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Old 28-11-2013, 11:13   #721
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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
Also,

VMG is everything. Any other decision in which you choose to make a routing decision that decreases your instantaneous VMG, ie tack onto a less advantageous VMG or foot or linch - is a strategy decision related to knowledge of future conditions across a geographical space for the course - such as chasing current relief across a strong tidal current.
This is all fine if your vmg is ground referenced velocity toward the mark, and includes the current effect on the vessel speed. (As it would be calculated by a GPS)

But I believe that DH and SL have been using a "water referenced" vmg, as velocity thru the water made good along the CTS. (As it would be calculated by wind/speedo instrument systems). In that usage/case, then vmg is NOT everything.

I have been using the water referenced definition, both because DH and SL are both focused on the water reference, and because that's how VMG is defined in my b&g manual.
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Old 28-11-2013, 11:36   #722
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Evans,

I am assuming you are using the ProNav B&G system???

You are right - we have been using the wrong terms or more precisiely one term to cover two concepts.

VMG = SpeedOverGround * COS(CurrentCourse - BearingToMark)
...derived without a ground reference and related to True Wind reference (excluding current)

VMC = SpeedOverGround * COS(CurrentCourse - DesiredHeading)
derived with a ground reference and related to a waypoint or makr reference (including current)

Still doesnt change the fact that Bow Effect doesn't exist - but perhaps we can use the correct terminology to make things more clear.
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Old 28-11-2013, 15:30   #723
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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 need not be upwind of the rhumb line. The principle is to try and spend the longest time (eg tacking) in the section with the least amount of adverse current.

sorry ment to say " up current of the rhumb" ... it is only every referred to lee bowing when pointing up wind

How do you see it being used here?

without the add of your electronic kit ..... try it you might see with a burgee and a compass

How on earth would you use the "lee bow effect" (whatever flavour you choose to give it) here?


Sorry, this is completely incorrect (note my restrained use of language, I could be much more colourful here ). The effect of the current is identical on either hull when you are not tied to ground. The current is simply moving the body of water the boat is in. Altering the boat heading to make the current fall on the lee or windward side of the hull has no effect other than slowing down or speeding up the boat as would usually occur with the change in orientation to the true wind. Lack of understanding this is very unfortunately a real stumbling block for you .

I don't seem to be stumbling ... just refusing to see how you can conseptulise everything . it you have ever tacked up a 3 cable wide estuary, we often sail in things called waves ..... and they have an effect on how I sail

did you note the comment on heel? thus more leeway .... do you still consider leeway is not altered by angle sailed to wind?

all i'm trying to point out is that there may be other factors to consider where lee bowing the tide is used


We have turned to CTS only because any beneficial effect to pinching to achieve the current on the lee bow has been dismissed.

may be the way you consider it , just saying its not purely used in cts equation and as I see it , if you just add the same leeway on each apparent wind angle plotted you really are not proving anything, but I maybe wrong..... as I said its my opinion



An far as I can gather, the true meaning of "lee bowing the tide" (eg when crossing the English Channel) means that when the cross current is variable (and therefore the true wind is variable), and you need to spend roughly 50% of the time on each tack, then the tack to be on is the one with the current closer to the lee bow when compared to the other tack. It has absolutely nothing to do with he action of the current on the boat, only the action of the current on shifting the true wind.
really ? the boat sits in the moving current..... there is conseptulising and reality but I do agree it makes working out a cts a lot easier to understand

no wind...... you move over the sea bed with the current , 2 knots of current and you move 2knts over the ground .... and I am sure of this
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Old 28-11-2013, 15:33   #724
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

sorry the multi quote didn't seem to work , you might need to read above in between your quotes
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Old 28-11-2013, 15:34   #725
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

^^ FS . . . . actually, according to b&g definitions both vmg and vmc reference boat speed thru the water and not sog. Eg it is water referenced and can be calculated by b&g systems without reference to a GPS. This is the definition I was using. It is also (I believe ) the definition DH and SL were using - water referenced. (And note the vmg point on polars are water referenced).

You are using the definition that a GPS mfg might develop, and I think you are the only one on the thread using a ground referenced vmg or vmc. Your various statements above are certainly true for a ground reference, but are not for a water reference.
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Old 28-11-2013, 16:16   #726
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Dropped pin
near River Fowey
http://goo.gl/maps/b9SX5

Have a look at this satelites image this would be a classic case where I would lee bow the ebbing tide
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Old 28-11-2013, 19:11   #727
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Quote:
Originally Posted by foolishsailor View Post
We are talking about the validity of an "effect" not about the cumulative results of a series of strategic decisions that include wind direction and velocity changes, and tidal direction and vellocity changes - not to mention all the variations of tactical decisions related to boat on boat events.

Obfuscating the problem by continually stating that one cant find a solution due to all the variables associated with getting from the leeward mark to the windward mark is another straw man.

Either the effect exists or it doesnt. Either it can be demonstrated in idealized scenarios or it doesnt exist.

I have shown very early in this thread, utilizing to scale drawings that can be used for a rwfutation, that it doesnt exist and no one has provided the same to prove otherwise.
I have countered that my drawing shows a bow effect , an improvement in wind angle over the situation of no current, several attempts by others mix up vector references. Whether one can turn a very small change in wind angle into a benefit on the course is another issue entirely.

Dave
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Old 28-11-2013, 20:44   #728
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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I have countered that my drawing shows a bow effect , an improvement in wind angle over the situation of no current, several attempts by others mix up vector references. Whether one can turn a very small change in wind angle into a benefit on the course is another issue entirely.

Dave
Dave, in recent discussions nobody has mixed up the vector references, except possibly you (it was hard to tell, given the terminology problems). And, nobody has disputed that given a fixed Ground Wind a current will change the True Wind(*) vector. That's the definition of True Wind -- the combination of the Ground Wind and Current vectors.

Now if somebody wants to define "Bow Effect" perhaps we can discuss this.

(*) I am using True Wind to refer to the water-referenced wind.
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Old 28-11-2013, 23:07   #729
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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every boat will spend the same amount of time over a given leg on starboard and port tacks
Estarzinger, FoolishSailor's quote needs to be put in context. Here is a little more of what you rather unfairly clipped off. His comments are correct

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There is no benefit gained by staying on a given tack longer to take advantage of the lift caused by the current as all boats have to sail the same distance on the same tack - on a race course with marks where there are no changes in wind and current over time or geography every boat will spend the same amount of time over a given leg on starboard and port tacks whether you sail to the layline on a long tack or short tack up the middle it is irrelevant - smae distance and time for every boat.
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Old 29-11-2013, 01:10   #730
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Thanks SL and what I said is factually accurate and there was a scaled drawing attached that illustrated it. It is only true in the scenario I presented.
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Old 29-11-2013, 01:46   #731
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

^^

SL, nothing I clipped changed the meaning. Both the bold part and the full paragraph were ambiguously/poorly written. I realized after I wrote my post that he probably meant that the different boats spent the same time as each other on each tack, rather than that one boat spends equal time on each tack. I deleted my post rather quickly and it only lived because paul managed to quote it.

That diagram is (apparently) a calculation of one particular case. Are you both suggesting that "proves" all cases? A mathematician would certaintly not accept that as a proof.

Further, to my simple point here, FS simply asserts that sailing higher will not help. He does not prove it and cannot without using a polar. He has tried to say that it is obvious because vmg will be lower. But "it is obvious" is not a proof. And remember that the optimal upwind angle is related to water referenced vmg, when he is asserting a point about ground referenced vmg (eg faster to the mark). Again, remember that I am not saying he/you are wrong . . . I am simply saying you cannot analytically prove the case without using a polar.
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Old 29-11-2013, 02:04   #732
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

Evans we cant all be well written authors now can we?

If a mathematician makes a claim that a principle exists another mathematician only has to provide a single instance where that claim does not work to refute the claim.

The scenario (apparently) presented an optimized situation to demonstrate a benefit opportunity for the Lee Bow Effect and it doesn't exist. It is not a calculation but is is to scale and accurate.

We don't need to prove all cases - if one where the effect should be maximal doesn't work then that is enough to prove the case.

Dave and others have provided drawings that illustrate their views on the effect - however what is needed is a scale drawing that illustrated the benefits of the effect from start to origin. - even one on a chart with accurate angles and distances or calculations of angles and distances that can be checked by others for accuracy.

Something like I provided when we were discussing the idea of a favored tack principle showing the actual legs, time and distance covered for two different boats.

We still await this.
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Old 29-11-2013, 02:09   #733
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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^^

Further, to my simple point here, FS simply asserts that sailing higher will not help. He does not prove it and cannot without using a polar. He has tried to say that it is obvious because vmg will be lower. But "it is obvious" is not a proof. And remember that the optimal upwind angle is related to water referenced vmg, when he is asserting a point about ground referenced vmg (eg faster to the mark). Again, remember that I am not saying he/you are wrong . . . I am simply saying you cannot analytically prove the case without using a polar.
Fair enough. I have a bunch of polars for different boats - I will draft up a drawing using the polars and current showing time to mark for different strategies.

If anyone would like to give me the premises of a particular scenario that they think will show the max benefit for this effect please let me know and I will draft it up.

Dave, Evans, Paul, Hoofsmit let me know and I will draft a drawing over the weekend.
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Old 29-11-2013, 02:11   #734
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

All i really need is the following

Current Speed
Current Direction
Wind Speed
Wind Direction

Direction from origin to destination

I will assume the following
1. I will choose boat polars with upwind boats speeds in the 7-8kts range
2. We are dealing with a "steady state universe" - no change in current or wind
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Old 29-11-2013, 02:14   #735
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Re: Let's Get to the Bottom of the Lee-Bow Effect Once and For All

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If a mathematician makes a claim that a principle exists another mathematician only has to provide a single instance where that claim does not work to refute the claim.
Fair enough . . . You are only saying that you are making a claim, not to have proved it. That's exactly what I have been saying.

I suspect you are right. Which means that it would be rather difficult to demonstrate a counter instance (I have not looked at Dave's case). But, again, my simple point was that the claim had not been proven, which we now seem to agree on.

Frankly, you and I agree on the most essential point, which is that the steady state case in mostly unrealistic, and the real world effect/opportunity here is playing the shifts.

As an aside, by education (graduate degree), I actually am a mathematician (thesis in number theory).
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