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Old 17-11-2013, 06:47   #511
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by europaflyer View Post

Current between 0 and 180 relative to true wind - starboard tack favoured.
Current between 180 and 360 relative to true wind - port tack favoured.

IE.

1-Find bearing of true wind (easy to do with modern instruments)
2-Find the bearing of the tide from the tidal atlas.
3-Deduct the true wind bearing from tide bearing.
4-Add 360 if negative.
-If the result is between 0 and 180, sail on starboard.
-If between 180 and 360, sail on port.



Therefore -
-always sail the favoured tack when possible
-if no favoured tack exists, sail the tack which gives you the most direct COG.
Europa - thanks for putting your idea into a workable method, I understand what you are saying now.

Lets apply it to the below diagram I posed earlier.

1 Bearing of true wind

45°

2 Bearing of the tide

270°

3 Deduct the true wind bearing from tide bearing

270 - 45 = 225

4 Add 360 if negative

n/a

-If the result is between 0 and 180, sail on starboard.
-If between 180 and 360, sail on port.

225° = Sail on Port.

According to this you would sail on port tack - however Starboard tack when leaving the origin is the correct decision for this situation.

Now there may be some confusion in the above equation that would make the right answer. For example Current/Tide Bearings are determined by where the tide is going versus where the tides is coming from and if you meant the later - as in you meant the bearing of the current the same way one would take a wind bearing then the tide bearing is 90° and that makes the following equation:

90 - 45 = 45

45° = Sail on Starboard.

Can you clarify? Thanks
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Old 17-11-2013, 07:09   #512
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Quote:
Originally Posted by foolishsailor View Post
Europa - thanks for putting your idea into a workable method, I understand what you are saying now.

Lets apply it to the below diagram I posed earlier.

1 Bearing of true wind

45°

2 Bearing of the tide

270°

3 Deduct the true wind bearing from tide bearing

270 - 45 = 225

4 Add 360 if negative

n/a

-If the result is between 0 and 180, sail on starboard.
-If between 180 and 360, sail on port.

225° = Sail on Port.

According to this you would sail on port tack - however Starboard tack when leaving the origin is the correct decision for this situation.

Now there may be some confusion in the above equation that would make the right answer. For example Current/Tide Bearings are determined by where the tide is going versus where the tides is coming from and if you meant the later - as in you meant the bearing of the current the same way one would take a wind bearing then the tide bearing is 90° and that makes the following equation:

90 - 45 = 45

45° = Sail on Starboard.

Can you clarify? Thanks
45° is the bearing of the ground wind, not the true wind. Because of the tide, the true wind will be a little less than 45°. No matter - the result of 'port tack' is the same and your calculation is a good one. It's just that, not actually being on the boat, you didn't have the true wind bearing to hand.

I'm not sure starboard tack when leaving is the correct decision. As myself and Seaworthy are now both saying (I think) the important thing is to sail the favoured tack. In this case, the current makes that the port tack (following my point about true wind being a little less than 45°).

When on port tack, the tide in this situation will make your COG pretty direct to the destination. Certainly, starting on port is the decision I would make, then tacking when much nearer the destination if needed.

I really only added the calculation to prove my point that the 'true wind current angle theory' works. I think it does, but in real life sailing the favoured tack is what matters, whether this is influenced by the tide or not.

Your example is an interesting one because of the variable tide speed with position. It is not the 'classic' example of 'the bow effect' because it does not involve a current reversal. If it did, favour would switch to starboard tack when the current changes.
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Old 17-11-2013, 07:46   #513
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Ground Wind - understood.

See attached, i have added the rlelvant tacking angles and two courses one using starboard - my choice and one using port - your choice based on calculation.

I have guestimate the actual course with the current influence - but they are close enough for this.

What you see is that no matter what initial tack you make you still have to cross the tidal stream - and no matter what initial tack you make you eventially have to sail up current.

The fallacy in a fixed rule based on current and wind is illustrated here clearly.

If you start out on port tack you are sailing upcurrent in 3 knts versus if you start out on startboard tack you cross the tidal stream and then when you tack you are heading upcurrent in 2kts.

This would be basic tidal strategy for racers in the Solent and it is what wins races - however cruisers can benefit from this as well.

All you need is a good tidal book for the area you are in. This same method works in SF very well where there are similar currents to that illustrated here - there they have the added variable that they are not only discrete differences in speed but direction as well.
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Old 17-11-2013, 07:58   #514
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Originally Posted by foolishsailor View Post
Ground Wind - understood.

See attached, i have added the rlelvant tacking angles and two courses one using starboard - my choice and one using port - your choice based on calculation.

I have guestimate the actual course with the current influence - but they are close enough for this.

What you see is that no matter what initial tack you make you still have to cross the tidal stream - and no matter what initial tack you make you eventially have to sail up current.

The fallacy in a fixed rule based on current and wind is illustrated here clearly.

If you start out on port tack you are sailing upcurrent in 3 knts versus if you start out on startboard tack you cross the tidal stream and then when you tack you are heading upcurrent in 2kts.

This would be basic tidal strategy for racers in the Solent and it is what wins races - however cruisers can benefit from this as well.

All you need is a good tidal book for the area you are in. This same method works in SF very well where there are similar currents to that illustrated here - there they have the added variable that they are not only discrete differences in speed but direction as well.
Hmm, very pretty diagram but a tricky example to use. The lines might be representative of the tacking angles and course through the water (?) but they don't seem to alter when they pass from one current zone to another, but the wind angle would in reality be shifted in this scenario. Also, am I right in saying that they don't account for current? Confusing, as what you are really aiming for is COG lines, and for that you need the current vectors adding to the CTW lines. If you did this, I think you would find that setting off on port tack gives a COG pretty direct to destination.

Have I got the wrong end of the stick here?
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Old 17-11-2013, 08:05   #515
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Quote:
Originally Posted by foolishsailor
Ground Wind - understood.

See attached, i have added the rlelvant tacking angles and two courses one using starboard - my choice and one using port - your choice based on calculation.

I have guestimate the actual course with the current influence - but they are close enough for this.

What you see is that no matter what initial tack you make you still have to cross the tidal stream - and no matter what initial tack you make you eventially have to sail up current.

The fallacy in a fixed rule based on current and wind is illustrated here clearly.

If you start out on port tack you are sailing upcurrent in 3 knts versus if you start out on startboard tack you cross the tidal stream and then when you tack you are heading upcurrent in 2kts.

This would be basic tidal strategy for racers in the Solent and it is what wins races - however cruisers can benefit from this as well.

All you need is a good tidal book for the area you are in. This same method works in SF very well where there are similar currents to that illustrated here - there they have the added variable that they are not only discrete differences in speed but direction as well.
Yet another completely different set of problems. This is probably the hardest thing about this thread. Maybe let's summarize what different problems we have discussed:

1. Lee bow effect 1 - if you can get the current fine on your lee bow, the current will push your keel and give you a lift. By general consensus, utter bullocks.

2. Lee bow effect 2 - if you sail to a windward mark favoring the tack which makes you end up with the current on your lee bow, you are more likely to avoid ending up downtide from your mark. In my opinion bullocks, because it's confusing a sailing problem with a navigation problem, but I am open to corrections to my statement of the theory, as Im not sure I completely understood it. However, the reference to "pinching" in this one strongly indicates bullockshood.

3. Lee bow effect 3 - or - Channel Sailor's Lee Bow Effect. If you have to tack across the Channel, time your tacks to keep your lee bow into the current as much as possible (the less dead up-groundwind your desired average average heading is, the less you will be able to always sail the favored tack). That way, you will optimally distribute your tacks according to shifts in wind direction. Bingo, this one works as advertised. It's a specific application of the "in wind shifts, always sail the lifted tack when you can" rule. It may even work when the current is on the weatherbow. Maybe should be called the "further towards the lee effect".

4. All of the above is thinking without considering currents which vary by space as well as time, like in the Solent. In that case, all bets are off, as another layer of complexity is added. If there's some variant of LBE which is supposed to deal with this, Id like to hear about it, but I doubt any general rule can apply.

All these things are very different from each other, despite similar names! N.B.!
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Old 17-11-2013, 08:05   #516
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

You may have a point.

In my original drawing for discussion the port tack was directly up current so there was no lateral set that would be favorable to your desired COG - on this there is. The other example I showed was when the wind was parrallel to the current and you would have to tack upwind on either shore - where this would work as well.

I may have outfoxed myself this time as I think you may be right in this example as the current set may be more favorable.

Damn, I am going to have to work this one out - I am no longer sure Im right with this scenario.
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Old 17-11-2013, 08:20   #517
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Here is an 'example solution' to your problem. I have attempted a complete solution with CTW, tidal vectors and COG. Because I don't have time for a lot of complex trigonometry, it is necessarily accurate but not precise, if you get my meaning.

The boat starts off on port tack in the 3kt current. This produces a moderate shift of true wind to port compared to no-current situation, making port the favoured tack.

Once in the 4kt stream, the wind shift is somewhat more pronounced, as shown by the slight shift of the CTW line to port. Coupled with the stronger current, this results in a big shift of the COG line to port.

Entering the 2kt stream, the wind shifts back to starboard, in fact more to starboard than in either of the other streams. This results in the CTW line being to starboard of either of the previous lines (ie. has highest bearing). The weakest current also makes the COG shift further to starboard than the other two.

Finally, in this example, once the boat can lay the destination on starboard tack, it tacks. This is just how things have worked out in this sketch of mine, if things are tweaked than the destination could me made all on port, or it might even be necessary to bear away a bit. It's just an example.

Not really the traditional 'lee bow effect' or whatever we call it now, but a useful example of how different currents can shift the true wind, and create a favoured tack where one might not otherwise exist. It is worth noting that taking this favoured tack also allows the boat to 'punch' the tide rather than get swept along downstream by it, and so is clearly the sensible decision whether you concern yourself with true wind shifts and favoured tacks or not. Starting on starboard tack would be a total disaster.

Tell me if you think I have made any serious errors in my reasoning here.
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Old 17-11-2013, 08:22   #518
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Quote:
Originally Posted by europaflyer View Post
Oh come on, really? It's not a 'single moment', it's true for as long as the conditions on the diagram exist. The diagram shows the lee bow boat making far more ground to windward than the lee bow one, because it is poingting far closer to the ground wind (incorrectly called true wind in my diagram ). Even in a constant-current scenario it will get to the other side of the channel faster, and that is all the effect is. We call it an 'effect' because it is a little counter-intuitive - you wouldn't think that a current at 90 degrees to where you want to be could possibly help you get there, but it does do by altering the true wind and allowing you to sail there more directly. A little logical leap but really nothing more complex than that. As I say, taught as part of RYA shorebased courses and really pretty trivial stuff once you look at wind vectors. If you don't get it then that's fine, just get someone else to work as your tactician.
Europaflyer, I made the same argument as you,and was roundly slapped le down for not knowing what the LBE is. What your excellent diagram shows is not what they call the LBE, but rather the favored tack, or sailing to the current. The LBE they are referring to is stupid and involves pinching to move the current to the lee bow which will never be faster unless maybe the current is stronger than the wind.

No one is disagreeing with the validity of the well known principle you diagrammed so well they just call it something else. It's more than a little confusing.

Their definition of true wind is different too. They call true wind referenced over the bottom ground wind. True wind is referenced to the surface of the water, which will be moving with the current. While this is all very accurate I suppose, it is a little tricky and makes it easy to get tripped up over semantics. This thread is a semantics minefield, watch your step.
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Old 17-11-2013, 08:22   #519
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

What Dockhead said. The above is an example primarily of (2). (3) is an excellent definition of the true 'lee bow effect', or just 'bow effect' taking Seaworthys nomenclature.
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Old 17-11-2013, 08:27   #520
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Europaflyer, I made the same argument as you,and was roundly slapped le down for not knowing what the LBE is. What your excellent diagram shows is not what they call the LBE, but rather the favored tack, or sailing to the current. The LBE they are referring to is stupid and involves pinching to move the current to the lee bow which will never be faster unless maybe the current is stronger than the wind.

No one is disagreeing with the validity of the well known principle you diagrammed so well they just call it something else. It's more than a little confusing.

Their definition of true wind is different too. They call true wind referenced over the bottom ground wind. True wind is referenced to the surface of the water, which will be moving with the current. While this is all very accurate I suppose, it is a little tricky and makes it easy to get tripped up over semantics. This thread is a semantics minefield, watch your step.
That's the problem right there, good point. We've still got people worrying about a non-existent pinching effect, and thinking that by rightly disproving this they have disproved the 'lee bow effect'.

Semantics minefield couldn't be more right. I may have made the odd error here, but my last few posts get it right. Wind over the ground or fixed objects is ground wind, add tide in and wind over the water or free floating objects is true wind. Add speed to that and wind over water-mobile objects is apparent wind. A naming system I now thoroughly agree with.
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Old 17-11-2013, 08:45   #521
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I'm ok with the wind names too, good for getting super precise about what you are trying to explain. I agree with SL that this dead horse is well beaten and that this thread has morphed into "effects of variable current on CTS". Who's starting the new thread?
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Old 17-11-2013, 08:46   #522
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Europa,

See the attachment - I have worked out the angles for the situation and it does bear out my original reasoning.

My chart seems to be different from the one you just presented however.

My calcs are very accurate and all the information is on the image for you to double check.

I will work out the other tack now, the one I prefer - however i think it already shows the fact that starting on starboard tack will be faster.

You do not need to calc CTS to sail this as in each setting you will be beating into the wind and will naturally follow the wind as the apparent wind whisfts with the current - however to make things clear and to facilitate someone else challenging my drawing and to find flaws I have invluded as much as possible.

Key point is that the boat has a STW of 10kts
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Old 17-11-2013, 08:49   #523
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Originally Posted by I.Grind View Post
I'm ok with the wind names too, good for getting super precise about what you are trying to explain. I agree with SL that this dead horse is well beaten and that this thread has morphed into "effects of variable current on CTS". Who's starting the new thread?
I disagree - I dont think the theory works at all except in an environment with no variable tide over distance as my illustration shows - in this very common situation the opposite tack of the one recommended by this theory is the correct one.

CTS over variable current is only part of it.
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Old 17-11-2013, 08:59   #524
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Europa,

I see you are using a boat speed of 6 kts which is different to the 10kts Im using - which raises something very intersting as well interms of STW for this situation that I hadn't thought about.

however your headings dont seem to be correct assuming a 90 tacking angle relative to the wind. The wind shifts for the different current speeds are 6, 9 and 12° respectively.

I will finish working out the other tack and then redo it with 6kt boat speed to see the difference
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Old 17-11-2013, 09:02   #525
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Europa,

See the attachment - I have worked out the angles for the situation and it does bear out my original reasoning.

My chart seems to be different from the one you just presented however.

My calcs are very accurate and all the information is on the image for you to double check.

I will work out the other tack now, the one I prefer - however i think it already shows the fact that starting on starboard tack will be faster.

You do not need to calc CTS to sail this as in each setting you will be beating into the wind and will naturally follow the wind as the apparent wind whisfts with the current - however to make things clear and to facilitate someone else challenging my drawing and to find flaws I have invluded as much as possible.

Key point is that the boat has a STW of 10kts
Clearly a very carefully worked out example, and another solution of the same problem. In my example, boat speed was perhaps 6kt or maybe a little less. Having a 10kt boatspeed produces a very different solution, of course having to tack on to starboard a lot earlier. With your boatspeed, starting on either tack is possible as a fair time needs to be spent on each tack. With mine, if you started on starboard you would be almost immediately laying the destination on port, but working out the tacking point would be flipping difficult in the real world, and you risk wasting time by sailing beyond the lay line. Far better to start on port and wait until the end to make the tiny bit of starboard tack, where the layline is much more obvious. I'm pretty sure this is the only advantage to a port tack start though. For your example, where the layline is encountered far earlier on, this point is irrelevant.

What would be really useful is if you added the course for starting on starboard and tacking on to port to your diagram, just so we can see that there is no advantage either way.

As I say, not really the 'lee bow' or 'bow' effect, as there is no current shift. It's just a navigation and layline problem, as per Dockheads point (2), albeit a fiendishly difficult one.

As there is a slight favour to port tack in this scenario, it might be worth starting on port purely for standard 'guarding against future wind shift through making favoured tack first' reasons, but really nothing to do with the 'bow effect'.
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