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

[estarzinger]

The lee bow trade-off to discuss is vmg vs distance to sail.

You cannot analytically answer that trade-off without using polars.

And yes, slow vs fast mode tends to have to do with variable conditions, which is really the only realistic case. The "constant current" case is almost never a real world case. The current almost always changes across the course and/or time. We all agree there is a positive opportunity when the current is variable.

[Seaworthy Lass]

Originally Posted by estarzinger

The lee bow trade-off to discuss is vmg vs distance to sail.

You cannot analytically answer that trade-off without using polars.

And yes, slow vs fast mode tends to have to do with variable conditions, which is really the only realistic case. The "constant current" case is almost never a real world case. The current almost always changes across the course and/or time. We all agree there is a positive opportunity when the current is variable.

CTS applies for variable current over time not variable current over different positions on the course at the same time.

Evans, could you explain what you think the "lee bow effect" actually is? This is not clear from your posts. The term is used to describe several scenarios.

[foolishsailor]

Evans,

There is a difference between pinching versus fast/high.

Fast/high and low/slow or whatever varient of terminology works in a racers sailing area are related to changing your upwind pointing angle to optimize VMG for conditions of varying wind speeds and swell.

For example, in flat conditions with moderate wind you will flatten your sails and point high. You are trading "torque" for "top speed". In bumpy conditions you may foot off and power up your sails as you need more "torque" since your boat is constantly required to reaccellerate due to chop or swell conditons.

You also have "gears" like in a car which are used in situations like coming out of a tack as you are reaccelerating.

However none of these are pinching. Pinching is actually clearly defined as sailing upwind closer than optimal VMG for the conditions.

If by pointig a boat higher it sails faster to the mark then you arent pinching, you were instead sailing too low and are now on an optimal course and trim.

Since pinching by its definition is a less than optimum method of going upwind then one doesnt need polars to state that any thosry that utilizes pinching to acheive a gain is mathematically false.

One can take this position as it is also provable, without polars, that the lee bow effect does not exist.

[foolishsailor]

Further to my last post:

If
VMG = x
and
Pinching = x - y ( with y equalling the loss in VMG due to pinching)
and
Current assist to VMG by pinching = c

Then
Lee Bow Effect exists if c > y

We need to prove there is a current assist through pinching first and then worry about polars.

It is not possible to do the former.






c > y

[Hoofsmit]

Originally Posted by Seaworthy Lass

CTS applies for variable current over time not variable current over different positions on the course at the same time.

Evans, could you explain what you think the "lee bow effect" actually is? This is not clear from your posts. The term is used to describe several scenarios.

Could I butt in at this point? ( I know .... not him again)

I've spent sometime digesting this subject, what is very noticeable is that the term is traditionally used buy sailors of old (prior to electronics) as a rule of thumb
The word " effect" has only been added by modern authors, many of them use it as a add on to sailing in tidal stream chapters and is generally used with a counter buff that the general sailing course and trim should not be made detrimental for its use, but as is obvious if the wind drops In a channel with tidal stream, sailing upwind of the rhumb line is a good seaman like decision in shallow areas , say like the thames approachs with drying banks
The TRUE meaning comes from " lee bowing the tide" and was used with generally long keeled tradition heavy displacement vessels not our modern low wetted area cruisers.
I sea it being used both in
1) out of sight of land in a tradition manner with burgee/flag and compass as your main course to steer equipment

2) Narrrow strong tidal channels that have varying stream rates and eddies and variable wind strength and directions.

from my practical experience of living and sailing in these enviroments the effect on the hull is more important than setting a cts, if the current is producing a wave on my windward bow I know if it ment pinching a tad to get that wave gone I would.

I still think this thread has looked far to much at the cts and not enough on the effect it has on whats in the water and you cannot conceptulise this into a wind vector, because it rules out apparent wind angle to keel laterial movement, remember also the wind increases with the current on your windward bow so heel would be increased and loss of laterial resistance
we need to look else where ( but this is my opinion .... !)

[estarzinger]

Originally Posted by foolishsailor

Evans,

There is a difference between pinching versus fast/high.

This is a definitional issue. What I was simply trying to point out, purely as an aside, is that there is a spectrum of angle/speed trade-offs used for different upwind (and downwind) situations, defined by the polar. "Pinching" is probably "officially defined" as somewhere beyond the normal "high/slow" end of this spectrum. However, This is not critical to the discussion at hand. I just mentioned it because SL seemed to think there was just once single optimal point for close hauled, when in fact strategically there is a range to consider.

Originally Posted by foolishsailor

Further to my last post:

If
VMG = x
and
Pinching = x - y ( with y equalling the loss in VMG due to pinching)
and
Current assist to VMG by pinching = c

Then
Lee Bow Effect exists if c > y

We need to prove there is a current assist through pinching first and then worry about polars.

It is not possible to do the former.

c > y

No, nowhere in that formulation is the distance sailed thru the water. Since the CTS varies based on speed, the distance sailed will vary. And there is then (potentially) a trade-off between higher vmg over a longer distance vs lower vmg (higher/slower) over a shorter distance (shorter because of the change in CTS not only because it is higher). To analytically calculate that trade off you MUST know the polar.

You guys have entirely focused on speed and ignored the fact that the CTS and distance changes based on speed.

Please note: nowhere am I suggesting the answer. I am simply suggesting to prove the answer analytically you must include the polar because it determines the changes in CTS and thus distance sailed as you vary AWA.

[Seaworthy Lass]

Originally Posted by Hoofsmit

In a channel with tidal stream, sailing upwind of the rhumb line is a good seaman like decision in shallow areas , say like the thames approachs with drying banks

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.

Originally Posted by Hoofsmit

I sea it being used both in
1) out of sight of land in a tradition manner with burgee/flag and compass as your main course to steer equipment

How do you see it being used here?

Originally Posted by Hoofsmit

2) Narrrow strong tidal channels that have varying stream rates and eddies and variable wind strength and directions.

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

Originally Posted by Hoofsmit

from my practical experience of living and sailing in these enviroments the effect on the hull is more important than setting a cts, if the current is producing a wave on my windward bow I know if it ment pinching a tad to get that wave gone I would.

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 .

Originally Posted by Hoofsmit

I still think this thread has looked far to much at the cts and not enough on the effect it has on whats in the water and you cannot conceptulise this into a wind vector, because it rules out apparent wind angle to keel laterial movement, remember also the wind increases with the current on your windward bow so heel would be increased and loss of laterial resistance
we need to look else where ( but this is my opinion .... !)

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

Originally Posted by Hoofsmit

The TRUE meaning comes from " lee bowing the tide" and was used with generally long keeled tradition heavy displacement vessels not our modern low wetted area cruisers.

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.

[foolishsailor]

DTW is a subset of VMG, DTW is as irrelevant as STW.

VMG and COG are everything when determining optimal routing.

If your STW is higher on port tack but your VMG is higher on Starboard tack that tells you alot about current and wind - but ultimately you are looking at VMG as your decison bias.

VMG is simply a summation of vectors and this whole problem is about a summation of vectors. The confusion lies in people mixing ground and water references to evaluate the process.

Current and the apparent wind it generates are equal regardless of which direction a boat points. The additional apparent wind a sailboat generates through its own wind generated motion through the water is equal to the apparent wind it would generate moving through water with a ground reference speed of 0kts.

Since there is no benefit found in pinching in water not moving there is also no benefit found in water moving.

This is not to say there is not a benefit found to choosing an initial tack when dealing with current.

I posted VERY early on in this thread a graphical and numerical refutation to this theory via a link that showed an accurate representation, including polars associated with pinching.

A refutation requires either a testable formula or a scale illustration of wffect. Neither of which has been provided for this whole thread.

[goboatingnow]

Originally Posted by foolishsailor

Further to my last post:

If
VMG = x
and
Pinching = x - y ( with y equalling the loss in VMG due to pinching)
and
Current assist to VMG by pinching = c

Then
Lee Bow Effect exists if c > y

We need to prove there is a current assist through pinching first and then worry about polars.

It is not possible to do the former.






c > y

( and Im sorry so many postings since I last posted here, hard to follow)

No , That might be described as the lee bow benefit. However its described as the lee bow "effect". Whether it benefits you on a race course might come down to lots of factors not just VMG, ( because VMG is simplely not the ONLY or even necessarily useful metric to use to determine whose is going to get to the mark first. )

for example VMG cannot see adverse currents ahead, eddies, bad wind, physical obstructions, other boats etc. Hence an advantage or disadvantage may arise that may not be indicated by VMG

dave

[foolishsailor]

Dave,

None of that is relevant to the discussion.

The "lee bow effect" either works in an instantaneous measurement or it doesnt exist.

Everything else you mentioned is about routing and is easily solvable as long as the conditions are predictable. In fact I would go so far as to say it is a straw man to this discussion.

[foolishsailor]

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.

In the absesence of foreknowledge or geographical knowledge of conditions - ie the theroetical problem we are discussing - there is no strategic be efit not tl be sailing optimal VMG upwind.

[goboatingnow]

Originally Posted by foolishsailor

Dave,

None of that is relevant to the discussion.

The "lee bow effect" either works in an instantaneous measurement or it doesnt exist.

Everything else you mentioned is about routing and is easily solvable as long as the conditions are predictable. In fact I would go so far as to say it is a straw man to this discussion.

Bow effect gives a small increase in wind angle over the situation where there is no effect. Whether you can get an advantage from that effect is an entirely different situation. At the end of the day the purpose of utilising these "effects" , are to give you a perceived "advantage" not just an improved VMG

You a racing boat coming up slightly because of better wind angle due to current , may confer upon itself an advantage by way of position, for example, even if it might be moving slower. If it success in reaching the mark, it workd , if not it didn't.

But I have never heard the lee bow effect delineated in terms of VMG anyway,

dave

[foolishsailor]

Whether it is a fixed mark for a race or a waypoint for a cruise - if by altering your course to engage an "effect" you dont see an increase in VMG them tje effect is not benefiting your ETE to that waypoint.

[goboatingnow]

Originally Posted by foolishsailor

Whether it is a fixed mark for a race or a waypoint for a cruise - if by altering your course to engage an "effect" you dont see an increase in VMG them tje effect is not benefiting your ETE to that waypoint.

That is where progress to the mark can be ascribed purely in terms of VMG. Ive not been in a race where it can be soley used to measure success to the next mark

dave

[foolishsailor]

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.