jack Sparrow
Date Posted: 11 Oct 11 at 2:22pm
Stuffing (pinching) into wind for height. Footing... Easing off the wind for speed.

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Posted By: didlydon
Date Posted: 11 Oct 11 at 2:33pm
Ahhhhh..... Thanx! I do know how to sail - honest - just not familiar with the (Slang?) terminology.

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Vareo 365



Posted By: JimC
Date Posted: 11 Oct 11 at 2:44pm
The lee bow thing is largely a myth...

The theory goes that if you point up so that the tide is one degree on the lee bow instead of 1 degree on the windward bow then you'll magically go much better. Of course if you sail your [expurgated] off so that you point two degrees higher *without slowing down* then you don't need any complicated theories about tide to explain why you're suddenly going better up the beat...

Yes, the tide pushes you across the track, but the apparent wind clocks back the other way as a result. Nothing special happens when the tide goes from 0.1 degrees one side to 0.1 degrees the other. It takes ages and a lot of drawing of force triangles to explain why, but that's what happens.

Remember all that counts is what the wind is doing relative to the sails and the water is doing relative to the foils. The boat hasn't got a clue what the sand on the bottom of the river is doing... In tide the effective wind direction and speed is not what you get in a moored boat, but what you get in a boat drifting in the current.

Of course if the tide is not constant in speed and direction across the race track there are enormous gains to be made by exploiting the differences, but that's something else again: not what was traditionally called the lee bow effect.


Posted By: Oli
Date Posted: 11 Oct 11 at 3:19pm
jim my understanding of it is that i dont point higher with a tidal lee bow but for the same tacking angle i can effectively sail a shorter distance to the mark as the tide is pushing me into the "as the crow flys" line (crabbing).  a velocity diagram may show you as sailing slower but at a better angle, its all down to vmg to the mark as to whether its beneficial.

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Posted By: Rupert
Date Posted: 11 Oct 11 at 4:17pm
The lee bow effect is one of those things like asymmetric spinnakers lifting the bow - easy to disprove with science, but less easy to explain why it still appears to work in real life!

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Firefly 2324, Lightning 130, Puffin 229, Minisail 3446


Posted By: G.R.F.
Date Posted: 11 Oct 11 at 4:27pm
Lee Bow is my particular expertise, have had arguments with Olympics coaches then demonstrated my point by taking ten minutes, yes ten minutes out of the entire Olympic Squad in one instance by it's correct application as against the advice the coaches gave to the squad, a delicious moment.

Anyone who doubts it's effect hasn't had thirty years of tidal racing and sailing, including that muppet that tried to write a lame book debunking it.

On a sailboard you actually get to 'feel' the effect it has on the driving forward force of the board, it is very much less in a dinghy.

The other Lee Bow effect is that force prevalent at the start of the race when all the boats (boards) are lined up, the sail on boat to your lee bow, if allowed to creep ahead, diverts the wind between both boats and effectively heads the windward boat. So you fight to get your nose ahead, to stuff the guy upwind of you with lee bow, whilst covering the boat to leeward, it's how the fleet divides by approx 1/3rd within minutes of the gun firing.

Back to the tide, leaving the line, imperative to take the tide on the lee bow initially to help speed if at all possible, even though the water you're on should be thought of as a conveyor belt, the boats speed over it is not the same. (assuming in this instance a left to right tide within which tidal lee bow would play a part.

Kind of difficult doing this in words, sailing in tide is an entire lecture in it's own right and events get won and lost by the correct application and timing of when to 'go for' tide on the lee. Tide on the lee bow can be like a lift whereas tide on the weather bow is almost certainly heading. Just as with wind shifts there are times to play either or, depending on your position in the fleet at any given moment.


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Posted By: Presuming Ed
Date Posted: 11 Oct 11 at 4:38pm
THERE IS NO LEE-BOW EFFECT - Dave Perry
One of the most fascinating and timeless controversies in our sport is over what effect current has on how we sail and race our boats. Beginning in early 1979, Peter Isler and I filled hours of time debating the effects of current, and it wasn't until mid-1980 that he finally parted my clouds and shook me loose from years of misconceptions and incorrect assumptions. Here then is my understanding of the effects of current, substantiated by several of my more mathematically-clever friends.

Assuming that we're sailing in constant current direction and strength, No! As we've determined, the direction and strength of the current created wind is the same no matter at what angle the boat is aiming or at what speed it is moving. The presumption of the lee-bow effect is that if you are sailing directly into the current you can pinch slightly, putting the current on your leeward bow, and the current will push you up to weather. This is obviously false because the only direction the current can move you is in the direction it is going (the stick on the river).

The presumption of those who believe that in current a boat will have a different apparent wind direction and strength on opposite tacks, is that on one tack the boat will be slowed more by the current than on the other. The extreme example is when port tack takes you right into the current, and starboard tack takes you across it. The illusion is that on port tack it would seem that the boat is still going forward toward the wind, but that on starboard the boat is being swept away from the wind by the current. Therefore, the apparent winds must be different on the two tacks.

The fallacy here, though, is that the judgment of going toward the wind and being swept away are made in reference to fixed objects such as the mark, an anchored boat, or land. In reality, both boats are being affected equally by the current and the wind "sees" both boats in the same way. In other words, if you were following the race in a motorboat and were in the ocean where you couldn't see any land for reference, the boats would look identical on either tack, and in fact you would have no clue that there even was current unless you knew from charts or perhaps from the surface condition of the water. Put another way, if you're sailing on a boat with wind strength and direction instruments, they'll read the same on both tacks because the boat is affected in the same way by the current on either tacks (the stick in the river again).
-- Excerpt from Winning in One-Designs by Dave Perry,
http://www.ussailing.org/member/library/wiodcurrent.htm

Of course, there is an argument that if your opposition _do_ believe in the "lee bow effect", then the last thing you actually want to do is disabuse them of their belief. If they believe that stuffing the bow up and sailing slowly is the best way to win a race, then who are we to try and dissuade them....


Posted By: JimC
Date Posted: 11 Oct 11 at 4:45pm
One of the main sources of confusion in this topic is that not everyone's talking about the same thing...


Posted By: rb_stretch
Date Posted: 11 Oct 11 at 4:46pm
Just to add to others, it's definitely very real. I've mostly thought about it in keel boats. As anyway knows tide rarely goes in a straight line for any distance near shores. The navigator will often be thinking about the course that maximises any lee bow effect and minimises the opposite. Of course you are still having to balance that against tidal strength, eddies, gates etc for overall maximum gain, but it is one of the factors.




Posted By: tgruitt
Date Posted: 11 Oct 11 at 4:59pm
Originally posted by JimC

One of the main sources of confusion in this topic is that not everyone's talking about the same thing...


I always thought a lee bow was tacking under someone and being sucked up to windward a bit by the air rushing between the close proximity of the rigs?


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Posted By: rb_stretch
Date Posted: 11 Oct 11 at 5:08pm
I don't type fast enough and just seen the other answers. I think there is probably is confusion about the definition. I think the point about just pointing a little higher making a difference is probably the myth part as I kind of recycled it as I heard it.

For me the bit I experience as real is the difference between a tide coming in on the lee bow versus the windward bow. That's why you try on be on the tack with lee bow rather than the opposite.




Posted By: rb_stretch
Date Posted: 11 Oct 11 at 5:12pm
Originally posted by tgruitt

Originally posted by JimC

One of the main sources of confusion in this topic is that not everyone's talking about the same thing...


I always thought a lee bow was tacking under someone and being sucked up to windward a bit by the air rushing between the close proximity of the rigs?


Have to say I've only heard lee bow in reference to tide. I believe the air ahead of a boat is only affected for a very short distance so you would have to be incredibly close, like the distance between main and jib.

Having someone just under your lee bow I always assumed to be the start of when you are getting dirty air...




Posted By: G.R.F.
Date Posted: 11 Oct 11 at 5:17pm
Perry, that's the chap, be cool to race against him in some tidal venues..

One point he makes is true, pinching rarely helps, not even to just get the tide on the knock because it doesn't help what you're looking for. Which is an increased power supply by combining the true wind with the created tidal wind thus increasing the apparent wind on one tack. The water may well be moving over the bottom and away from the mark, but one of the boats on it will be moving faster over that water than the other simply down to the increase in wind - sure they'll have to tack, but where and when, becomes important. (the tide might be faster offshore than inshore, the wind might be less inshore than out to sea, the current very rarely is acting constantly over the entire course (you tell this by looking at the water).

Trust me, he's wrong.

We once sailed an entire windless race without pumping, up the Thames, entirely on the wind created by the currents movement against still air - that's the effect you're looking for, created tidal wind to combine with the true wind, think of it that way.. In that instance it was a favourable lee bow tide on both tacks, the guys that did better were those that spotted where the current was fastest (on the inside of the bends).

Many a day back home in a drifter, we sail home choosing the tack that puts the tide on the lee bow, those that don't just drift downtide. (You can't tell some folk)


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Posted By: rb_stretch
Date Posted: 11 Oct 11 at 5:38pm
Glad it isn't just me who has experienced this as real!


Posted By: r2d2
Date Posted: 11 Oct 11 at 5:41pm
Originally posted by rb_stretch

Originally posted by tgruitt

Originally posted by JimC

One of the main sources of confusion in this topic is that not everyone's talking about the same thing...


I always thought a lee bow was tacking under someone and being sucked up to windward a bit by the air rushing between the close proximity of the rigs?


Have to say I've only heard lee bow in reference to tide. I believe the air ahead of a boat is only affected for a very short distance so you would have to be incredibly close, like the distance between main and jib.

Having someone just under your lee bow I always assumed to be the start of when you are getting dirty air...


I have heard both the tide thing and the issue of tacking just under another boats lee bow. 
 
I understood that the idea of tacking just under another boats lee bow was that it slowed them down enough either to make them tack off or enough so that after a short while you can then tack across them
 


Posted By: Rupert
Date Posted: 11 Oct 11 at 5:45pm
You are talking about 2 totally different things, Jim and GRF. Of course where you are in the tide has a massive effect, but the very specific effect of the tide pushing just on the lee side of your bow having a magic effect compared to it coming straight on is what the lee bow effect is, so making pointing very slightly higher worthwhile.

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Posted By: Presuming Ed
Date Posted: 11 Oct 11 at 5:45pm
Originally posted by rb_stretch

Just to add to others, it's definitely very real. I've mostly thought about it in keel boats. As anyway knows tide rarely goes in a straight line for any distance near shores. The navigator will often be thinking about the course that maximises any lee bow effect and minimises the opposite. Of course you are still having to balance that against tidal strength, eddies, gates etc for overall maximum gain, but it is one of the factors.

That's different. If the tidal flow varies across the course, then obviously the wind shifts resulting from the tide will vary across the course, and a good (or even semi competent) navigator should take advantage of that - looking for the maximal tidal lifts, and minimising the knocks. 

If, however, the tide is even across the area of the course, then the lift you get on one tack (relative to the ground wind) will be offset by the knock you get on the other tack (again, relative to ground wind). What you gain on the swings, you lose on the roundabouts. 


Posted By: Presuming Ed
Date Posted: 11 Oct 11 at 5:52pm
Originally posted by Rupert

You are talking about 2 totally different things, Jim and GRF. Of course where you are in the tide has a massive effect, but the very specific effect of the tide pushing just on the lee side of your bow having a magic effect compared to it coming straight on is what the lee bow effect is, so making pointing very slightly higher worthwhile. 

And there was me thinking that the lift/drag curve for a particular foil was a fixed thing. A foil of NCAA section xxxx at an angle of attack (= leeway angle) of x will generate lift of F and drag of D. All physics. 

If you're saying that there is some magic way the keel becomes "aware" of the angle between the tidaf flow and the seabed, then could you tell me where the brain in the keel is because I haven't found one yet. Also, if there is some way we can "switch on" this "extra lift", the again, that would be really useful because I'll just switch it on all the time. 


Posted By: Presuming Ed
Date Posted: 11 Oct 11 at 5:55pm
Originally posted by tgruitt

Originally posted by JimC

One of the main sources of confusion in this topic is that not everyone's talking about the same thing...
I always thought a lee bow was tacking under someone and being sucked up to windward a bit by the air rushing between the close proximity of the rigs?

IME, it's not that the boat ahead is lifted, it's that the boat behind is knocked. In the same way that the main has to be sheeted closer than the jib, because it's knocked by the jib. I've always called tacking into someone's lee bow "lee bow-ing" someone, never the "lee bow effect".

To me the "lee bow effect" is this mythical thing about magically generating extra lift out of nowhere. 


Posted By: Presuming Ed
Date Posted: 11 Oct 11 at 6:07pm
Originally posted by G.R.F.

Perry, that's the chap, be cool to race against him in some tidal venues..

Well, he does have a rough idea how to sail.... http://www.sailing.org/biog.php?id=USADP4 - http://www.sailing.org/biog.php?id=USADP4


Posted By: Rupert
Date Posted: 11 Oct 11 at 6:09pm
Originally posted by Presuming Ed

Originally posted by Rupert

You are talking about 2 totally different things, Jim and GRF. Of course where you are in the tide has a massive effect, but the very specific effect of the tide pushing just on the lee side of your bow having a magic effect compared to it coming straight on is what the lee bow effect is, so making pointing very slightly higher worthwhile. 

And there was me thinking that the lift/drag curve for a particular foil was a fixed thing. A foil of NCAA section xxxx at an angle of attack (= leeway angle) of x will generate lift of F and drag of D. All physics. 

If you're saying that there is some magic way the keel becomes "aware" of the angle between the tidaf flow and the seabed, then could you tell me where the brain in the keel is. I haven't found one yet. Also, if there is some way we can "switch on" this "extra lift", the again, that would be really useful because I'll just switch it on all the time. 
Not me saying it, just trying to define what we are discussing - pointing too high into tide sounds like a dumb thing to do. In fact, as someone who grew up sailing on rivers with current, the only way of making ground faster against current is to get where it is less, and I assume tide works just the same.

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Posted By: RS400atC
Date Posted: 11 Oct 11 at 6:19pm
The lee bow effect debate is largely as JimC says, people talking at cross purposes.

Some people cannot add up the vectors to understand how tide affects your VMG
Some people forget the mark is moving through the tide
The deniers can't explain what they are denying because the proponents can't explain it properly either, usually because they are working on a badly worded explanation in a sailing book that was not written by a mathematician.

There is nothing magic about it, but a tide on your bow definitely has an effect.
What seems to a science graduate as the boring predictable unsurprisng addition of vectors is the same magic effect that surprises people who can't add vectors.
It's also used to explain any inconvenient/remarkable divergence of courses.

A couple of seasons sailing leadmines, well iron mines in fact, in the solent help you realise that the tide is always significant, never constant and doesn't obey the little charts they sell.


Posted By: SoggyBadger
Date Posted: 11 Oct 11 at 6:21pm
Originally posted by Presuming Ed

IME, it's not that the boat ahead is lifted, it's that the boat behind is knocked. In the same way that the main has to be sheeted closer than the jib, because it's knocked by the jib. I've always called tacking into someone's lee bow "lee bow-ing" someone, never the "lee bow effect".


Actually, the leeward boat is lifted slightly WRT to the apparent wind and also experiences a slight increase in wind speed.


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Posted By: JimC
Date Posted: 11 Oct 11 at 7:00pm
These sorts of discussions probably feel really intimidating to beginners, but they should in fact be enormously encouraging, because they so often demonstrate that a lack of understanding of the underlying science is frequently no obstruction to sailing a boat fast...

I have to admit I've never really quite understood what GRF is getting at with respect to tides and lee bows: I have no idea whether he's saying the same thing as me or something quite different.


Posted By: Presuming Ed
Date Posted: 11 Oct 11 at 7:01pm
I've never really quite understood what GRF is getting at with respect to pretty much anything. 

I'll get me coat. 


Posted By: Rockhopper
Date Posted: 11 Oct 11 at 7:49pm
Don just come over and join us when the winds from the south or south east its perfect for that lee bowing the tide


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Posted By: didlydon
Date Posted: 11 Oct 11 at 8:36pm
Ok.... Thanks everyone for the really informative repliesThumbs Up It's just as I thought...... Bloody complicated to understand, but there is something definitely there. I can feel it when pushing into the tide (& breeze)... It's just working out how to harness it to your advantage to either make the mark without tacking again or getting that extra bit of "lift" & catching the bloke in front by surprise. So can anyone suggest a practical way of demonstrating it's effect to yourself when out practising so it's effect or otherwise is really noticeable? 




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Vareo 365



Posted By: sargesail
Date Posted: 11 Oct 11 at 9:50pm
I find myself in two minds on this.

I believe that the tide is a moving carpet and that it should make no difference whether the tide is on my lee or weather bow.

But I find that I still use lee bow theory to gain an advantage - given equal other factors I will take the lee bow in the stronger tide.  Unless other factors come in to play I have never known this fail.

And yet I have just spent half an hour doing the vectors to comprehensively proof I'm talking rubbish!

But that doesn't help me explain why it feels so much faster when I put the tide on the lee bow with a liitle squeeze!

AAAAARRRRGGGGHHH!

But I observe that 


Posted By: Presuming Ed
Date Posted: 11 Oct 11 at 10:26pm
If you concentrate really hard, and put that extra effort into hiking, to get the bow up so that you can "take the lee bow", then you might find yourself sailing higher at the same speed as your competitors. 

Of course, if you're sailing inland in zero current, and concentrate really hard, and put extra effort into hiking, so that you're pointing higher than your opposition but going the same speed as them, then you might just find yourself doing pretty well also. 

A speed or height advantage make things much easier. Speed AND height..... 


Posted By: Medway Maniac
Date Posted: 12 Oct 11 at 12:24am
Originally posted by sargesail



But I find that I still use lee bow theory to gain an advantage - given equal other factors I will take the lee bow in the stronger tide. 

This makes sense without any magical effect. You will get a lifting windshift from a lee-bow tide compared to sailing in no, or less tide. So it makes sense to sail with the tide on the lee bow when you are in the stronger tide, and then to sail on the other, headed tack when you are in less tide.

But this is a result of sailing in tide which is uneven across the course. All you are doing is, in effect, playing the shifts as you experience them. If the tide is the same strength everywhere (actually rarely the case in practice) it makes no difference which tack you take first.


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Posted By: sargesail
Date Posted: 12 Oct 11 at 8:03am
Maniac,

Except that when you try and prove that it doesn't work.

And again I agree with your last point - but that doesn't explain why in those circumstances one "feels" the lee bow effect.


Posted By: Rockhopper
Date Posted: 12 Oct 11 at 8:14am
The tide at our place is never the same strengh over the course we have rips when the ground is say five six feet shallow than other areas creating more tide closer to the shore you get less tide and to top it all off on the back of the pier the tide goes the opposite way Shocked
It does take a while to learn the tides and how and when to use the lew bow effect but ti does work wonders in three knots of tide


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Posted By: Pierre
Date Posted: 12 Oct 11 at 8:15am
Can someone please draw me a picture?



Posted By: osproject
Date Posted: 12 Oct 11 at 8:58am
You wont physically point higher. You can only sail as high as your sails will let you. But your COG will be higher. Try sailing in 7knots of tide and see what happens on opposite tacks. :-)


Posted By: didlydon
Date Posted: 12 Oct 11 at 9:13am
Would I be right in saying that you'd get extra "lift" on one tack with the action of the additional tide flow over the foils pulling them to windward? Whereas, on the other tack, you wouldn't cos the tide will be pushing you away?  We need some sort of animation / pictures to show this.... anybody?Ermm

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Vareo 365



Posted By: 2547
Date Posted: 12 Oct 11 at 9:49am

Definition: "The lee-bow effect" is a term used to describe what may or may not happen when sailing directly into tide upwind and what may or may not happen if you pinch up to put the tide on your lee bow ... assume linear tidal flow over the area.

Some silly gibbons think something magical happens if you pinch up and have the tide on the lee bow as to opposed to footing off and having it on the weather bow. These are the same people who are deluded enough to try and invent perpetual motion machines Wink

Clever monkeys like JimC and Mr. Perry know how to add vectors and know it does not exist.

Now of course it is a jungle out there and clever monkeys know that the tide is often stronger or weaker in different places and taking advantage of this can make huge differences but this is not some juju magic of the mythical leebow effect ...

I think Perry's mid ocean example should be able to be understood be even the most feeble minded gibbons.






Posted By: G.R.F.
Date Posted: 12 Oct 11 at 4:15pm
OK just for the slow (and probably ever so slightly retarded) I will explain why, even in a constant tide, it is better to take the lee bow first, if it is on offer.

Read any tactical book, they'll all explain about getting away in clean air at the start, and as I used to say during those heady days when people would queue for hours just to hear me speak on the subject, in a 60 strong start line, make any kind of mistake at the gun and potentially you can lose 59 places in one seconds hesitation, make that same mistake at the weather mark and you might not even lose one place.

So at the start there are two tacks you can take (assuming you can get out the back door of the pin end) there is say a 5 knot cross tide (And ignoring any time/tidal calculation you might have made re time of the beat and virtual position of the weather mark in relation to the end of the line you start at.) there is only 5knots true wind, so on the one tack lets say starboard with the tide going right to left across the course, with the water heading your way you'll be going down tide at five knots and <insert appropriate tide v wind calculation to establish boat speed over the water> say six knots over the water, whereas on the other tack you'll be maintaining station over the bottom but making windward ground at <insert calculation of combined forces of five knots tide and five knots wind, say nine knots, so those elements of the fleet that have taken the down tide tack first will be travelling faster over the bottom sideways, but slower through the water toward the weather mark than the boats that take the uptide tack initially, they move slower sideways over the bottom but make ground to weather and over the water faster, so at the half way mark are closer to their final destination.

OK, logic says yeah but.. the reverse will be true in the second leg, but hey you're a tactical racer, where would you rather be at the half way mark? And how many races have you ever experienced where conditions remain exactly constant throughout an entire beat, never mind the whole race.

Sure in deep water Ocean racing with miles of mind numbing boredom going on, it may make no odds, but this is about the cut and thrust of small boat tactical division by error and he who makes least wins..

I know what I'd do.


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Posted By: JimC
Date Posted: 12 Oct 11 at 4:36pm
OK, GRF I think I see what you are talking about... To my mind that's an entirely different thing to the classic "lee bow/tide" fallacy. Am I right in thinking you're saying there's (often?) a tactical advantage to taking the tack that gets you nearest to the mark most quickly...


Posted By: 2547
Date Posted: 12 Oct 11 at 5:42pm
Poor old GRP gibbon ... he's having a bit of trouble with his maths ...

If you have a true wind of 5 knots (i.e. the wind over the ground) and a 5 knot cross tide (i.e. 90 degrees to the true wind running left to right) you have a apparent wind of 7.07 knots which is 45 degress different to the true wind ... in the gibbons example you sail straight to the windward mark on starboard ... if it's laid square to the line by a race officer who has laid a windward leg to the true wind as he is anchored to the land and isn't experienceing the apparent wind which all sail boats do ...



This example is nothing to do with the lee bow effect .... which as JimC says is a fallacy ...


Posted By: I luv Wight
Date Posted: 12 Oct 11 at 6:11pm
But doesn't the RO notice the tidal current, and offset the windward mark?

It can be good to 'lee-bow ' the tide, but only because the tide is always variable across the course.

If you can stay close to the shore in weaker current by pinching a bit, it does gain a lot over those who foot off into stronger adverse current, - reducing the apparent wind, so they sag off even more - and they also have to do extra tacks.


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Posted By: rb_stretch
Date Posted: 12 Oct 11 at 6:34pm
Originally posted by seamonkey



This example is nothing to do with the lee bow effect .... which as JimC says is a fallacy ...


Clearly there are different definitions around as I've always used it and been around people who have used it to describe tide coming in on your lee bow and pushing you into the wind.


Posted By: 2547
Date Posted: 12 Oct 11 at 7:42pm
Originally posted by I luv Wight

But doesn't the RO notice the tidal current, and offset the windward mark?
if the course is square to the apparent wind then it dosn't make any difference.
 
The other examples you describe are just tactical tidal issues to manage.
 
Of course current is a key tactical issue but this thread is about the so called lee bow effect


Posted By: G.R.F.
Date Posted: 12 Oct 11 at 7:47pm
Originally posted by JimC

OK, GRF I think I see what you are talking about... To my mind that's an entirely different thing to the classic "lee bow/tide" fallacy. Am I right in thinking you're saying there's (often?) a tactical advantage to taking the tack that gets you nearest to the mark most quickly...

Eh? come on Jim, don't you be a chump.. It's not a fallacy, tide on your lee bow assists you, in some circumstances if the RO doesn't spot it, or it comes on during an event (say over lunch) you can single tack a beat and even then some idiots go the other way...

Tell me you'd go the right way now, i refuse to believe you don't 'get it'.

It's written large by Eric Twiname in start to win I'll wager..

I'm going to find that book and have a look.

I can't believe any of you are even doubting it.




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Posted By: JohnW
Date Posted: 12 Oct 11 at 8:04pm
Originally posted by G.R.F.

 
...... so those elements of the fleet that have taken the down tide tack first will be travelling faster over the bottom sideways, but slower through the water toward the weather mark than the boats that take the uptide tack initially, they move slower sideways over the bottom but make ground to weather and over the water faster, so at the half way mark are closer to their final destination.


At the half way mark you are closer to the final destination over the ground, but you are no nearer the mark through the water - the other guys get the tidal benefit when they tack as you start loosing out.

However...

Originally posted by G.R.F.

Originally posted by JimC

OK, GRF I think I see what you are talking about... To my mind that's an entirely different thing to the classic "lee bow/tide" fallacy. Am I right in thinking you're saying there's (often?) a tactical advantage to taking the tack that gets you nearest to the mark most quickly...

Eh? come on Jim, don't you be a chump.. It's not a fallacy, tide on your lee bow assists you, in some circumstances if the RO doesn't spot it, or it comes on during an event (say over lunch) you can single tack a beat and even then some idiots go the other way...



As above, it does make tactical sense, in the same way that it is (generally) better to take the long tack first on a very one sided beat, to insure against over standing the lay-line from a long way out.





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Posted By: sargesail
Date Posted: 12 Oct 11 at 8:08pm
Exactly - not lee bow but a tidal consideration.



Posted By: G.R.F.
Date Posted: 12 Oct 11 at 8:09pm
No, go back to my scenario.

Taking the lee bow effect tack first, straight away, lets say the tide eases for the 2nd part of the beat.. = you're ahead you win.

Lets say it doesn't, it increases, they've gone too far = they over stand.

Tide is never ever constant.


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Posted By: JimC
Date Posted: 12 Oct 11 at 8:15pm
Originally posted by G.R.F.

It's not a fallacy, tide on your lee bow assists you, in some circumstances if the RO doesn't spot it, or it comes on during

Same term, different meanings... it sounds as if you've never come across the traditional lee bow fallacy.

That says that if the tide is more or less bang on the nose you should pinch up so that the tide is fractionally on the lee bow because even though you'll go more slowly the tide will push you up to windward.

Its nothing to do with the choice of which tack is favoured and thus which you should be on, which is what I believe you are talking about.


Posted By: G.R.F.
Date Posted: 12 Oct 11 at 8:17pm
In the meanwhile I've had another look at Eric Twinames section on tide, he does deal with it, but he also misses the essential part of the tidal equation, which maybe back in those days wasn't as relevant and probably because the stuff we sail amplifies the effect (Literally a favourable tide in certain conditions can make the difference between planing up wind and displacement speed).

Wether you believe what I'm saying or not and lets face it the back end of the fleet is full of doubting thomases, it is a fact as sure as day follows night.

I'm available for lectures on tactical sailing in tidal situations...


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Posted By: Peaky
Date Posted: 12 Oct 11 at 8:20pm
Originally posted by rb_stretch


Originally posted by seamonkey


This example is nothing to do with the lee bow effect .... which as JimC says is a fallacy ...


Clearly there are different definitions around as I've always used it and been around people who have used it to describe tide coming in on your lee bow and pushing you into the wind.

That is the fallacy. The orientation of the boat's hull does not affect the direction it gets "pushed" by the current.

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Posted By: rb_stretch
Date Posted: 12 Oct 11 at 8:46pm
Originally posted by Peaky

 
That is the fallacy. The orientation of the boat's hull does not affect the direction it gets "pushed" by the current.


It depends on the angles. As soon as there is any component of the tide against the wind, you will get lifted. Basically means difference between tide and wind needs to be greater than 90 degrees and you will get some Lee Bow effect. At least in the definition I know.....


Posted By: G.R.F.
Date Posted: 12 Oct 11 at 9:30pm
It's not to do with the boat or it's hull especially, it's the effect on the sail, via the increased pressure on the foil. The term is just a slang definition for ease of explanation.. er to the ESSR which it seems most dinghy folk appear to be at times..

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Posted By: sargesail
Date Posted: 12 Oct 11 at 9:31pm
No it doesn't!  The apparent wind is a combination of a moving volume of air (True or Ground wind) and a moving carpet (OK volume but in practice it's 2D) of tide (the tidal wind), and your boat speed and your angle to the wind (I'll call it induced wind).   Pinching or footing changes the induced wind but it can not change the tidal wind.




Posted By: sargesail
Date Posted: 12 Oct 11 at 9:35pm
Originally posted by G.R.F.

No, go back to my scenario.

Taking the lee bow effect tack first, straight away, lets say the tide eases for the 2nd part of the beat.. = you're ahead you win.

Lets say it doesn't, it increases, they've gone too far = they over stand.

Tide is never ever constant.

Taking the lee bow tide first, or lee bowing in the strongest tide is just a way of relating another of the great tactical rules - "Sail the major, dominant, long tack first"  - to tidal factors.


Posted By: sargesail
Date Posted: 12 Oct 11 at 9:36pm
But we need someone clever to do some vectors!


Posted By: G.R.F.
Date Posted: 12 Oct 11 at 9:50pm
Originally posted by sargesail

No it doesn't!  The apparent wind is a combination of a moving volume of air (True or Ground wind) and a moving carpet (OK volume but in practice it's 2D) of tide (the tidal wind), and your boat speed and your angle to the wind (I'll call it induced wind).   Pinching or footing changes the induced wind but it can not change the tidal wind.



dude.. sailing 101.

True wind generates movement.
Movement generates "created wind" opposite to movement direction
Both True and Created combined = Apparent wind.


So
Favorable tide on lee bow increases apparent wind by increased vector pressure on true wind
Tide on weather bow decreases apparent wind inversely.

Edited for accuracy..


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Posted By: RS400atC
Date Posted: 12 Oct 11 at 10:01pm
Originally posted by JimC

....
Same term, different meanings... it sounds as if you've never come across the traditional lee bow fallacy.

That says that if the tide is more or less bang on the nose you should pinch up so that the tide is fractionally on the lee bow because even though you'll go more slowly the tide will push you up to windward.

Its nothing to do with the choice of which tack is favoured and thus which you should be on, which is what I believe you are talking about.


Quite succinctly put, Jim.


One element that messes with your head is the fact that because the boat is slow relative to the ground, the apparent wind does not move forward as you expect it to, therefore you seem to be able to pinch without slowing the boat.
This feels like a strange effect, which some people seem to roll into 'the lee bow magick'
We tend to think in the 'land and marks' frame of reference, whereas the boat is operating in the water frame.


Posted By: sargesail
Date Posted: 12 Oct 11 at 10:37pm
+1 Yep that explains the phenonomen I experience.


Posted By: sargesail
Date Posted: 12 Oct 11 at 10:54pm
Originally posted by G.R.F.

Originally posted by sargesail

No it doesn't!  The apparent wind is a combination of a moving volume of air (True or Ground wind) and a moving carpet (OK volume but in practice it's 2D) of tide (the tidal wind), and your boat speed and your angle to the wind (I'll call it induced wind).   Pinching or footing changes the induced wind but it can not change the tidal wind.



dude.. sailing 101.

True wind generates movement.
Movement generates "created wind" opposite to movement direction
Both True and Created combined = Apparent wind.


So
Favorable tide on lee bow increases apparent wind by increased vector pressure on true wind
Tide on weather bow decreases apparent wind inversely.

Edited for accuracy..

Dude - more accurate but still wrong.  

I wish we were drinking beer so that we could karate sail this or use tablecloths and or napkins to show the moving carpet effect of the tide.  Thus allowing me to prove to my satisfaction, but probably not yours (I was a persistent heretic once too) that the angle of the boat's motion to the tide is irrelevant in determining the "vector pressure".

Try it this way: it would be more accurate to say apparent (the wind felt by the boat) wind generates motion, but for a boat at rest movement generated wind is nil.

Now let's stop the boat from being at rest by adding some tide.  That creates a new wind which is a constant input to our equation.

Now when we take the brakes off no matter what direction and speed we sail at the true wind and tidal input to the forces we experience has not changed.  We can not generate increased pressure by taking the tide on our lee bow, or less by taking it on our weather bow.

Anybody else with me?


Posted By: G.R.F.
Date Posted: 12 Oct 11 at 10:58pm
Dude you use the tablecloths

I'll stick to the experience of thirty five years and countless victories gained sailing in tides of all different descriptions and directions in all manner of craft.


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Posted By: rb_stretch
Date Posted: 13 Oct 11 at 7:02am
I think the best way to have this debate is to drop the term "lee bow effect" all together. As concluded earlier different people are using it to mean different things, so we are arguing at crossed purposes. The question I think we debating is can the tide have an effect on your apparent wind?

I'm of the camp definitely yes. Simple geometry can demonstrate it by saying: if you have East going tide of 5 knots with a westerly wind of 10 knots (so both in the same direction) and you are floating (pointing north for arguments sake) and stationary in the water, how much wind do you feel? Answer should be 5 knots. If the tide turns and you have a 5 knot west going tide against the wind you will feel 15 knots of wind.

As you are not moving through the water do you call this apparent wind? Doesn't matter really, because in the same situation with a moving boat the tide is making a difference for the same reason as above. The difference will be less because of apparent wind due to boat speed. Now you can take this further by saying as long as their is a component of tide against the wind and it is pushing from the leeward side of the boat you will get some lift towards the wind.

Can we agree on that?




Posted By: sargesail
Date Posted: 13 Oct 11 at 7:42am
Originally posted by G.R.F.

Dude you use the tablecloths

I'll stick to the experience of thirty five years and countless victories gained sailing in tides of all different descriptions and directions in all manner of craft.

While I might not be a God like figure of the early days of windsurfing I have a few wins myself - some of which I have attributed to lee bow effect.  But I have seen the error of my ways....


Posted By: sargesail
Date Posted: 13 Oct 11 at 7:44am


I'm of the camp definitely yes. Simple geometry can demonstrate it by saying: if you have East going tide of 5 knots with a westerly wind of 10 knots (so both in the same direction) and you are floating (pointing north for arguments sake) and stationary in the water, how much wind do you feel? Answer should be 5 knots. If the tide turns and you have a 5 knot west going tide against the wind you will feel 15 knots of wind.

As you are not moving through the water do you call this apparent wind? Doesn't matter really, because in the same situation with a moving boat the tide is making a difference for the same reason as above. The difference will be less because of apparent wind due to boat speed. Now you can take this further by saying as long as their is a component of tide against the wind and it is pushing from the leeward side of the boat you will get some lift towards the wind.

Can we agree on that?


[/QUOTE]

You will get pushed towards the wind - yes.

But can we agree that that push is the same no matter where the boat is pointing?


Posted By: Pierre
Date Posted: 13 Oct 11 at 8:56am
Still no pictures I see....


Posted By: bert
Date Posted: 13 Oct 11 at 10:21am
so for this to have any effect the wind & tide have to be at diffent angles to each other.
correct? or Not?
May be a couple of people who understand this effect & would be willing to write a summary for the mag & these would be considered to be publised. 

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Posted By: Steve411
Date Posted: 13 Oct 11 at 10:46am
Anyway, I'm looking forward to the Inlands this weekend...LOL

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RS300 411
D-Zero 11

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D-Zero page


Posted By: Ian29937
Date Posted: 13 Oct 11 at 11:00am
A wonderfull rambling thread as usual. 
 
The question is not whether the tide has an impact on the apparent wind as clearly it will.  The question is whether or not pinching to shift from "tide on the windward bow" to "tide on the leeward bow" mode works or not. 
 
The theorists say no, some people say experience indicates yes.
 
I've certainly been overtaken whilst trying to pinch up and gain a leebow effect by people sailing normally to the true apparent wind so my experience dictates that the academics are correct.  Smile
 
Ian


Posted By: G.R.F.
Date Posted: 13 Oct 11 at 11:11am
Originally posted by sargesail




But can we agree that that push is the same no matter where the boat is pointing?

No, we can't agree on that either.

If it were a boat with no means of propulsion then yes we could agree with your 'theory'

But it isn't, it's a sailboat with two sets of foils set in a plane at the conjunction of two 'fluids' moving in different directions.

If the fluids were both moving in the same direction, then yes the push would be the same no matter where the boat was pointing.

But the moment you introduce a means of resistance against either fluid force, then that produces an equal and opposite response <insert all sorts of maths and physics bollox>

Now come on, you must see the sense of this...


In all seriousness here for a second and joking and banter aside, have there been no books or lectures or videos about sailing in tide, tidal currents, waves fast and slow, how to spot them all this stuff we're bantering about here? I can't believe over the years somebody hasn't already detailed it, somebody y'all might actually believe that is.. I bet if my name was Bethwaite y'all would be going ooh ahh it must be true then...


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Posted By: r2d2
Date Posted: 13 Oct 11 at 11:16am
Originally posted by G.R.F.

Originally posted by sargesail




But can we agree that that push is the same no matter where the boat is pointing?

No, we can't agree on that either.

If it were a boat with no means of propulsion then yes we could agree with your 'theory'

But it isn't, it's a sailboat with two sets of foils set in a plane at the conjunction of two 'fluids' moving in different directions.

If the fluids were both moving in the same direction, then yes the push would be the same no matter where the boat was pointing.

But the moment you introduce a means of resistance against either fluid force, then that produces an equal and opposite response <insert all sorts of maths and physics bollox>

Now come on, you must see the sense of this...


In all seriousness here for a second and joking and banter aside, have there been no books or lectures or videos about sailing in tide, tidal currents, waves fast and slow, how to spot them all this stuff we're bantering about here? I can't believe over the years somebody hasn't already detailed it, somebody y'all might actually believe that is.. I bet if my name was Bethwaite y'all would be going ooh ahh it must be true then...
 
GRF you'll never get that v-twin thing finished if you stop to write a book for us!


Posted By: JimC
Date Posted: 13 Oct 11 at 11:36am
To have a mag article you'd have to agree on what "this effect" is first... A sailing in tide series wouldn't be a bad idea for the mag, hink its been a while.

GRF seems to be talking about tactical considerations affecting which tack you should take first. I hope no-one is going to disagree that the tidal stream profoundly affects tactical considerations, most especially since by and large tidal sailing is done in places where the tidal stream and direction is anything but constant.

The direction and strength of the tide also affects the wind direction felt on the boat: if you've got 5 knots of tide from East to West and 5 knots of wind from Nth to Sth then the boat is effectively sailing in about 7 knots of wind from the North West...

If the boat can do 5 knots though the water on a beat then on one tack it will be making 10 knots across the ground heading west, and on the other tack about 7 knots across the ground heading north west, but on both tacks it will be making 5 knots through the water...

Why this gets complicated is that the marks are fixed relative to the ground, not relative to the water, so in this example, compared to the water the boat is sailing on the windward mark is travelling east at 5 knots.

What GRF seems to me to be saying is that when in doubt always sail the tack that is getting you nearer to the mark quickest, because if the tide stream isn't constant you win, and if it is constant then you don't lose. Seems uncontroversial to me...


Posted By: G.R.F.
Date Posted: 13 Oct 11 at 11:38am
Originally posted by Ian29937

A wonderfull rambling thread as usual. 
 
The question is not whether the tide has an impact on the apparent wind as clearly it will.  The question is whether or not pinching to shift from "tide on the windward bow" to "tide on the leeward bow" mode works or not. 
 
The theorists say no, some people say experience indicates yes.
 
I've certainly been overtaken whilst trying to pinch up and gain a leebow effect by people sailing normally to the true apparent wind so my experience dictates that the academics are correct.  Smile
 
Ian

No, that wasn't the question, the question was 'what is lee bow effect'. and what does it do?

The answer to which I've already given.

What then developed was itty nitty picking, the answer to your bit was no not always, pinching in tide is never a good idea, unlikely that you could even pinch to that large a degree that the tide would shift from the weather to the lee bow, it might shift from the nose to the lee bow and tide on the nose is better than tide on the weather side of the foil for all those fluid dynamics equations that I failed to list above..


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Posted By: G.R.F.
Date Posted: 13 Oct 11 at 11:50am
I've had a thought, trying to get folks head round wind bends and when and why you sail into a header, imagine trying to explain where you go on a tide bend into a wind bend (we have one at our club).. At certain states of the tide, you do all the text book stuff get the tide on the lee bow, but as you near the mark it knocks, you get fooled into thinking its a wind bend when it isn't so you persist, (because the curved coastline is classic wind bend geography and does produce it at times) you sail what is now a tidal knock (although you still think the tide is on the lee or at the very worse the nose, until you're on what should be the lay line, flack over and bosh the tides now on that weather bow as well...

Nightmare.


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Posted By: fudheid
Date Posted: 13 Oct 11 at 1:41pm
Undecided on who to believe on this one. Certainly sailing in alot of tide on the east coast the 'lee bow' theory certainly helps your COG whether it makes the boat faster - No i don't think so.
If the theory is wrong why do gybing boards work surely its the same theory the angle of attack of the blade on the water. Its all changing the angle of upwash on the foils????


Posted By: Oli
Date Posted: 13 Oct 11 at 2:00pm
lee bowing the tide is really just cross track error.

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Posted By: fudheid
Date Posted: 13 Oct 11 at 2:12pm
i think i'll go with thatClap


Posted By: Ian29937
Date Posted: 13 Oct 11 at 2:24pm
Originally posted by G.R.F.


No, that wasn't the question, the question was 'what is lee bow effect'. and what does it do?

OK. to rephrase
 
What is the lee bow effect - the lee bow effect is non existant, there is only a tidal effect on your apparent wind which effects you consistently whether you are sailing high or low, port or starboard
 
What does it do - nothing, it doesn't exist!  However for those who believe, it is the fictional notion that there is an advantage gained (the lee bow effect) by pinching higher to get the tide pushing on the leeward bow.
 
There I've said it!
 
Ian


Posted By: Ian29937
Date Posted: 13 Oct 11 at 2:28pm
Originally posted by fudheid

Undecided on who to believe on this one. Certainly sailing in alot of tide on the east coast the 'lee bow' theory certainly helps your COG whether it makes the boat faster - No i don't think so.
If the theory is wrong why do gybing boards work surely its the same theory the angle of attack of the blade on the water. Its all changing the angle of upwash on the foils????
Do gybing boards work?  I used to sail fireballs which allowed them at the time.  They were only used by a few people and then only in some very specific circumstances  (e.g. flat water, medium breeze when the height gain exceded the extra drag and speed loss)
 
This sounds like a whole new topic
 
Ian


Posted By: Rupert
Date Posted: 13 Oct 11 at 3:09pm
505's use gybing boards to great effect.

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Firefly 2324, Lightning 130, Puffin 229, Minisail 3446


Posted By: Rockhopper
Date Posted: 13 Oct 11 at 3:14pm
I cant belive its got the 8 pages so far on this topic
Every sea club where tides are will at some point have a lee bow effect some people master it and  others cant seem to understand how it works even at our club some know how to make the most of it just like reading the tide on the water others cant but does have an effect it does to a certain point i know used to use it all the time when sailing slower boats


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Retired now after 35 seasons in a row and time for a rest


Posted By: Garry
Date Posted: 13 Oct 11 at 3:22pm
Sailing the tack that gets you to the mark quickest works for me, the rest is academic - thanks for that insight Jim

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Garry

Lark 2252, Contender 298

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Posted By: tgruitt
Date Posted: 13 Oct 11 at 3:23pm
So what angle from the bow does the tide have to be for it to have this effect? Let's just say it's on the starboard bow, what does the angle need to be between the centreline of the boat and the angle of the tide?

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Posted By: Presuming Ed
Date Posted: 13 Oct 11 at 3:34pm
Originally posted by Ian29937

Do gybing boards work?  I used to sail fireballs which allowed them at the time.  They were only used by a few people and then only in some very specific circumstances  (e.g. flat water, medium breeze when the height gain exceded the extra drag and speed loss)
 
This sounds like a whole new topic

Couple of articles in Seahorse recently from David Hollom about why gybing boards are a chimera. Worth digging out (if you can ever understand his articles). 


Posted By: JimC
Date Posted: 13 Oct 11 at 3:56pm
Suggest that if you want to talk about gybing boards it goes in a new topic, because its an even more complicated subject... So I've statred one...


Posted By: sargesail
Date Posted: 13 Oct 11 at 8:28pm

Dude - Great post.  It sums up the issue for me.  There are broadly 3 camps:

 

1.  The believers who have "experienced" lee bow effect, or been told about it and just accept it.

2.  The unbelievers.  I'll explain why below.

3.  The technical believers - GRF's post nearly put me back in that camp and might yet when I go and get my copy of "The Symettry of Sailing" off the shelf tonight.

 

The unbelievers have a "simple" model in which the tide is like a conveyor belt.  (And I'm looking at a system without variables here to ease the explanation).  In that model the angle of the boat to the tide is irrelevant and there can be no lee-bow effect.

 

The technical believers use a complex model in which altering the angle of attack of the hull and foils to the tide alters the force applied to them by the tide.  This in turn affects the force applied by the sails because the forces acting in the two fluids must remain in balance. 

 

GRF's post almost convinced me - but then I stepped back and remembered that the lower fluid in which those forces are working is not the tide but water.  If the water is moving east at three knots and the boat has a force applied to it by the tide which would move it east at 3 knots in the absence of other forces moving it in different directions then there is no angular force on the hull and foils.




Posted By: fab100
Date Posted: 13 Oct 11 at 8:31pm
Originally posted by tgruitt

So what angle from the bow does the tide have to be for it to have this effect? Let's just say it's on the starboard bow, what does the angle need to be between the centreline of the boat and the angle of the tide?

Forget apparent wind, a red herring. The name lee-bow effect is part of the problem too, it gets over interpreted, as has rightly been said. 

Anyway. Imagine sailing close-hauled directly into the tide, doing 1.01 knots thru the water, against a 1 knot, on the nose, adverse tide. If you point slightly high of the tide, over 10 minutes you will go hardly forwards, over the ground, at all. But the 'high' element means you will crab to windward, over the ground. This too is simple vectors.

Conversely, if you point slightly low of the tide direction, you will go hardly forwards at all, but again a long way sideways, but this time to leeward.

Apply this and as if by magic, you are suddenly miles to windward, so well ahead of the low pointer. Tweak the numbers for boat speed and tide flow how you like, the effect remains.

So, much as it hurts me to say it, GRF is spot on. I too have been there, struggling to get around a mark in the middle of the tide. Seen it, done it, got the t-shirt. 

Don't care about the mathematical models. If they don't agree with the empirical evidence, the model is wrong, not the real world. (I'm trying not to say 'just like global warming.' Doh! It slipped out)

But if anyone sees me ordering a V-twin, please shoot me before I can sign the cheque!




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Posted By: JimC
Date Posted: 13 Oct 11 at 8:47pm
Originally posted by fab100

doing 1.01 knots thru the water, against a 1 knot, on the nose, adverse tide.

You're missing a few things. First, if you are doing 1.01 knots through the water when you are slugging that 1 knot tide then if you point up 1 dgree you will drop speed, say to 0.91 knots. Conversely if you point down 1 degree you may go up to 1.11 knots. Then lets consider how big the lateral component of this tide actually is. If its a 1 knot tide and you are 1 degree off bang into it the lateral component is going to be, oh I dunno, I can't be bothered to do the sums, but lets say 0.01 knot (bet its way less than that). So that means that if you point up the 1 degree then after an hour you will be one tenth of a mile downtide and 1 hundredth of a mile cross tide of where you started. Meanwhile the chap who pointed down is 3 tenths of a mile uptide but one hundredth of a mile cross tide. He then puts a covering tack in and sails the two hundredths of a mile needed to be bang in front of you in about a minute and a half in which he gets pushed back two hundredths of a mile by the tide to be that much less than three tenths of a mile ahead of you...

Ah yes, you say, but I can point 2 degrees higher than the other chap and not go any slower. In which case I say congratulations, well sailed, but that means you'd beat him just as thoroughly if there were no tide at all...


Posted By: sargesail
Date Posted: 13 Oct 11 at 8:56pm
Which would be fine if all we did was fetched....but the boat which points high sees its gain neutralised as soon as they both tack.  Because the boat that footed should be moderately faster so is slightly further to windward on the new tack - and because the tiny gain to windward is off set by the speed on the new tack which is root 2 at 135 degrees to the old heading, making any gain almost irrelevant.

This is just sail the lee bowed (major) tack first.  

But it is a good way of explaining the phenonomen which has been obsereved without resorting to the fallacy that you get "lift" from the angle to the tide.


Posted By: leebow the letter
Date Posted: 13 Oct 11 at 10:30pm


Posted By: leebow the letter
Date Posted: 13 Oct 11 at 10:31pm

Above I have drawn a very basic diagram.  Imagine there is a NE wind. 

The thin black, green and red lines are the starting points of three boats. They are at the same place, but on different headings.  To make it simple assume they are all going the same speed (not velocity) and there is no tide.  After 10 minutes (and 1000m) they end up in the places with the thick outlines. Not suprisingly, the red boat has sailed to windward of the others. 
 
Now suppose there is a tide.  The boats would end up after 10 minutes at the locations with the dashed lines, because all the boats are pushed back equally by the tide. 

So, after the same number of minutes, the red boat is the same amount of metres to windward.  But all the boats have moved forward 800m rather than 1000m due to the tide.  After 1000m (but, say, 12 minutes) the red boat will be further to windward than if there was no tide. 



Posted By: Presuming Ed
Date Posted: 13 Oct 11 at 10:42pm
But in this magic boat of yours - which can point like a badger, and lose no speed at all, why would you sail at anything but the top of the groove? ITYF that with most boat, if you foot you go faster.


Posted By: G.R.F.
Date Posted: 13 Oct 11 at 10:47pm
Originally posted by sargesail

 

GRF's post almost convinced me - but then I stepped back and remembered that the lower fluid in which those forces are working is not the tide but water.  If the water is moving east at three knots and the boat has a force applied to it by the tide which would move it east at 3 knots in the absence of other forces moving it in different directions then there is no angular force on the hull and foils.



You're not thinking about the 2nd fluid, which is air, even still air. Try to move a foil sideways and the fluid dynamics will propel it forward. When the Fluid A) The tidal water, moves your craft, Fluid B)the (even dead still )Air propels your sail forward.

So even in totally stationary air, so there is no 'upwind', the action of a sideways force of the tide against one foil, will move it at a vector of the two angles, so effectively at 90 degrees to the direction of the tidal current if the foil in fluid a) is presented to the flow at 45 degrees, doh this is hopeless trying to explain, it needs a diagram or even better a test tank.


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Posted By: sargesail
Date Posted: 14 Oct 11 at 7:52am
Yes - but those forces act as a totality not with changes of direction when the angle of the boat to them changes.


Posted By: G.R.F.
Date Posted: 14 Oct 11 at 8:37am
Originally posted by sargesail

Yes - but those forces act as a totality not with changes of direction when the angle of the boat to them changes.
Absolutely correct, right up until the moment the 'other' foil, the sail, gets orientated against the direction of the tidal flow, even in totally still air.

I totally understand where your coming from, and it is true if a boat moves with the flow of the tide it matters not which direction it points in, it just moves along.

But once you engage the sail, you introduce another force, then if you orient the foil under th boat in opposition to that second force, you get a resultant movement of the combined forces.


There...


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Posted By: 2547
Date Posted: 14 Oct 11 at 10:08am
Originally posted by JimC

Originally posted by fab100

doing 1.01 knots thru the water, against a 1 knot, on the nose, adverse tide.

Ah yes, you say, but I can point 2 degrees higher than the other chap and not go any slower. In which case I say congratulations, well sailed, but that means you'd beat him just as thoroughly if there were no tide at all...


... and there JimC has made the critical point to debunk the fallacy ... lee-bow effect gibbons rely on the assumption that they can point higher with no loss of speed or that the footers don't go any faster.

When sailing we are all on a moving carpet ...

I'm waiting for GFR to tell us next how he exploited the rotation of the earth to beat Robbie Nash ...




Posted By: G.R.F.
Date Posted: 14 Oct 11 at 11:34am
Do you really want to set me off on the Coriolis effect and it's application to sailing in advanced racing tactics?

Y'all can't cope with simple Lee Bow effect and I used to think we were the ignorant bozos in matters of sailing tactics..

I wouldn't expect folk who sail inland to even consider tide, why would they need to?

But at sea, on the coast, we view it as a secondary power source, simply that, it's just third dimensional thinking. If there aint enough power from the airflow then there is always power from the tidal flow, in a way it can be more relied upon than the wind, every day constant as the moons rotation around our planet they even make watches that time it.. tell me you don't have one.


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Posted By: Presuming Ed
Date Posted: 14 Oct 11 at 11:49am
Next you'll be saying that you can sail in a 3kt easterly with a tide setting 3 kts to the west....


Posted By: I luv Wight
Date Posted: 14 Oct 11 at 12:02pm
But it can be faster to sail downwind in a 3 kt adverse tidal current in a 10 kt wind ( compared to hugging the shore in a 1kt current ) LOL


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Posted By: G.R.F.
Date Posted: 14 Oct 11 at 12:04pm
Originally posted by Presuming Ed

Next you'll be saying that you can sail in a 3kt easterly with a tide setting 3 kts to the west....

A frequent occurrence where I sail...

You need to pump a bit to get the apparent wind working, then use that..Wink


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