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Old 16-11-2013, 06:09   #376
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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
......
This is one of the reasons I say CTS has no value in upwind upcurrent situations.
Very meekly chiming in here after that dreadful strategy I proposed (any chance you could forget what I stupidly wrote? LOL). The reason I led myself up the garden path with the example you gave was that any CTS calculations work only for current changing over time not in different spots on the course.

I still maintain that if you conducted a race in the middle of something like the Channel (avoiding shipping lanes ) that a CTS calculation is needed and the course is divided either side of the and all the normal strategies of staying on a certain side of the course apply.

I will make up an example (all very realistic data) and do the calculations and drawings at some stage just to see if it works or not.

I will give you the same data and see how you would tackle it, if you have the time to look at something like this.

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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 irrelevant.
I will ponder on this, but off the cuff I think something like this would only apply if current cancelled out for the journey.
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Old 16-11-2013, 06:11   #377
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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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
This was the main discussion with the method explained and lots of examples with calculations and diagrams for each, including how to plot your predicted ground track while sailing a straight line through the water:

Inaccurate RYA Teaching : CTS - Quest For a New Method

I think there was a link in there to a YouTube for the RYA method and I also provide all the calculations and diagrams for that method along with mine.
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Old 16-11-2013, 06:12   #378
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

SF:
BRILLIANT ..... what but how to I draw on it to get it back into Cf ?

sorry..... now you see my problem both in writing and drawing lol
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Old 16-11-2013, 06:14   #379
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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.
Yes, that is what I meant, without a portion of the course experiencing current relief, it is a sliding tablecloth that doesn't matter.

However, the point of my post was to point out that there are two sets of foils at work here and depriving the upper foils of wind in order to reduce current induced drag on the lower foils may not produce optimal results.

It's is my experience that windward shores give relief not only from current, but also from the wind. Therefore, I agree with your shore hugging strategy to avoid current, but not your choice of shores. I would propose that (unless the land is very flat) a course along the leeward shire, while indeed riskier, would likely produce better results.
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Old 16-11-2013, 06:16   #380
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Thank you Seaworthy

to quote scot of the antarctic....... I may be gone sometime !
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Old 16-11-2013, 06:18   #381
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Yes, that is what I meant, without a portion of the course experiencing current relief, it is a sliding tablecloth that doesn't matter.

However, the point of my post was to point out that there are two sets of foils at work here and depriving the upper foils of wind in order to reduce current induced drag on the lower foils may not produce optimal results.

It's is my experience that windward shores give relief not only from current, but also from the wind. Therefore, I agree with your shore hugging strategy to avoid current, but not your choice of shores. I would propose that (unless the land is very flat) a course along the leeward shire, while indeed riskier, would likely produce better results.
ho ho ........ im getting excited now
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Old 16-11-2013, 06:19   #382
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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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
No, I don't think you would use CTS at all if current varied in different spots at the same time (unless it was for the last part of your leg where you were no longer planning to deviate). Even you were laying the mark and gybing was not advantageous, you may well ignore you CTS if there was deeper or shallower water nearby giving you possible a huge advantage if you deviated to encounter it.
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Old 16-11-2013, 06:26   #383
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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.........
It's is my experience that windward shores give relief not only from current, but also from the wind. Therefore, I agree with your shore hugging strategy to avoid current, but not your choice of shores. I would propose that (unless the land is very flat) a course along the leeward shire, while indeed riskier, would likely produce better results.
Yes, I agree that wind alters near shore. I added a note about that what must have been a few dozen posts ago now .

But, it needn't necessarily be the lee shore that has the stronger wind. Wind can rip over mountains and create extremely strong winds on the leeward side. You just need to be familiar with local conditions to know how different general wind directions and strengths will affect the local wind on a particular shore.
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Old 16-11-2013, 06:27   #384
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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No, I don't think you would use CTS at all if current varied in different spots at the same time (unless it was for the last part of your leg where you were no longer planning to deviate). Even you were laying the mark and gybing was not advantageous, you may well ignore you CTS if there was deeper or shallower water nearby giving you possible a huge advantage if you deviated to encounter it.
OH NO

Now we get to what is called Traditionally 'stemming the tide'

Lets not get into ' ferry gliding'

we might start thinking about moving through the water !
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Old 16-11-2013, 06:28   #385
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Angela,

Here is the original image I posed to challenge the CTS theory for Upwind Upcurrent sailing.

I have revised it to make it more sketch friendly and have included a graph.

Here are the premises to make this straightforward, we can add these variables back in later if we wish:

1. There is no change in current over time
2. To avoid anyone breaking out the calculus on us the currents are discrete vectors. When you cross the line represented by the change in color you instantaneously go from one current speed to the next
3. The grid lines are perfectly aligned N/S and E/W
4. Top is N
5. The wind is blowing from 45
6. The destination is dead upwind
3. Boat STW is 7kts.

Europaflyer,

Would these conditions work for you to demonstrate the course you would take to illustrate your ideas? If not let me know what set of conditions would work and I will draw them up so we can work it out on a grid.
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Old 16-11-2013, 06:28   #386
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I was trying to keep the variables to a minimum.
Is not this entire thread based solely upon nit picking the multitudinous variables? Where is the sport in reducing variables? We need something to do when not sailing, I thought this thread might well occupy my idle hours for a few more weeks.
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Old 16-11-2013, 06:36   #387
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Very meekly chiming in here after that dreadful strategy I proposed (any chance you could forget what I stupidly wrote? LOL).
You're a moderator, can't you just delete it? Or is that considered an abuse of power? If you'd like I could go back and report it as offensive, then you'd be justified : )
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Old 16-11-2013, 06:38   #388
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Yes, that is what I meant, without a portion of the course experiencing current relief, it is a sliding tablecloth that doesn't matter.

However, the point of my post was to point out that there are two sets of foils at work here and depriving the upper foils of wind in order to reduce current induced drag on the lower foils may not produce optimal results.

It's is my experience that windward shores give relief not only from current, but also from the wind. Therefore, I agree with your shore hugging strategy to avoid current, but not your choice of shores. I would propose that (unless the land is very flat) a course along the leeward shire, while indeed riskier, would likely produce better results.
It would be a tough call - as there is an additiional 1kt of current relief on the far side (windward shore) versus the near side (leeward shore).

This is where local knowledge and/or personal experience plays a role - and not tidal stream books.

For example if you know that on the windward shore there is a 5kt drop in windspeed but you are able to maintain 6kts but on the lee shore you are able to maintain your full polars for pointing (6.7kts) then you know that the far show will yield a net gain of .3kts - which is a huge gain.

However it is also a risk as you have a known element, the near shore with known wind speed versus the gamble of the far shore wind speed.

If you were in the lead - you would likely tack up the near shore and watch for those near you in points and if they went across you would tack over to cover and go.

If you were in the back of the fleet and looking to move up - you would take the gamble and go across.

You have to weigh the value of risk and reward in addition to all the other variables known and unknown...

...This is what makes tactians age faster than helmsman.

But at the end of the day you cant make anydecisions if you dont know the variables and how they interact and what to do with them.

This is why i didnt add wind variables into the equation as we wanted to focus just on current relief tactics...
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Old 16-11-2013, 06:40   #389
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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Is not this entire thread based solely upon nit picking the multitudinous variables? Where is the sport in reducing variables? We need something to do when not sailing, I thought this thread might well occupy my idle hours for a few more weeks.
No, I thought this thread was trying to evaluate and understand important concepts that can play a huge role in passage times.

By removing certain variables it is easier to determine the validity of the ones that remain.
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Old 16-11-2013, 06:43   #390
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

TALKING ABOUT VARIABLES :
when do we start bringing this lot in ?

I copy and pasted this from www.simarnet.com re keel design , this is there preface !


1.2.2 Acting forcesThe forces acting on a yacht can be divided in six groups:
  1. gravity force
  2. hydrostatic force of buoyancy on the hull
  3. aerodynamic forces on any emerged item, mostly on the sails
  4. hydrodynamic forces on any submerged item, specially made up by side forces on the keel and the rudder
  5. wave drag on the hull, due to the presence of the free surface of separation between air and water
  6. inertial forces.
It must be said here that for side force we intend generally the component of the fluid-dynamic force perpendicular to the far field flow direction and to the vertical axis of the boat. For the similarity of the phenomena involved it is often called lift even if, unlike on aircraft, it is not needed to balance the gravity but other forces.



Remembering this thread does not understand lift from keel dynamics
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