Sea State? Wind against tide

[lw395]

Or those who dont understand simple vector maths

I have a fair grasp of vector maths, thanks.
There's some things in real world sailing that aren't on the CSE maths syllabus though.
You need to think about how the water in a wave moves, and how it interacts with the sea bed.
Or just observe.

 

[jdc]

The idea that wind generated waves produced on a moving sheet of water will be just the same as a wind of the relative speed acting on still water seems logical but is actually making the assumption of an infinite sheet of water moving at uniform speed. This is clearly not the case, so talk of simple vector addition is over simplified (pun intended).

What happens I think is that waves are generated over a considerable fetch and propagate quite a bit faster than any current. The rough water one experiences in wind against tide situations occurs when waves propagating along meet⁽¹⁾ water flowing against the direction of propagation. At the interface the waves shorten, ie wavelength falls because the frequency has to be preserved: there's no creation or destruction of waves in a train.

Since gravity waves in water break when the ratio of height to wavelength exceeds about 0.07, with quite an abrupt threshold, it's actually quite easy for waves to change from not breaking to breaking simply through a change of speed of only a couple of knots.

Were the waves to have been generated in the moving water by wind a couple of knots stronger they'd be just as high, but not so steep and so would be less likely to break.

(1) this interface is the key to the rough water I believe, ie the waves have come from elsewhere and have then met the moving water.

 

[lw395]

...
(1) this interface is the key to the rough water I believe, ie the waves have come from elsewhere and have then met the moving water.

I think that is one effect, certainly waves coming in from afar make their presence felt at places like St Aldhelms Ledge when the tide is against.
That can be rough even if the wind has died down a lot.
In fact, a significant change of depth can affect waves without any input from the wind.
The waves systems in rivers are interesting, often mostly about current and depth changes, not much wind involved.

But there is a pure wind v current effect which does not need the waves to be coming from elsewhere.
We get it in the face where we race dinghies!

 

[mike1956]

I am the OP. Thanks for replies.

I will try and get my head round all this in due course.

Mike

 

[sighmoon]

Interesting thread. Anyone notice how all the snide remarks are from those who maintain that adding vectors solves it, whereas those who have asserted that it's to do with the mechanics of waves have avoided being rude and patronising.

 

[Daydream believer]

Interesting thread. Anyone notice how all the snide remarks are from those who maintain that adding vectors solves it, whereas those who have asserted that it's to do with the mechanics of waves have avoided being rude and patronising.

Yes but there is still an undercurrent of (e)motion

Wave to me when I go I am a swell guy with a great depth of character - you can bank on it !!!!

 

[rogerthebodger]

Have a look at the work done in studying rogue waves.

This is of interest to us on the east coast of Africa as we have a current of 4 to 5 knots running from the north east down the coast and winds blowing up the coast from south west and can produce "rogue waves" so as any one who have sailed down the east coast of Africa to Cape Town timing in very important to avoid the wind against current situation.

http://en.wikipedia.org/wiki/Rogue_wave

 

[LeoLind]

I don't think it's as complicated as some make it sound. The effect can happen in deep water (e.g. Gulf Stream) and without wind, so some explanations proposed don't apply. 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 are converging towards the center where they add their energy and height. The opposite effect exists when a swell curves around an island which spreads out the wave-train and weakens the waves. Some harbours exposed to large ocean seas use specially designed jetties to the same effect. I have illustrations here http://www.3dym.com/waves/waves.htm