Sea State? Wind against tide

[mike1956]

It is generally accepted that in a wind against tide situation the sea state is rougher. I have never seen any suggestion as to why this is but does this sound likely.

With a 15 knot wind and a 5 knot tide in the same direction the speed of wind over water is 10 knots.

However with a 15 knot wind and the 5 knot tide in the opposite direction to the tide the speed of the wind over the water is 20 knots.

In the first example you get a sea state associated with 10 knots of wind and in second example a sea state associated with 20 knots of wind?.

Mike

 

[earlybird]

The relative speeds are not the main cause. In wind against tide, the water movement is moving wave energy upwind and concentrating it. The wavelength becomes shorter and steeper, waves higher. Opposite occurs for wind with tide.

 

[macd]

Long thread on this a few years ago. Much wind expended.

 

[savageseadog]

Wind over tide:
Wind is slowing the water down at the surface. The water has lots of kinetic energy so any ripples or waves will steepen and break if steep enough.

 

[GregAC]

Maybe another way to look at is you have two fluids moving in opposition, so you see turbulance at the interface between the two. When they are moving together the flow is more laminar.

 

[CFarr]

Wind over tide:.

I've seen this written a few times on here.
I always thought 'wind over tide' meant both in the same direction and 'wind against tide' as, well, what it says.

 

[savageseadog]

I've seen this written a few times on here.
I always thought 'wind over tide' meant both in the same direction..................

Not in English it isn't

 

[RupertW]

It is generally accepted that in a wind against tide situation the sea state is rougher. I have never seen any suggestion as to why this is but does this sound likely.

With a 15 knot wind and a 5 knot tide in the same direction the speed of wind over water is 10 knots.

However with a 15 knot wind and the 5 knot tide in the opposite direction to the tide the speed of the wind over the water is 20 knots.

In the first example you get a sea state associated with 10 knots of wind and in second example a sea state associated with 20 knots of wind?.

Mike

It's clearly exactly this and no more. The turbulence and the "bunching" must logically be precisely the same if it's a 20 knot wind over a zero knot tide, or a zero knot wind with a 20 knot tide, and all combinations in between. Neither the wind nor the water are aware of what speed they are going, nor does it matter any more than the high speed both are travelling through the galaxy as the Earth moves. Their relative speed is all that matters.

Obviously a tide can cause turbulence by its relative speed over obstacles and narrowing/widening channels, but the wind/tide effect then adds it's own changes based on the relative speeds.

 

[guernseyman]

As was explained in the previous thread, the phrase 'wind against tide' is a bit of a misnomer, and leads to many misconceived explanations.

'Wave train against tide' is better. The wave train will have been generated by a wind which may subsequently have died or changed direction. In the case of swell, the wind may have been thousands of miles away.

Concentrating now on the wave train encountering an opposing current, what remains constant is the wave period i.e. the frequency. The wavelength is correspondingly reduced, and the wave height increased. Thus the steepness of the waves has to increase, giving rise to the yachtsman's experience of a rougher sea.

 

[CFarr]

Not in English it isn't

Well, in English, It doesn't exactly mean they're in opposing directions either does it?

Good explanation Geurnseyman.

 

[johnalison]

"Wind over tide" has changed its meaning during my sailing career from wind with the tide to wind against, at least in popular usage, which, I am constantly being told, is the only arbiter in language. Don't get me going on "topsides".

 

[neale]

Surely the wind is always 'over' the tide, whichever direction it's going. Wind 'under' tide would be interesting! I use wind 'with' and wind 'against'.

 

[Resolution]

Surely the wind is always 'over' the tide, whichever direction it's going. Wind 'under' tide would be interesting! I use wind 'with' and wind 'against'.

Biggest smile of the evening so far! Thank you.

 

[Daydream believer]

It's clearly exactly this and no more. The turbulence and the "bunching" must logically be precisely the same if it's a 20 knot wind over a zero knot tide, or a zero knot wind with a 20 knot tide, and all combinations in between. Neither the wind nor the water are aware of what speed they are going, nor does it matter any more than the high speed both are travelling through the galaxy as the Earth moves. Their relative speed is all that matters.

Obviously a tide can cause turbulence by its relative speed over obstacles and narrowing/widening channels, but the wind/tide effect then adds it's own changes based on the relative speeds.

Not entirely correct as the speed of the tide over the sea bed contours will have an effect on the surface as well so a 20 knots over zero tide will not be the same as 15 kts over a 5Kt tide

 

[Boathook]

Best way to find out about it is in the western Solent with a SW6 and an ebbing spring tide. I've done it a few times and try to avoid if possible !

 

[lw395]

Best way to find out about it is in the western Solent with a SW6 and an ebbing spring tide. I've done it a few times and try to avoid if possible !

Yes, anyone who sails much in the Solent, or anywhere else with decent tides will know that there's a lot more to it than simply adding a few knots of tide to the windspeed.
In shallow water, even a knot or two of tide can make a F4 very rough.

While the Western Solent is somewhere where we need to respect wind vs tide, if you want a graphic proof of the effect, the place to look is either Chichester or Langstone Harbour in a Northerly, say F4 or F5.
The flood tide will kick up spectacularly, despite the waves only having a mile or two to build up, as the wind is off the land. As soon as the tide starts to ebb, the chop subsides.
The Western Solent obviously has the effects of waves coming in with a long fetch from Christchurch Bay or beyond on top of that.

'Wind over tide' may not always mean 'wind against tide', but wind with the tide is a relative non-event, if people are remarking on it, it's the wind vs tide effect they will be remarking on, give or take a few interesting places where you get strong winds coming down a valley across the tide.

 

[Birdseye]

It's clearly exactly this and no more. The turbulence and the "bunching" must logically be precisely the same if it's a 20 knot wind over a zero knot tide, or a zero knot wind with a 20 knot tide, and all combinations in between. Neither the wind nor the water are aware of what speed they are going, nor does it matter any more than the high speed both are travelling through the galaxy as the Earth moves. Their relative speed is all that matters.

Obviously a tide can cause turbulence by its relative speed over obstacles and narrowing/widening channels, but the wind/tide effect then adds it's own changes based on the relative speeds.

Relativity? No **** Einstein!

Lots of bored old men on here - 16 posts to tell the OP that what he thought was right, is right.

 

[lw395]

Surely the wind is always 'over' the tide, whichever direction it's going. Wind 'under' tide would be interesting! I use wind 'with' and wind 'against'.

Sometimes lumps of 'tide' get well up into the 'wind'.

 

[lw395]

Relativity? No **** Einstein!

Lots of bored old men on here - 16 posts to tell the OP that what he thought was right, is right.

Always good to have one of these threads from time to time, it sorts out those who either don't sail, or aren't very observant!

 

[Birdseye]

Or those who dont understand simple vector maths