just why, exactly, is wind over tide rough?

[ QUOTE ]
I sailed on the Humber where fast tidal streams and shallow water are the rule.
I didn't need to know why it was rough, just that it would be.

Life's difficult enough without trying to work out if the doppler effect applies. /forums/images/graemlins/wink.gif

[/ QUOTE ]Yes - but was it just the fast tidal streams that made it rough, or the fact it was shallow water as well?
So - wouldn't you say the more we understand about a natural phenomenum the more we can predict what may happen in other circumstances?
 
[ QUOTE ]
Life's difficult enough without trying to work out if the doppler effect applies. /forums/images/graemlins/wink.gif

[/ QUOTE ]

But why is your life difficult? Or do you just accept that it is? /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
 
Isn't this an apparent wind and friction issue?

Waves are a result of wind friction on water, more wind > more friction > bigger wave.
Wind with tide = less apparent wind on the surface of the water = smaller wave.
Wind against tide = more apparent wind = bigger wave.

This can sometimes be seen when the tide changes direction it can affect the wind at lower levels.

Changing from wind over tide to wind <span style="color:blue"> with </span> tide, reduces surface friction and can increase the surface wind for the duration of that tidal flow.

Or is that too simple?
 
If you follow the threads on straightforward black and white things like Col Regs on here you will see that working out when you are likely to encounter wind over tide conditions and why will be a life's work for some people. And they'll still get it wrong! /forums/images/graemlins/wink.gif
 
Now I don't understand your position!

It is an effect caused by the relative velocity of the listener and the sound source. It is not an 'illusion', there really is a frequency shift at the listener. You are correct that someone at the side hears less frequency shift, if they are infinitely far away so that there is no movement towards or away from them, they do not experience any frequency shift.

Someone driving past a staionary listener at the same speed as the police car hears the 'true' note while the stationary listener hears a shifted note even in the instant when they are at exactly the same distance and direction from the siren. Their velocity difference causes a change in the frequency of the sound hitting their ears.

In the same way, stars moving away from earth look redder than they would if they were staionary relative to earth, stars moving towards us loook bluer.
 
Sorry Benbow, I agree with what you say exactly. If you don't understand its because I stated it badly.

However I don't think Doppler is the issue with w over t.

Peter
 
Fin,
I think it is more than friction. Re-read the original post. Why are cases 2 and 3 different?

In other words why does a certain apparent surface wind speed give rougher sea when a higher proportion of that speed is contributed by tide? When the tide turns the change in apparent wind is only a few knots and yet kicks up a hell of a sea. That same number of knots more on the wind speed (in slack water) would have little effect.
 
Completely agree!

I thought I understood w vs t, but as soon as some asked what was going on I realised I didn't.
 
It's baffled me too for years, for the same reason but I eventually resolved it (in my own head at least) with the thought that it is all about the apparent wind.

In other words your statements 2 & 3 are not both true. Either 13 knots apparent makes a particular body of water rough, or it doesn't.

For any given tidal passage with overfalls and tidal effects then there's likely to be one direction which is rougher than the other. When you add in an opposing wind then the existing ripples and swirls from the current get exaggerated by the extra apparent winds.
 
Apparent wind? is apparent because of the direction one is travelling in? I.e. if you travel North at two knots and wind is southly two knots No apparent wind? if wind is Northly 2 knots then apparent wind four knots?
 
Sorry I was talking about apparent wind at the water surface as in the original post, i.e. actual wind 5 knots, water speed 3 knots so apparent wind 8 knots against the tide or 2 knots with it.
 
Now we are getting somewhere....

"In other words your statements 2 & 3 are not both true. Either 13 knots apparent makes a particular body of water rough, or it doesn't. "

Do we agree with this or not, from our experience?

I certainly would have said this before I started sailing, but my experience does not bear it out. I would say, from experience, that cases 2 and 3 *are* different. It's counter-intuitive, but they are.
 
Re: Now we are getting somewhere....

[ QUOTE ]
I would say, from experience, that cases 2 and 3 *are* different. It's counter-intuitive, but they are.

[/ QUOTE ]

Absolutely agree. The effect of wind over tide is definitely greater than just the change in apparent wind speed. Also the 'shape' of the waves changes: much shorter and steeper and more likely to break. Come on! - you must all have noticed this.

I think I'm going to take Lakesailer's advice and just accept that it happens and avoid it above F4 whenever I can.
 
I tend to agree the point re: friction. If totally off the mark - I apologise in advance (as I am no physicist) - However the way I see it is there are two opposing forces. Sitting at my desk reading the above - i considered a piece of paper going across my desk from left to right (tide in one direction) - if the wind was travelling in the opposite direction and thus an opposite force it would result in paper billowing up (like a wave) - try it will a piece of paper - you may see what I am trying to explain.
 
If the wind is blowing over the sea at 10 knots over a distance of 100 miles, it will take 10 hours to reach you. It is well documented that that will create waves a of a certain height.

If there is a current of say 3 kts flowing with the wind, then the wind is actually only exposed to an effective surface fetch of 70miles - smaller waves.

The tide turns and the current now flows against the wind at 3 knots, the effective reach becomes 130 miles - 60 miles more than it was before - substantially bigger waves.

Then, a wave travelling with the tide towards you at - for the sake of argument 7 kts, (and I ave no idea whether that is a realistic figure) will have an SOG of 10kts, so will not be so big as the same wave pushing against the tide with an SOG of only 4kts. Theoretically it would take 25 hours to reach you over the 100 miles stretch and would be a great deal bigger than the wave which has only had 10 hours to build.

Another consideration assumes that the wave absorbs power from the wind in direct proportion to the wind strength. If therfore it is compressed in the wind over tide situation, it will gain height and speed in direct relation to the power it has absorbed. If it is then 'stretched' again over a larger area of sea as the current starts moving wth the wind again, then it will appear to decrease in size as the frequency slows down.

This effect is particuarly noticeable with a swell which may be barely noticeable in deep water, but as it enters shallows the ground has a similar effect as an adverse current, and it returns rapidly to the 'big wave' state it started off as.

'Least, thats how it looks to me after watching it happen for the best part of 50 years.
 
The comparison between schmoo's cases 2 and 3 is misleading. Wind and wave are not independant and not symmetrical with regard to direction: waves are generated by (a following) wind. There is no reason to think that an opposing wind is going to behave in the same way, and certainly not to gently enhance the wave as a following wind would.

The comparison between cases 1 and 3 is more significant. The drag force exerted on the wave by the wind will be proportional to the square of the relative velocity. In case 3 the relative velocity is nearly twice that in case 1: the drag force will be nearly four times as great. Quite significant!
 
looking at the cross section of a wave reveals a circular motion inside the wave as the energy passes throught the water. it moves the water forward, up then down and back where it started (in a perfect case)

so, wind (energy) against the wave (energy) slows the face down, makes a steeper face and heaps up the crest. (wavelenght, period, speed and height)

wind energy behind the wave decreases the wave height but increases the speed - changes the wavelenght / period etc. /forums/images/graemlins/cool.gif
 
Thanks for the thoughtful contributions. However I need to tax your patience still further since I am still not satisfied.

My original question seems to have misled guernseyman into thinking the wind direction changes in the senarios. Forgive me: I meant to keep the wind direction constant, only change stength to contrive to keep the apparent surface wind the same in both tide states.

Also, even if the drag forces are four times as great for 13kt wind as for a 7kt one, that doesn't explain the increase in roughness when a significant part of the apparent wind speed is contributed by tide, rather than just by wind.

I would expect an apparent surface wind of a particular force, (either 7kt or 13kt) and direction to give rise to a particular sea-state, regardless of how much of the apparent speed is contributed by tide. By observation, this is not the case. By observation, the greater the amount of the apparent surface wind that is contributed by the tide, even for a contant apparent surface wind, the rougher it is.

I am not sure that scope is that relavant for a couple of reasons. 1) The tide will have turned a couple of times while your wave was covering 70-100 miles 2) The wind over tide effect seems to turn on and off pretty quickly as the tide turns, and can be quite local in races, even with little surrounding swell

By the way...
shmoo dosn't have a "C". The previous owner of the name (a very cunning cat) would be turning in his grave....
 
when looking at the scenario I assumed it was coastal with no fetch. it is of course more complex in open waters.

a good example of the oceanic situation is off s africa, where the aghulas current meets the prevailing westerlies. here we have evidence of 'double troughs' or killer waves which sink web page - mystery sinking etc ... and which damage even the largest modern vessels.

the same principles as mentioned before exist but are compounded by large seas (wind born waves) interacting with the southern ocean swell interacting with the contra current.

this doesnt fully explain your query but hopefully will lead you to explore this phenoma - which will satisfy your curiosity.

any specific queries then please revert, I may be able to point you in the right direction - but most can be answered by studying oceanography, a facinating subject.
 
Yes I see, shmoo, I was confused by your use of the word 'water'. In reality there are two things happening in the water: (1) the waves travelling in the direction they were generated, and (2) the tidal current, which may be in the same direction as the wave travel, or not.

I need to think that case through again.
 
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