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

[Seaworthy Lass]

Originally Posted by Hoofsmit

Agreed , never knew he was a east coast sailor, he ought to have Seaworthy as navigator , that would be interesting

No, I would be in the hands of a master here LOL. Dockhead is an expert in navigating the Channel .

[I.Grind]

Question:

Having never crossed the English Channel, is there an optimal timing? My guess would be to try and time it so the tide is slack at the midway point, providing what one hopes to be symmetrical current.

[Seaworthy Lass]

Originally Posted by foolishsailor

A really great thread. Capt Force's incredibly graceful and hilarious capitulation is one of the best posts of all time on this forum.

Not only that, Capt Force opened the eyes of many (myself included) how the ground track would be sinusoidal on a passage where the wind was coming all from one direction, just varying in amount, if you were following a single CTS.

Before all of you say that is only of academic interest it is NOT. If you are following a single CTS it is important you plot your expected ground track beforehand to make sure it doesn't take you into any dangerous areas (eg land ).

[Freedom45]

Arrrghhh.....
I say keep the boat pointed the best you can in the direction you want to go, you''ll get there eventually.
It's sailing, enjoy the ride, finish reading or writing that book.
If I wanted to get there in a hurry, I would have bought a power boat.

Chuck

[Hoofsmit]

Originally Posted by I.Grind

Question:

Having never crossed the English Channel, is there an optimal timing? My guess would be to try and time it so the tide is slack at the midway point, providing what one hopes to be symmetrical current.

its normally dictated by the height of tide of your destination , ie : can you get into harbour, I forget without looking, but some have 12 ft tidal range , also if heading to Guernsey you have some amazing tidal races which can run over 14 knts , you don't wanna hit those the wrong way !!!

Some times you just have to lay off and have a cup of tea ( or a beer if you are a us citizen ) and wait for the tide

[Hoofsmit]

you would normally try to make the passage with a equal tide both ways.

but think of the channel as a big river, the ebb is weaker than the flood so there is that to consider dependant on which side of the centre as in which bit of France you want to be, but your rhumb line normally points you to that clew very early on in your passage plan, if you can find a tidal stream atlas and have a nose, it will help with a lot of the opinions on this thread

sorry flood weaker than the ebb

[Hoofsmit]

Originally Posted by Seaworthy Lass

Not only that, Capt Force opened the eyes of many (myself included) how the ground track would be sinusoidal on a passage where the wind was coming all from one direction, just varying in amount, if you were following a single CTS.

Before all of you say that is only of academic interest it is NOT. If you are following a single CTS it is important you plot your expected ground track beforehand to make sure it doesn't take you into any dangerous areas (eg land ).

Dame .... I really didn't realise some skippers thought like this , I feel I have risen in the ranks lol

[Hoofsmit]

Having read this complete thread though,

some thing is obvious, ( apart from I am as good at written as a bull in a china shop )

We have an international and educational divide --- we cannot even agree that

A tonne is not a Ton

A gallon is not a gallon

A knot is a measurement of nautical velocity, or a method of seizing a rope. unless we highlight in which context it is used

Displacement is not the weight of your boat ( I am just making a point not a discussion)

what hope do we have ? lol

[Paul Elliott]

Originally Posted by Dockhead

Umm, the theory is no less valid as a theory, just because it's hard to implement when sailing around the cans.

But sailing around the cans, neither will you encounter changing tide. [...]

One of the races I do is called the "Three Bridge Fiasco". This single or doublehanded takes place in San Francisco Bay, and the start/finish line is off the cityfront (near the America's Cup finish line). We can start and finish by crossing the line in either direction, and have to round three marks (on port or starboard) in any order. These marks are near the Golden Gate Bridge, the Richmond Bridge, and the Bay Bridge. The shortest path around is about 22 miles. Some years there are over 300 boats participating.

We hold this race at the end of January, when the wind is often very light, and the currents are very strong. Some races our best strategy has been to anchor and have lunch while we wait for the wind or the current to change. The race starts in the morning, and the "did not finish" deadline is 7:00PM. Many boats take longer than that.

So don't tell me that we don't encounter changing tides! And this is a situation where the CTS method is completely irrelevant. I would be crazy to use it.

But so what? We really ought to understand the principles, and that means we simplify the conditions. You don't learn how to calculate a rhumbline path by throwing an island in the way and saying: "See? It doesn't work!"

If you are interested, here's a VALIS blog entry from one of the Three Bridge Fiasco races.

[Dockhead]

Originally Posted by foolishsailor

Good luck with the docking Camaron - that is a big boat to drive in yourself.

Thanks! In the event, it went fine. The last time I docked single-handed in Yarmouth, a small crowd gathered to watch what they thought would be the inevitable crash I was happy to disappoint them Had a nice sail today; glad I didn't stay and work on the boat as I planned

[Dockhead]

Originally Posted by Freedom45

Arrrghhh.....
I say keep the boat pointed the best you can in the direction you want to go, you''ll get there eventually.
It's sailing, enjoy the ride, finish reading or writing that book.
If I wanted to get there in a hurry, I would have bought a power boat.

Chuck

Crossing the Channel at springs in a slow enough boat, it is actually possible to never arrive We proved it mathematically in the other thread

[Paul Elliott]

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.

But in your diagram, in both cases the TWS(water) velocity changes with current, and in both cases the boat speed will therefore change, thus changing the Apparernt Wind. For that matter, when the wind speed changes, so does our boat's optimum angle change (per the polars), also resulting in a different Apparent Wind speed /angle.

The calculations are simplified if you first take the Ground Wind, and apply the current vector to derive the True Wind (water). You can then use True Wind (water) and your polars (or your experience) to chose your desired heading and target boat speed (which results in a particular Apparent Wind). We then work out the current issues and our track over ground.

Of course at the end of the day we are trying to reach a point on shore, and the forecast wind and current are both ground-referenced. These are the underlying factors. But if we translate this data to a water-based frame of reference we can better understand certain aspects of the problem.

At some point we may decide that the currents, winds, and obstacles are too varied and chaotic for these simpler techniques to handle. You always have to use judgement and experience. This is the reason that most (all?) routing programs (such as Expedition) use a directed Monte Carlo style of route optimization. (The Monte Carlo method essentially throws random numbers at the problem, looking for the best results.)

[Dockhead]

Originally Posted by I.Grind

Question:

Having never crossed the English Channel, is there an optimal timing? My guess would be to try and time it so the tide is slack at the midway point, providing what one hopes to be symmetrical current.

Yes, some departure times will give you better times than others. The reason for that is that the length of your water track will be longer if you are set off further in one direction than the other. And yes -- slack tide in the middle is best if you can do it within two tides (about 12 1/2 hours), and if you actually get to the middle at slack tide

Actually one of the most ironic outcomes of the CTS thread earlier in the year was that I -- who had always thought I really had the manual method down -- have abandoned doing it by hand and now only use a computer program called Neptune. The analysis in the thread allowed me to understand how inaccurate the hand methods are. Now after a season doing it that way, I can say it rocks

[Hoofsmit]

Originally Posted by foolishsailor

Those are already all added in - the boats COG and SOG are already a summation of all those vectors of force.

This is my point, they are ground referenced

If you use the tablecloth as suggested by some we are mixing it up, we then start to ignore that our hull is in the water at an angle to the current, we have lift because of that angle, I just am not intellectual enough to prove it

This is what makes dock heads post I referred to so wrong

[Dockhead]

Originally Posted by Seaworthy Lass

No, I would be in the hands of a master here LOL. Dockhead is an expert in navigating the Channel .

Blushing here. Well, I know how to do it, but there are many more experienced Channel skippers. This is just my fifth year sailing here.