Lee bow effect when running?

Simondjuk said:
Goodness me! All that nonsense over the difference between saying, "the effect of lee bowing" and, "the lee bow effect". And to cap it all, everyone but the main protagonist was clear that the former was what was being discussed. Still, I suppose it makes for a hobby, of sorts.
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Thank goodness we've got that out of the way. Now we can move on to the original question which, if I can remember that far back, was about was about the effect/use of tide when running downwind.

That situation is where the lee quarter effect comes into play! ;-)
 
LittleSister said:
Thank goodness we've got that out of the way. Now we can move on to the original question which, if I can remember that far back, was about was about the effect/use of tide when running downwind.

That situation is where the lee quarter effect comes into play! ;-)
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Then we can move on to the Lee Mack effect.
 
I think Gary Jobson states the answer to the original question most clearly
In the book 'Championship Tactics' IIRC
something like
On a leg which is long enough to expect the wind (or tide?) to shift, always be on the making tack..
The making tack being the one which gives the highest rate of closing with the target.
Which might mean going with the tide to be on a more headed tack downwind, a bigger component of wind across the boat equals more apparent wind, or the same forward movement of the apparent for less deviation from the calculated CTS.
The snag is that the true wind will be shifted by the tide to some extent.
In a channel crossing I would expect the true wind to shift enough in 8 hours to dominate in practice, and/or the true wind off Cherbourg to be different from the true wind off the Nab or wherever.
 
Some musings.

I found an interesting mental exercise was to imagine two boats at Bridge at slack tide, setting off X-Channel.

The wind at slack tide is bang on the nose. Mystic Meg has told us that the true wind at their destination and departure point and all points in-between will remain unchanged. The two boats merely have windguru's forecast, predicting what Mystic Meg has told you and I is certain to happen. [1]

One boat, "Tide Aware II", knows what time the tide created wind shift is predicted to arrive.

The other, "Blissful Ignorance", has been hypnotized and thinks the Channel is a lake with no tides.

How do they behave?

"Tide Aware II" puts himself on the best tack, knowing that as long as the forecast is right he will be changing tacks at the change of tide.

"Blissful ignorance" puts himself on the best tack for the current wind. He doesn't know how long he will remain on that tack, all he can do is remain on the best tack for as long as practically possible. He won't spend any time on the worst tack, until it's unavoidable.

So hours pass and we get near the change of tide.

"Tide Aware II" knows a wind shift is coming. However he doesn't tack at the predicted time of the change of tide. It's more accurate and easier to wait for the header he knows is coming. He tacks on the header.

"Blissful ignorance" doesn't know a wind shift is coming. However to his delight, by total chance, he finds he's slowly getting headed. Terrific, the other tack is now the better tack. He tacks on the header.

Two conclusions:

1) In the real world the boat that doesn't know the shift is coming, behaves broadly the same as the boat that does.

2) Moreover, in order for the knowledge of the time of the coming wind shift to shave significant time off a trip, you'd have to have spent those hours sailing on the worst tack? Why? Not even the last observant sailor will sail significantly off his best course for any length of time. If he does choose the worst tack over the best then tide awareness is clearly not his biggest problem.


[1] I've chosen wind guru because if we're planning our day around a small wind shift we need a forecast to given in degrees. A vague S to SW from the Inshore won't be good enough.
 
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But surely "Blissful Ignorance" was just lucky to have picked the right tack. If the wind was bang on the nose and expected to stay there it is a 50/50 decision for him and if he'd picked the other tack he would have found himself needing to tack mid channel just as a lift arrived and so he would have lost out.
 
DJE said:
If the wind was bang on the nose and expected to stay there it is a 50/50 decision for him
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The wind relative to something on land is on the nose. The wind experienced by the boat on the water does have a bias when the tide is running.

I accept that at slack tide that bias won't exist and "Blissful Ignorance" may initially choose the wrong tack for the wind change that we know is coming.

However as the tide fills in the bias will be detectable and "Blissful Ignorance" will know she has to tack (or sail miles off her best course).

If the bias is so subtle as to not be detectable it can't matter much.
 
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toad_oftoadhall said:
If the bias is so subtle as to not be detectable it can't matter much.
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Not so sure about that. The 5 degree shift which Flaming tells us that we cruisers cannot detect (and I tend to agree with him there) builds up to a difference of 2.7 miles made good after sailing 30 miles to windward. So about 5 miles for the whole crossing.

2 knots of tide and 20 knots of wind would give us a 5.7 degree shift from slack water. So the 11 degree shift when the tide turns will be more easily spotted but it might be too late by then.
 
I believe Toad is right that in the circumstances described, closely and accurately monitoring which tack is "better" and tacking when that changes would give broadly the same course as deciding ahead of time to choose the lee-bow tide.

Blissful Ignorance would I think need to do a few "trial" tacks to check which tack is better. As has been said, the effect may by fairly small and doing the math or using black boxes to predict COG and SOG on the the other tack may not be accurate enough.

"Better" here means the tack which makes the greatest ground speed across the tide, I suggest.

As for the OP, I would echo earlier comments that the downwind benefit is there, but less certain to be significant. For a lightweight boat with a huge Assymetric kite in a light breeze there's a massive gain. For almost any boat in loads of wind, the quickest route will be as close to the dead run as you can safely sail, and it really won't make much difference which gybe on which tide.
A.
 
DJE said:
Not so sure about that. The 5 degree shift which Flaming tells us that we cruisers cannot detect (and I tend to agree with him there) builds up to a difference of 2.7 miles made good after sailing 30 miles to windward. So about 5 miles for the whole crossing.

2 knots of tide and 20 knots of wind would give us a 5.7 degree shift from slack water. So the 11 degree shift when the tide turns will be more easily spotted but it might be too late by then.
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IMV if there's someone who fails to spot 5 degree wind shifts when sailing a long distance upwind then a) Considering and acting exclusively on wind shifts caused by the tide isn't going to help them because they're still missing 99pc of the wind shifts that occur and b) they can't be in a hurry anyway.

So yes, if someone doesn't care which direction they're going in then it will take them longer to get around and if slow passages don't bother them, fair enough. [1]

[1] Our experiences are obviously different, but I find it hard to credit that someone with a 60 miler in front of them would be so disinterested in their course as to fail to spot the fact they were sailing 5 degrees higher or lower than 30 minutes ago. It would be the prime focus of everyone I know for the majority of the sail. [2] Even if the chat was flowing to the exclusions of everything else many people would note a course in their log on the hour so they would pick it up then.

[2] You know the sort of thing that goes through your mind: "If it just lifts a fraction we'll make it on one tack."
 
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Alan ashore said:
"Better" here means the tack which makes the greatest ground speed across the tide, I suggest.
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I would argue better merely means better VMG.

In practice, let's say I needed 210 to get to Ormonville and I could comfortably make 180. The better tack to me would be the 180 tack, not the ~270 tack.

On that tack I'd say most folk would be staring at the compass and if they got lifted to 185 for a time they'd be smiling, if they got headed to 175 for a time they'd be frowning. I'd be amazed if they missed either change but DJE and Flaming's experiences are just as valid.

Yes, later on I might have to spend time on the ~270 tack but that's going to be left as late as possible in the hope that something has changed in 12 hours time.

It's all obvious, I know, but I've typed it now so it's staying. Sorry to patronise everyone.
 
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Alan ashore said:
Hmmm. For absolute clarity, what exactly do you mean by VMG?
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Rightly or wrongly in this case I meant the best speed at closest heading to the course I'd ideally like judged by the compass/log/gut feel which is 180 in my example when I'm trying for 210.[1]

That's what I meant, but post 70 is irrelevant to my point, and not intended to contradict any other criteria for 'best tack'.

[1] I don't always mean that by VMG though, sometimes I mean over the ground to a fixed point, shown on the GPS,
 
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