Wind against tide - physical reasons why this is dangerous ?
- Thread starter Boo2
- Start date
ghostlymoron
Well-Known Member
I think it's unlikely that anything new will be said on this topic after 13 pages of discussion spanning 5 years. I would suggest that old threads coulb be made read only although I don't feel strongly about it.
I've sometimes contributed to old threads, and felt rather silly when someone - usually Vic or Nigel - have pointed it out.
I've sometimes contributed to old threads, and felt rather silly when someone - usually Vic or Nigel - have pointed it out.
jac
Well-Known Member
As others have said, is anything new being said? It brings a dead topic back to life, loads of people respond to what they think is a live issue and the OP has long gone. (Although not in this case) so those thinking they are being helpful are rather wasting there time.
I think all it needs is for those who review posts by new members to also look at what they are replying to. If it adds value, or is a genuine reactivation of the thread then fine.
It doesn't stop oldies reactivating long dead threads but it tends not to be the old hands that do it.
LittleSister
Well-Known Member
I think I read this thread in its prime. (Either that, or one uncannily like it.) But it's still interesting to read it again now, and perhaps get a bit more out of it and see the new posts.
What does it matter if some of the posts are old? Wind against tide hasn't gone away.
It's not as if it's crowding out more vital and up to date threads. If you're not interested, then don't read it!
What does it matter if some of the posts are old? Wind against tide hasn't gone away.
It's not as if it's crowding out more vital and up to date threads. If you're not interested, then don't read it!
MagicalArmchair
Well-Known Member
For my part, its the first time I've seen the thread, so quite interesting reading going back to the first page, so thanks for 'necroing' the thread (I believe that is what the 'hip' dudes call it...
)
)
franksingleton
Well-Known Member
What we really need is a good search engine and then use it before posting questions. There have been several/many threads starting with the same weather query. I am always happy to try to answer them but sometimes go back and copy a post from a previous thread to do so.
There is chapter in Reeds Weather Handbook on the tidal stream/ wave issue.
There is chapter in Reeds Weather Handbook on the tidal stream/ wave issue.
macd
Well-Known Member
Do feel free to provide evidence that wind over tide is not a live issue.
For the life of me I can't see the problem with born-again threads. Opening them isn't compulsory.
ex-Gladys
Well-Known Member
Coming out of Vlissingen with a SW on the ebb, I defy anyone to say it wasn't dangerous, Gladys was being thrown around like never before, I couldn't hold on to the crash bar whilst steering. Stand? Sit? Very dangerous to move around.
MagicalArmchair
Well-Known Member
"first holiday" with the girlfriend (she loved it, so now she's the wife!), wind over tide on the Wallet on the East Coast. The conditions were pretty moderate, gusting praps 28-30 knots apparent. That short steep and wet chop shows the great fun that wind over tide heralds, you can see how that would ramp up pretty quickly given either a stronger tidal flow or the wind building.
LittleSister
Well-Known Member
That's all very well , but this is a discussion forum. If we only wanted to know 'the facts', there are probably better sources than YBW forum that Google and other search engines could find for anyone sufficiently keen enough to follow up.
I think the mix of views, observations, perspectives, solid expert information and insights from people such as yourself, plus a generous dose of blind prejudice and wilful provocation from hither and thither, is what makes the forum entertaining and informative. That, and the way that subjects come up (or resurface!) that one probably wouldn't have set out to research, but is pleased to have drawn to one's attention, engage with, and perhaps gain some insights from.
Long live old threads!
Floyd Raser
N/A
Absolutely, but I see where Franksingleton is coming from; there have been two threads about painting engines within the last couple of days. Yes this is a discussion forum but that makes it an excellent source of facts. I have used the search engine here to great effect, finding out how to get my tiller pilot working for instance.
Maybe the moderators could use the code system to illustrate that a particular thread is over a certain age?
The search engine would be more effective if some people put a bit more care into the titles of new threads; "What's happening here?" may prevent useful info being found easily.
And it is interesting how threads usually get lively after around nine pages, which is why I looked in here!
macd
Well-Known Member
Such a system has been in place for some considerable time. It's in the format DD-MM-YY and accompanies the name of the OP, just below the title of each thread. Granted, the reader would have to do some fairly tricky arithmatic for this elaborate code to fulfill the function you describe. Those not up to the task could perhaps get their carer to help.
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Floyd Raser
N/A
+1 :encouragement:
Buck Turgidson
Well-Known Member
It's almost a brilliant post.
Except that this thread is still active as indicated by the name and date of the LAST post ;-)
macd
Well-Known Member
Shucks....you are almost too kind.
You're also right, of course. But I daresay that there's not a single active thread on the main forums that was begun even one year ago unless it's had once of these frightful resuscitations.
franksingleton
Well-Known Member
It is difficult to know whether it is better to start a new thread or carry on with an old one. Using the search engine for a specific topic may take you to a thread with many pages. It is not always easy to scan through several pages and sort out the wheat from the chaff particularly when there is some banter. It can be s easier to start a new thread with your question and risk being told that the answer is at ……..
In any case the world moves on. In my specialism, over the past year NOAA has improved its model and is now able to provide GRIB data on a 0.25 degree grid and Météo France now makes its high resolution GRIB data freely available. Somebody wanting some information about GRIBs and going to an old thread rather than starting a new one might not find those facts. Nevertheless, it can be a little frustrating to answer a question, maybe at some length, only to see the question asked again a few weeks later, perhaps in another forum. As it is, I think that we just have to accept being frustrated at times. It is always best to ask a question rather being deterred from doing so.
It is easy to miss a discussion on some topic of interest until it I too late to make a, hopefully useful, comment or ask a follow-up question. I find it difficult to monitor forums for new and potentially interesting threads. It is not helped by the search engines not always working properly although they seem to have improved lately. Particularly when away cruising, what is the best way to look out for new threads on particular topics?
Apologies for rambling on but, like others, I see this from the point of view both as a provider of information and a searcher for help.
In any case the world moves on. In my specialism, over the past year NOAA has improved its model and is now able to provide GRIB data on a 0.25 degree grid and Météo France now makes its high resolution GRIB data freely available. Somebody wanting some information about GRIBs and going to an old thread rather than starting a new one might not find those facts. Nevertheless, it can be a little frustrating to answer a question, maybe at some length, only to see the question asked again a few weeks later, perhaps in another forum. As it is, I think that we just have to accept being frustrated at times. It is always best to ask a question rather being deterred from doing so.
It is easy to miss a discussion on some topic of interest until it I too late to make a, hopefully useful, comment or ask a follow-up question. I find it difficult to monitor forums for new and potentially interesting threads. It is not helped by the search engines not always working properly although they seem to have improved lately. Particularly when away cruising, what is the best way to look out for new threads on particular topics?
Apologies for rambling on but, like others, I see this from the point of view both as a provider of information and a searcher for help.
Hydrozoan
Well-Known Member
I’m adding to the resuscitation rather than starting afresh. It’s a long and rather techie post, and in the end it won’t tell you how big the waves will be. So don’t say you’ve not been warned!
Reading the thread, I was struck by the difference between A: waves entering a current, and B: waves generated in a current – the latter emphasized by lw395 in #42 and later posts, e.g. #97.
Lapworth’s paper (http://onlinelibrary.wiley.com/doi/10.1002/wea.606/pdf) cited by franksingleton at #89 covered A - and said of its subject: “... the phrase ‘wind against tide’ … should really be ‘swell against tide’ …”).
My digging into B has been constrained by the fact that much of the detailed material is behind paywalls and seen by me only in partial ‘previews’ - and by my old and limited physical and mathematical oceanography. But with those two caveats I’ll offer what I’ve found.
The introduction to a 2000 paper by Suh et al. emphasizes the distinction between A and B (full text of paper at http://www.researchgate.net/profile/Kyung-Duck_Suh/publication/245292866). It cites papers on A, in terms of the effect of current on wave spectra – papers much more mathematical than Lapworth’s I imagine, so I didn’t pursue them further.
Regarding B, Suh et al. cite two earlier studies and say that both of them ‘… found that when the wind is opposing the flow, the waves tend to be higher than when it follows the flow’. But Suh et al. also argue (and later show), contrariwise, that high frequency waves in the equilibrium range of the wave spectrum may be enhanced with a following (rather than opposing) current. However, I believe that the ‘equilibrium range’ means microscale (cm to m wavelength) waves (Ref 1 below).
A book chapter by D H Peregrine in 1976 (Ref 2 below) cites amongst others the same two sources as Suh et al. It notes that a paper (of 1975 by Vincent, which I failed to find) rationalized their results and pointed out that the effective fetch was increased by an opposing current because the wave energy had to travel more slowly, and that large amplitude waves could arise from very short fetches with an opposing current equal to the group velocity of the waves (IIRC). A book by I Lavrenov (Ref 3 below) also refers to studies of wave generation in a current, to the effective fetch in the presence of a current and to the problems for ship handling of ‘wind against current’ waves.
Those sources all support lw395’s repeated point about the importance of B, as well as A (with ‘current’ as inflowing tidal current in his case). Regrettably, with what I’ve been able to see and do, I cannot use them to put numbers to the amplitudes likely with various winds, currents and fetches.
Perhaps someone with full access to the sources and the expertise will be able to do that - or at least check and correct any omissions or misunderstandings in this post, as I’m conscious of being outside my technical comfort zone. But I hope - with that warning - it may be helpful. And as the late John Ebdon used to say: ‘Anyway, if you have been, thanks for listening’.
(1) In introduction of paper by Dai et al. ‘The equilibrium range of wind wave spectra: an explanation based on white noise’, Journal of Ocean University of China, 2007, Vol6, 345-348.
(2) ‘Interaction of Water Waves and Currents’ in Advances in Applied Mechanics, Vol. 16, Academic Press, 1976.
(3) ‘Wind Waves in Oceans. Dynamics and Numerical Solutions’, Springer, 2003.
Reading the thread, I was struck by the difference between A: waves entering a current, and B: waves generated in a current – the latter emphasized by lw395 in #42 and later posts, e.g. #97.
Lapworth’s paper (http://onlinelibrary.wiley.com/doi/10.1002/wea.606/pdf) cited by franksingleton at #89 covered A - and said of its subject: “... the phrase ‘wind against tide’ … should really be ‘swell against tide’ …”).
My digging into B has been constrained by the fact that much of the detailed material is behind paywalls and seen by me only in partial ‘previews’ - and by my old and limited physical and mathematical oceanography. But with those two caveats I’ll offer what I’ve found.
The introduction to a 2000 paper by Suh et al. emphasizes the distinction between A and B (full text of paper at http://www.researchgate.net/profile/Kyung-Duck_Suh/publication/245292866). It cites papers on A, in terms of the effect of current on wave spectra – papers much more mathematical than Lapworth’s I imagine, so I didn’t pursue them further.
Regarding B, Suh et al. cite two earlier studies and say that both of them ‘… found that when the wind is opposing the flow, the waves tend to be higher than when it follows the flow’. But Suh et al. also argue (and later show), contrariwise, that high frequency waves in the equilibrium range of the wave spectrum may be enhanced with a following (rather than opposing) current. However, I believe that the ‘equilibrium range’ means microscale (cm to m wavelength) waves (Ref 1 below).
A book chapter by D H Peregrine in 1976 (Ref 2 below) cites amongst others the same two sources as Suh et al. It notes that a paper (of 1975 by Vincent, which I failed to find) rationalized their results and pointed out that the effective fetch was increased by an opposing current because the wave energy had to travel more slowly, and that large amplitude waves could arise from very short fetches with an opposing current equal to the group velocity of the waves (IIRC). A book by I Lavrenov (Ref 3 below) also refers to studies of wave generation in a current, to the effective fetch in the presence of a current and to the problems for ship handling of ‘wind against current’ waves.
Those sources all support lw395’s repeated point about the importance of B, as well as A (with ‘current’ as inflowing tidal current in his case). Regrettably, with what I’ve been able to see and do, I cannot use them to put numbers to the amplitudes likely with various winds, currents and fetches.
Perhaps someone with full access to the sources and the expertise will be able to do that - or at least check and correct any omissions or misunderstandings in this post, as I’m conscious of being outside my technical comfort zone. But I hope - with that warning - it may be helpful. And as the late John Ebdon used to say: ‘Anyway, if you have been, thanks for listening’.
(1) In introduction of paper by Dai et al. ‘The equilibrium range of wind wave spectra: an explanation based on white noise’, Journal of Ocean University of China, 2007, Vol6, 345-348.
(2) ‘Interaction of Water Waves and Currents’ in Advances in Applied Mechanics, Vol. 16, Academic Press, 1976.
(3) ‘Wind Waves in Oceans. Dynamics and Numerical Solutions’, Springer, 2003.
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LittleSister
Well-Known Member
Excellent stuff! It's great that you've highlighted that there are two different issues, given that a lot of the thread seemed to me to be people talking past each other because they had missed the distinction.
guernseyman
Well-Known Member
Hydrozoan
Well-Known Member
Thank you. Some people did see it and lw395 made a good point at #102, that it was useful to try to look at real situations in which one mechanism is operating largely in isolation (if one can find them, of course!).
Yes, I agree that is a better description; I was quoting Lapworth himself to emphasize that the wave generation side was not covered, in what is indeed an admirably clear paper.
I hoped to find a similarly clear exposition of the ‘wind generation in current’ case, but I could not get the primary sources cited by Suh et al. and Refs 2 and 3 (nor a full view of the latter texts themselves). I wonder if general estimation of enhanced wave generation by estuary/coastal currents might have been addressed (e.g. by a body like the US Corps of Engineers) in guidance for the preliminary assessment - i.e. before any detailed modelling - of proposed port or other developments. Perhaps someone with access to that literature can turn up something.
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