Wind against tide - physical reasons why this is dangerous ?

[Boo2]

Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

 

[fergie_mac66]

difference in speed

In simplified terms the tide changes the shape of the wave. It slows the deep part of the wave in relation to the top of the wave, sort of trips the wave up.The waves become higher and unstable and start to break

 

[FullCircle]

Basically, the wind gets under the waves and makes them stand up, so they will be steep and choppy, hindering your forward progress.
Excellent thread here on Cruisers Forum....

http://www.cruisersforum.com/forums/f131/challenge-explain-the-physics-of-wind-over-tide-32570.html

 

[Channel Ribs]

We had an interesting one yesterday while drifting in a tide of 2kn, a ferry went past (at 3 cables) and when the wake eventually arrived it was moving diagonally across the tide. You could plainly see the waves grow in height and then fall over themselves into breakers.

I hope that description helps?

 

[LONG_KEELER]

Hi,

I keep reading that conditions often become rough when the wind is against the tide and I'm curious to know the physical reasons why this is so ? Obviously you need to add the windspeed to the tide speed to get an idea of how much of a sea will result, but if the wind is say 30 kt and the tide is 4 kt, does the resulting effect amount to worse conditions that a wind of 34 kt would have given ? From what I've read the answer is "yes", but I'm interested to know what is going on for that to be true ? Is it an interaction with the sea bed or what ?

Thanks,

Boo

It does seem to get a bad press.

But the other direction, down wind, is a vote grabber !

In a F6 & F7 it's as good as sailing gets. Finding a good enough reason for going in an opposite direction than you intended is often the problem though. Normally to do with going back to work.

 

[dt4134]

It shortens the wavelength of the waves, making them steeper.

The wind is not necessary for the effect, albeit it does make things worse, as a train of waves from the open sea encountering an area of strong tidal flow will steepen and tend to break. It can get pretty bad even on a calm day in some specific areas where a large swell encounters a strong ebb.

If the tidal flow is in the opposite direction, i.e. wind with tide, the wavelength will increase and the sea will feel smoother and calmer.

 

[Skysail]

A previous thread:

http://www.ybw.com/forums/showthread.php?t=31281&highlight=Here+in+Cook+Strait

 

[Roberto]

It does not explain anything about physics, but this graph (I think it comes from the Scripps Institution of Oceanography) gives some insight into the relationships between wave characteristics (length/height, curves L and H) and current speed (related to wave period/celerity, horizontal axis); it may also give an idea why in a tide against wind situation some waves break and some do not.

[/URL]



the full description can be found here

http://sybrancaleone.blogspot.com/2009/04/eng-waves-and-tidal-streams.html

 

[Ricd]

It does not explain anything about physics, but this graph (I think it comes from the Scripps Institution of Oceanography) gives some insight into the relationships between wave characteristics (length/height, curves L and H) and current speed (related to wave period/celerity, horizontal axis); it may also give an idea why in a tide against wind situation some waves break and some do not.

[/URL]



the full description can be found here

http://sybrancaleone.blogspot.com/2009/04/eng-waves-and-tidal-streams.html

Very simple and clear description..useful thanks

 

[savageseadog]

Sea bed interaction nearer the shore is extremely important. The other effect, not mentioned is that the relative wind speed is increased by the current

 

[AliM]

Well, i am a physicist and I am still quite puzzled - but this is my intuitive explanation:

In my experience, the effect is much worse in shallow water. The wavelength of the waves, and therefore the steepness of them, has a strong dependence on depth (shallow water leads to short waves which are therefore steeper).

No wind:
The molecules of water actually move in a circular trajectory within the wave, with a bit of extra motion with the tide - so end up in a spiral, with their fastest motion forward at the top of the wave.

Add the wind:
The wind hits the front of the wave (in the wind against tide case) and blows the water molecules back, steepening the wave.
If the wind is in the same direction as the tide, it hits the back of the wave blowing the molecules forward, flattening it. The difference is only a few knots, but the difference is very marked - as we all have experienced!

 

[bazobeleza]

'Sea bed interaction nearer the shore is extremely important. The other effect, not mentioned is that the relative wind speed is increased by the current'

Don't forget to add in overfalls, wonderful things that can turn a mere wind over tide situation in a really hair raising experience when added to the mix.

In a F7 on a spring tide the overfalls off carmel head wales once brought Beleza to a shuddering halt and she's a 20 ton boat and motorsailing.

 

[Salty John]

Wind-over-tide works on an oceanic scale too, so seabed interaction is a separate issue. For instance, the Gulf Stream runs north between Florida and the Bahamas at around 3k. Wind over 15k from the north creates very uncomfortable conditions whereas wind from a southern quarter leaves the sea state quite benign. Water depth is around 1800 ft.

 

[dt4134]

Well, i am a physicist and I am still quite puzzled - but this is my intuitive explanation:

Hi AliM,

I'm an ex-physicist. I've just tried dusting the cobwebs off an old textbook on waves to see if there were any diagrams I could still understand, but there was none that was appropriate to this.

I suspect the main effect is pretty much the same as the Doppler Effect. The waves are generated at a certain frequency based upon wind speed (and of course fetch etc.) and the velocity of propogation is determined by that (could look up all the numbers for that I guess but haven't yet).

When the waves enter an area of moving water the wave is slowed down by the same amount as that water is moving (I'm assuming for simplicity that the moving water is moving in completely the opposite direction to the waves) yet the frequency remains the same. Therefore the wavelength will shorten.

The above assumes constant depth of water. When you combine that with the effect of the shallowing of the water (which is often the case where you get waves encountering a strong ebb) thats when that bit of sea becomes famous and gets its picture in the pilot books.

Of course there's also the effect of the wind on the steeper seas, the increase in the apparent wind speed (from F7 to F8 in the OP's example) and any disturbance caused by the topology of the sea bed, particularly abrubt changes in depth.

 

[AliM]

JCP,

Yes, but when you look at it as a simple Doppler effect, then I don't think it can be the full explanation. In a 20kt wind in a river with a 1kt current, say, the sea state with the wind against tide is a lot nastier than in a 21kt wind with slack tide. Similarly, with the 20kt wind with the 1kt tide, it feels a lot nicer than a 19kt wide against 1kt tide. It must be very non-linear.

The Doppler effect gives the shortening of the wavelength (linearly), which is more obvious when the water is shallow, and this tells you about the gradient of the wavefronts (again linearly) along with the fetch which gives the amplitude.

Perhaps I'll look up the scientific papers, when I've done some of the stuff I'm supposed to be working on!

Ali

 

[Channel Ribs]

Are you sure your not over complicating it? The water under the surface is doing one speed and the air above the water is going in the opposite direction, therefore you end up with a turbulent area where the two interact. Given that water forms waves whenever it's flow is not the subject of a single force, you get waves that increase dramatically as soon as the forces increase.

 

[Quandary]

The Malthouse answer (without molecules) seems to make most sense to me.

 

[dt4134]

JCP,

Yes, but when you look at it as a simple Doppler effect, then I don't think it can be the full explanation. In a 20kt wind in a river with a 1kt current, say, the sea state with the wind against tide is a lot nastier than in a 21kt wind with slack tide. Similarly, with the 20kt wind with the 1kt tide, it feels a lot nicer than a 19kt wide against 1kt tide. It must be very non-linear.

The Doppler effect gives the shortening of the wavelength (linearly), which is more obvious when the water is shallow, and this tells you about the gradient of the wavefronts (again linearly) along with the fetch which gives the amplitude.

Perhaps I'll look up the scientific papers, when I've done some of the stuff I'm supposed to be working on!

Ali

Hi Ali,

I was thinking more of the comparison between the wave speed in stationary water and the tidal flow. That I think is more significant than the wind speed itself & the rate of tidal flow.

I remember seeing a table of the relationship wave height, wavelength, wave speed, fetch and wind speed but can't find it now.

Anyway, I'll have to drop out of the debate now as I'm off to do some applied chemistry (applying expensive chemicals from a tin of International Mega-gunk to the bottom of the hull).

Regards,

JCP.

 

[TQA]

Don't care about the physics just know that I do not want to do it again.

20 to 30 knots from the North while out in the Gulf Stream south of Cape Hatteras [ mini depression that was not forecast ] gave sea conditions that had large amounts of green water crashing aboard from all sides and the only time I ever worried about the stability of my tough 38 foot steel ketch.

That was a nasty day!

 

[Sans Bateau]

Both wind and water provide friction. Putting your hand out of the car window you will 'feel' the wind. Likewise, put your hand in the water from on a moving boat, you will 'feel' the pressure of the water against your hand.

Next, get a glass of water and blow across the top of it, you will produce little wavelets. If the wind is blowing across water that is not moving then also wavelets will be created. That is, the same as the glass of water, the friction of the wind dragging the water. If the water is moving (tide) and the wind is in the same direction the effect of the wind on the water is reduced. For example if the wind is blowing at 20 knts and say the water is moving at 5kts in the same direction the effect on the water will be as if the wind was only doing 15knts. The wind will also smooth the water and push it in the direction it is already flowing.

The reverse is the case when the wind and tide are in opposing directions (wind against tide). Say the wind is blowing at 20knts and the water is moving at 5knts in the opposite direction, not only is the effect on the wind on the water equal to 25 knts, more importantly the friction of the wind will be trying to push the water back in the direction it is flowing, this will cause the surface of water to build up into steeper wavelets, these in turn, depending on the strength of the wind are providing a rougher surface for the wind to push against, so the wavelets continue to build until the force of the flow of water allows it to flow in the direction it is trying to go. Depending on conditions, sometimes the waves can be very steep. And you dont have to go off shore to experience this, try running down the Solent past Cowes on spring ebb with a strong SW blowing.

The stronger the wind the steeper the waves. On a boat, its a wave that is more likely to cause you a problem than the wind.