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

[foolishsailor]

Originally Posted by europaflyer

Here is a diagram showing four scenarios. The upper boats on both diagrams are in a no-current situation, and have exactly the same apparent wind angle. In the left hand diagram, a current is added at 180 to the ground wind. It produces a change in apparent wind angle for the lower boat. On the right hand diagram, a current is added at 180 degrees to the apparent wind, with no effect on its direction. The effect relates to the alteration of apparent wind angle by the current. What matters, then, is the angle of the current to the apparent wind.

It does occur at a tangent (ie. 90 or 270 degrees) to the circle. We agree. The 'circle' is your current vectors. Having the true wind (equal to apparent wind as your diagram includes no boat speed) at a tangent to it proves my point about the effect being based on the angle of the current to the apparent wind.

The problem with this is that the effect exactly cancels out once the boats are on the opposite tack - which is why it doesnt exist or work as a tool.

If your destination is upwind and cant be laid in a single tack (i.e. a race course) then there is benefit from the current as the lift gained on one tack is exactly canceled out by the header on the other tack.

For all intents and purposes "Ground Wind" is a non-existent concept.

True Wind = Ground Wind
Apparent Wind = True Wind + All Vectors of motion

True wind is not the wind over the surface of a boat that is not moving in the water if the water has a current.

True wind is calculated by your GPS by subtracting your direction and velocity to give you the wind if you were fixed in space on the surface of the earth = hence Ground Wind.

I think this idea that Ground Wind and True Wind are different things is leading to many wrong assumptions in the discussions we are engaging in.

If you wish to speak of the difference between current generated apparent wind and apparent wind generated by the sum of all vectors you will have to invent somehting - but why? It has no purpose for optimizing vectors.

For example when we speak of Apparent Wind for a sailboat we dont break it down to each of the individual vectors such as drift, keel lift, keel resistance, hull resistance, etc. All of these are each separate vectors, just like current, and each, if we kept to the idea of separating current, could have its own defined apparent wind.

We don't however as there is no point.

My position is as follows. Once I have more time later today I will draw more diagrams that illustrate these points - however we are not inventing the wheel here. There is a tremendous amount of racing literature out there that covers this in detail.

1. By itself, with no other mediating factors, such as changes in wind direction or velocity, the apparent wind shift caused by current can not be utilized for a gain when as the effect is canceled out once the boat flips on to the other tack.

2. In an upwind and up current situation where one leg may be longer than the other so that the lifted and headed gains do not cancel each other out - the favored tack will not be decided by the advantageous lift gained but instead by a current relief strategy where the initial tack and points chosen to tack will be determined by the speed and direction of the current


edit: Mea Culpa. Just realized I can set my True Wind display on my boat to use either STW and CTW or SOG and COG to calculate True Wind. One True Wind is water referenced True WInd and the other is Ground referenced True Wind or "Ground Wind".

I still stand by the position I outlined about the irrelevance of water referenced True Wind. edit

[Hoofsmit]

I know you all think i'm a k..b head and I bring little to your calculations.
But I have sailed in strong tidal areas all my life, I am Dyslexic, writing this lot means I have to write it 5 times and read it 10 times to make sure what I am saying can be read. I will never make a scolar

The fact to me that by the time you lot have calculated you various angles and plotted them in the real world with the true wind changes and tidal eddies, you would be so far off course to you destination I would have drunk all the beer in the club house.

I would rather be on board a boat with a skipper that perceived the enviroment around him and sailed accordingly, than one sat at the chart table trying to figure out why he is aground.

If you just want to disprove a traditional term you have to understand who used it.

it is apparent that floating around in the med or atlantic stream is not where lee bowing the tide has come from

there is no effect..... only rule of thumb guides to sailing in an environment the majority of you have never experience

mods..... you can kickout me now if you wish ..... I will continue to drink the beer before you get in

[foolishsailor]

Originally Posted by Hoofsmit

The fact to me that by the time you lot have calculated you various angles and plotted them in the real world with the true wind changes and tidal eddies, you would be so far off course to you destination I would have drunk all the beer in the club house.

Exactly!

This is one of the reasons I say CTS has no value in upwind upcurrent situations.

You sail to your telltales - course is irrelelvant.

You cross high current sections on the tack that presents closest to your beam and in sections where there is less current you choose the tack that takes you maximally up current.

There are no calcs required - only having a good set of Tidal Stream drawings.

The image I posted several pages ago along with the image of Cowes illustrates this.

Beam to Big Currrent, Bow to Small Current. That is all you need to know to optimize your crossing for that situation and any like it - boat speed, wind speed, CTS are all irrelelvant.

Doesnt mean that someone with a good computer with real time tidal data couldnt calculate a better course - but not by much as your course is dictated by your pointing angle and there fore your course doesnt need to be calculate - only the most optimal time to tack to mitigate current

Which can be done on the fly, in a skiff - as you do every race!

[Hoofsmit]

Fs...... that is music to my ears... thankyou

This is one of the reasons I say CTS has no value in upwind upcurrent situations.

But if i wanted to go from The needles to Guernsey up wind .... I think I might do one ....... lol

[I.Grind]

[QUOTE="foolishsailor;1391326"]I.Grind - I think you are right we have all moved past Lee Bow Effect I think??? I hope??? But we are now entering a new area that i think is beneficial to cruisers which is optimizing vectors upwind in adverse current. What Lee Bow Effect wishes it was Angela, See attached drawing. The dotted red lines are your laylines and your tacking angles. I am assuming a tacking angle of 90degrees. If you calculated a CTS and short tacked the CTS line you would not be sailing an optimal course by a long shot as once you started entering the middle of the channel every port tack would be dead into the peak current. A much faster route would be to sail on starboard tack the whole way across the channel until you reached near your layline and then tack so that now you are in lesser current for the entirety of your port tack .[/QUOTEt]

Unless this is Holland, or some such place where the shore is below the water level, the wind will most likely be blocked by the windward shore, possibly making your you selected course sub-optimal.

I would agree with plotting a course along a shore where reduced depth might produce current relief. I would also seek out irregularities in the shore that may cause eddies or reverse current.

Other than that it is a sliding table cloth that effects all boats equally, so it doesn't matter.

[foolishsailor]

Originally Posted by I.Grind

Other than that it is a sliding table cloth that effects all boats equally, so it doesn't matter.

The sliding table cloth doesnt work as well as an analogy when at a given point in time there are different speeds of current in a body of water. It only works effectively when the whole body of water is moving at same speed or at varying speeds as a whole.

If a boat is sailing upwind on a bearing of 200° @ 6kts STW straight against a current of 3 knots and a boat perpinduclar to that boat 500 yards away is also sailing upwind on a bearing of 200° @ 6kts STW against a current of 1.5kts

Who will reach their destination more quickly? Obviously the boat in lesser current. The table cloth does not move all boats equally - it moves the boats the speed of the current they are in.

[Seaworthy Lass]

Originally Posted by foolishsailor

Exactly!

This is one of the reasons I say CTS has no value in upwind upcurrent situations.

Just got back on board after a walk. I immediately added this note to my long ridiculous ramble regarding the strategy I proposed. Red faced here at the moment LOL.

Edited to add:
Scrap all the comments below. Incredibly stupid of me. Penny dropped, in fact it was more a cannon ball that dropped LOL.

CTS only applies for variable current over time, not variable current in different spots on the course. Of course you would head straight over to spend min time in adverse current . Red faced here.

[I.Grind]

Originally Posted by europaflyer

True for one tack. On the other tack the boat will be going against the current ('lee bowing') and will be freed ('lee bow effect').

This was my interpretation of LBE until I was shot down hundreds of posts ago. Are we going in circles?

[foolishsailor]

Originally Posted by Seaworthy Lass

Just got back on board after a walk. I immediately added this note to my long ridiculous ramble regarding the strategy I proposed. Red faced here at the moment LOL.

Edited to add:
Scrap all the comments below. Incredibly stupid of me. Penny dropped, in fact it was more a cannon ball that dropped LOL.

CTS only applies for variable current over time, not variable current in different spots on the course. Of course you would head straight over to spend min time in adverse current . Red faced here.

edit: However you can still use CTS - if you are not gybing or tacking to reach your goal.

edit pt. 2 - But the calcs would be different

[Hoofsmit]

[QUOTE=I.Grind;1392099]

Originally Posted by foolishsailor

I.Grind - I think you are right we have all moved past Lee Bow Effect I think??? I hope??? But we are now entering a new area that i think is beneficial to cruisers which is optimizing vectors upwind in adverse current. What Lee Bow Effect wishes it was Angela, See attached drawing. The dotted red lines are your laylines and your tacking angles. I am assuming a tacking angle of 90degrees. If you calculated a CTS and short tacked the CTS line you would not be sailing an optimal course by a long shot as once you started entering the middle of the channel every port tack would be dead into the peak current. A much faster route would be to sail on starboard tack the whole way across the channel until you reached near your layline and then tack so that now you are in lesser current for the entirety of your port tack .[/QUOTEt]

Unless this is Holland, or some such place where the shore is below the water level, the wind will most likely be blocked by the windward shore, possibly making your you selected course sub-optimal.

I would agree with plotting a course along a shore where reduced depth might produce current relief. I would also seek out irregularities in the shore that may cause eddies or reverse current.

Other than that it is a sliding table cloth that effects all boats equally, so it doesn't matter.

This is just passage planning

Racing is tactics .... where you might cover another competitor to prevent an advantage

Heading up a tidal harbour is pilotage , even if it is seeing advantage in current headings

Different methods are used in different sailing environments, the problem with this thread is some are using the wrong method in the wrong enviroment

[Hoofsmit]

Originally Posted by I.Grind

This was my interpretation of LBE until I was shot down hundreds of posts ago. Are we going in circles?

PLEASE DONT,,,,,, welcome to my world

[I.Grind]

Originally Posted by foolishsailor

Exactly! This is one of the reasons I say CTS has no value in upwind upcurrent situations. You sail to your telltales - course is irrelelvant. You cross high current sections on the tack that presents closest to your beam and in sections where there is less current you choose the tack that takes you maximally up current. There are no calcs required - only having a good set of Tidal Stream drawings. The image I posted several pages ago along with the image of Cowes illustrates this. Beam to Big Currrent, Bow to Small Current. That is all you need to know to optimize your crossing for that situation and any like it - boat speed, wind speed, CTS are all irrelelvant. Doesnt mean that someone with a good computer with real time tidal data couldnt calculate a better course - but not by much as your course is dictated by your pointing angle and there fore your course doesnt need to be calculate - only the most optimal time to tack to mitigate current Which can be done on the fly, in a skiff - as you do every race!

One must also take land mass induced wind holes into consideration. We face this often here in San Francisco Bay. Angel island is one such example, great current relief, but unfortunately blocks the wind as well often trapping the unfortunate in its lee.

[foolishsailor]

Since we are covering interesting areas of sailing and it is very hard to present some of these concepts in writing, and also because some of the concepts presented may look to work when taken isolation - versus when worked out from origin to destination, I thought I would attache a nice grid that we can each use to demonstrate our positions by creating scenarios.

The each cell in the grid is 50px x 50px. I would say we make each grid 1 nautical mile - but you can do what ever you want. By having this grid we can incorporate vectors to demonstrate benefit but also for others to look at our position and have the ability to use the grid to validate or disprove a given position.

This should help get to the point of the discussion.

I am working out some scenarios now using this image.

Hope it helps.

[foolishsailor]

Originally Posted by I.Grind

One must also take land mass induced wind holes into consideration. We face this often here in San Francisco Bay. Angel island is one such example, great current relief, but unfortunately blocks the wind as well often trapping the unfortunate in its lee.

Absolutely, I raced in SF for 7 years before i moved to Ireland 8 years ago.

But where there are holes there can also be lifts generated by land as swell an you can also get substantial acceleration as well. I wasn't bringing those into play as I was trying to keep the variables to a minimum.

[Hoofsmit]

Seawothy:

could you provide a link to your CTS threads, I would be really interested on how tidal stream vectors are plotted, I am hoping to learn something