Wind against tide - physical reasons why this is dangerous ?

NormanS said:
Thats why I wonder why the Inshore Forecast, forecasts "Sea State". In practice, it doesn't mean anything.:confused:
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I agree with you. In my opinion, swell forecasts are far more useful. NWP models can predict swell and we sailors cannot. Sea-state is dependent on so many factors including tidal current that it is nigh impossible to give a meaningful value. However, it is a requirement defined as required under the GMDSS.

CROSS inshore forecasts give a sea-state but also swell. It is a shortcoming of UK forecasts that they do not. Is that because swell is more important as a determining factor governing entry to French (and Spanish) ports than too many British ones?
 
franksingleton said:
Probably the most definite comment that can be made about sea state is that it can certainly be complex. The variables are swell, wind-sea, bottom topography, coastal topography, current, reflection and refraction. Have I missed any? Near the coast, all can come into play. Which one or ones will be dominant will vary from time to time and place to place. Another definite statement – as ever, with anything connected to weather, there no hard and fast rules. There is no single answer.
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I agree.
Because there are usually so many factors applying at once, I feel it's useful to look at the cases where some of these factors are absent, like my Chichester Harbour example, where there is wind over tide but no incoming swell, or the way swell propagates around the Solent in no wind. Or to drift over St Aldhelm's ledge when the sea is a mirror one side but simmering the other.

I'm not sure there are 'no hard and fast rules' though. The laws of physics remain, it's just quite hard to apply them clearly.
 
lw395 said:
Conservation of energy
Conservation of angular momentum
If you can see Wales from Clevedon it is going to rain.
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Yes. Obviously. What I was trying to say, clearly inadequately, that sailors have no rules that they can apply. In principle, you can apply all these rules and get a forecast or a prediction of sea state. In practice that is not the case. NWP makes a pretty good fist of it but has to use many approximations, estimates and simplifications.
 
franksingleton said:
If a current develops opposing the swell then, as the period will be unchanged but the speed will be lessened by the speed of the current. Therefore, the wavelength must be shorter and the steepness (ie the ratio of height to wavelength) will increase.

If the current is with the swell, the wave speed increases and the wavelength must increase in proportion. The steepness will decrease.
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I've just read this whole thread (goodness knows why). That's the only completely correct passage anywhere in it.

There are a few considerations core to the concept here which haven't been mentioned in any post.

Surface friction (between water and air, and between water and the sea bed, between water of different densities)

Compressibility of water in comparison to air

The fact that most estuaries and all oceans are stratified.

Just my two penny worth.
 
bitbaltic said:
I've just read this whole thread (goodness knows why). That's the only completely correct passage anywhere in it.

There are a few considerations core to the concept here which haven't been mentioned in any post.

Surface friction (between water and air, and between water and the sea bed, between water of different densities)

Compressibility of water in comparison to air

The fact that most estuaries and all oceans are stratified.

Just my two penny worth.
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The thread wandered off from discussing why wind over tide is so horrible (tech term) to people denying that wind over tide in itself exists.
In my view, the seabed causes a velocity profile in the water, which changes the shape of the waves, causing more friction between air and water, hence more energy is transfered.
As the roughness of the sea increases, the air also becomes more turbulent, so the faster moving air a few metres above the water is mixed with the slow moving air at the boundary, hence there is faster air driving the waves.


Are you suggesting that strata in the water make it behave as shallow water, i.e. the waves only operate in the top layer? I'm not disagreeing, haven't considered that before.
 
I may be wrong but I get the impression that some on this thread are forgetting that, in a wave, the water goes up and down and not horizontally. It is the energy which is propagated in the form of wave action. The longer the wavelength, the further down will the wave effect extend. As the depth gets less, at some stage it becomes too shallow for the wavelength. The critical depth is about half the wavelength.

When the depth is less than the critical value the wave energy has to be realised somehow and that is by shortening the wavelength. This results in the steepness – the ratio of height to wavelength increases. When the steepness exceeds a critical value then waves break.
 
franksingleton said:
For the reasons at #89.
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Quite.. A lot of anguish .. and some useful analysis mind you.

In general, if tide or current / meets wind. Simple. generally nasty short waves ( but perhaps deep) meet boat. NOT comfortable on Boat. Much worse on close reach.
Avoid by looking at tidal atlas and weather?
Try going west against tide a Westerly in the North Channel?? madness - hide in Sanda or Rathlin. Use your head... your crew will thank you for it :D
Application of common sense solves it.
 
What we have never understood is why, when beating into a swell, one tack is often much more easy than the other. we have found this even when both tacks have a similar angle to wind or swell.

Any ideas? or should this be a new thread?

Of course, gentlemen do not beat but i have never presumed to be a gentlemen.
 
I don't think it's as complicated as you make it sound. The effect happens just as much in deep water and sometimes without any wind, so many previous explanations fail. As a mariner I have observed the effect often but in 1992 my eureka moment came one day while heading east on a motor yacht through the Current Rock passage in the Virgin Islands where I had a birds-eye view of whole effect clearly displayed in front of me. I could see that the current causes the waves in the center to travel a little slower than on the outsides, and that this bends the wave train, and now the waves on the left and right were converging towards the center where they added their energy and height. I have illustrations here http://www.3dym.com/waves/waves.htm
 
Having read the whole thread (I had nothing else to do) I think I sort of get it - my question is what is the effect of surface pressure on wave formation and indeed wind over tide conditions. Logic and intuition may suggest that low pressure may result in even higher waves and high pressure less - but do the models include this at all? Is there another force at work here we sailors should also be aware of?

I think so, but wondered if those far more technically nous may be able to educate me?
 
Peroo said:
Having read the whole thread (I had nothing else to do) I think I sort of get it - my question is what is the effect of surface pressure on wave formation and indeed wind over tide conditions. Logic and intuition may suggest that low pressure may result in even higher waves and high pressure less - but do the models include this at all? Is there another force at work here we sailors should also be aware of?

I think so, but wondered if those far more technically nous may be able to educate me?
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Pressure affects the height of the sea. An increase/decrease in pressure of 1 hPa results in lower. higher sea level heights of 1 cm. Changes in air pressure cannot affect wave height simply because the whole sea level will be lower/higher. Of course, with tidal surges, lower pressure will result in waves being higher but it is the sea as a whole and not individual wages.
 
jac said:
THis is a 5 year old thread - started again by a new person after a gap of 3 years. Can we not stop these somehow?
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They can be comical when 'advising' someone with, say, an engine problem that was probably rectified a decade ago. But, although LeoLind may now be wishing he'd paid more attention, is there really any time issue on general matters such as wind over tide? Unless, of course, you're suggesting it's no longer a phenomenon (being in the Med, I wouldn't know: plain old wind over water has been bad enough lately) ;)
 
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