Why is wind against current a problem? - SailNet Community
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post #1 of 80 Old 12-01-2018 Thread Starter
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Why is wind against current a problem?

There is lots of discussion (e.g. in the current thread of the Beneteau delivery crew being rescued in the Gulf Stream) about the dangers of a situation with the wind going against the current, like North wind in the Gulf Stream.

I do not doubt that this is true but my question is WHY? What is the underlying physics? I am actually trained as a physicist and one of the dogmas is that 'everything is relative.' I understand that the (velocity-) vectors of wind and current are added if they are contrary but that cannot be all. How much is the current in the Gulf stream at its highest, maybe 3 knots? So, if there is a 20knot wind against it this would mean there is a relative wind of 23 knots. From what I am reading here, the effect must be MUCH larger.

Again, I am not doubting that the effect is real, but can someone explain what is going on?

Thank you!
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post #2 of 80 Old 12-01-2018
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Re: Why is wind against current a problem?

Wind essentially causes a surface current on the water. The surface current is resisted by slower moving sub surface water. The friction of the subsurface water combined with the flow of the surface water creates a cyclical flow of energy. More wind on top results in faster surface current while sub surface current continues to create friction. The cyclicing energy causes waves to build in height.

A counter acting subsurface current results in greater subsurface friction counteracting the surface flow, resulting in steeper waves. The subsurface current reduces the frequency of the waves resulting in shorter frequency waves. As the frequency of the waves decreases the waves, instead of building in speed, they build in height. Higher shorter waves have less stability in their foundation resulting in relatively earlier breaking.

Shallow water works more or less the same way. Surface current on top, friction on the bottom forces the energy up causing steeper and eventually breaking waves.

Take a sheet of paper and push it across a flat surface like your kitchen table and the paper might stay flat. Push it faster and ripples may start to appear in the paper. Repeat experiment with a moderate friction surface like a carpet, the paper will show more folds. Repeat experiment again with a high friction surface like your driveway and paper will shpw even more folding and creasing.
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post #3 of 80 Old 12-01-2018
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Re: Why is wind against current a problem?

I sail in an area of current. I can't explain the physics the way you probably want. I will say that if the wind is with the current it can make for a very calm breezy day and sailing is pleasant, even on a blustery day.

Wind against current can make things surprisingly exciting, or fun, in reasonable conditions. A bit scary as it gets worse. The steep chop that develops as wind tries to push current in the opposite way it is meant to run can lead to pounding, falling off of steep waves even if they are relatively small by ocean standards.

There are places in New England I won't even bother attempting with a decent breeze against a strong current. Plum Gut or The Race with just a 20 knot wind against a strong current is something I would simply change plans rather that deal with.
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Re: Why is wind against current a problem?

The wind stacks up the waves against the current giving the waves a more vertical face and possibly taller. Sailing the channels in Hawaii are almost always wind against wave conditions. I'd swear that the waves have near vertical faces when the Trades kick up into the 30's. The channel between Molokai and Oahu is the worst because you are hard on the wind to lay the channel between Molokai and Lanai for almost the whole way till you get in lee of Molokai. My first crossing of that channel was in high wind trades and nearly every wave sent solid water over the bow. At least it was warm water.
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post #5 of 80 Old 12-02-2018
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Wind against current can cause waves, sometimes strong waves that create a situation in which one may win over the other and boats can go backwards and be tossed about. It sometimes creates a situation called confused seas.

I've experienced it kayaking and canoeing on the great rivers, the Missouri River. the Big Muddy. Once wind was blowing up river against an 8 knot current in the middle of the dredged river boat channel, creating white caps and pushing my boat up river. I wore myself paddling just to make headway.

The forces of current can run deep in the water. Wind may effect the top few feet of water. The top few feet of water surface may blow contrary to the current beneath the surface causing lots of turmoil.
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post #6 of 80 Old 12-02-2018
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Re: Why is wind against current a problem?

As others have said, itís not that the two force vectors are added. It is that water is encountering a counter force (wind) causing it to create standing waves. If thereís enough force the waves can be large and steep. This is what makes crossing the Gulf Stream challenging at times (not that Iíve ever done it ó yet).

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post #7 of 80 Old 12-02-2018
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Re: Why is wind against current a problem?

I ripped the following off the net...


The Scripps Institution of Oceanography (IIRC) has developed this simple graph describing the modifications of wave characteristics when under the effect of a current of given speed.
The two curves called H (height) and L(length) show how these two wave characteristics are modified by a favourable current (to the right of the vertical axis), or more interestingly by an opposing current (to the left of the vertical axis.

The horizontal axis shows the ratio between current speed and wave period, the vertical axis shows: 1. Hc/Hs, ratio between wave height with current and wave height without current; 2. Lc/Ls, wave length with current and wave length without current.

"R" is a measure of wave steepness, the ratio between height and length. It is usually accepted that beyond a 1:7 steepness the wave breaks. Suppose one is sailing in a 1:10 steep wave, say 2m high, 20m long, and all of a sudden an opposing current brings us along the dashed line in the graph.

Along line L one can find Lc (point A): it's 70% of Ls. The current shortens the wave, which is now only 14m long. Along line H one can find Hc: 1.4 times Hs, the opposing current brings wave height to 2.8m. The theoretical steepness of the resulting wave would be 1:5, in practice the wave would have broken already...

Of course, real world waves are not of constant length nor height, but rather a group of relatively smaller ones, followed by a group of higher waves and so on... in practice the boat finds herself like periodically jumping in a hole in the water, usually with the loudest crash. When water depth is sufficiently low, the effect is magnified.
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post #8 of 80 Old 12-02-2018
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Re: Why is wind against current a problem?

Quote:
Originally Posted by MastUndSchotbruch View Post
WHY? What is the underlying physics?
Thanks ofr a great question. Arcb seems to be on the money.

I did want to give the answer a go before I read his.
Then I thought I might search the internet and see theres bugger all there except for similar questions on a few sailing forums.

My favorite photo, and one of the very few that show the difference made with current and a bit of wind it the one attached in the Alderney Race off the coast of France and the island of Alderney.
The wind doesnt look that strong. The Race can move at up to 10 knots. The interesting bit is the exactness of the troubled water. Either side is flat as a tack. in it is crazy.
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post #9 of 80 Old 12-02-2018
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Re: Why is wind against current a problem?

It is wind that creates waves. No wind... no waves. Wind interacts with the water at the boundary layer the surface to create waves. Wave characteristics are driven by depth of water, and "fetch". So the wave height and period is constrained by the parameters of the body of water. You won't find "large" waves in small harbor. If it is open to the sea... large waves could enter but will like break up.

You can see / learn the behavior if waves in say Long Island Sound. It has current going in and out 2x a day. And often has a persistent SW breeze. If you've sailed all over LIS you can see how the wind and waves (current) interact and what the results are,

The Race, Plum Gut and Hellgate are know for strong currents and confused seas and short tall waves when the wind direction is opposing the current direction (more or less). When they more or less align... the sea is calmer waves are lower and fewer white caps.

Shallower waters near the lee shore have steeper waves... in the lee of the shore the sea is calmer.

I suspect wave and wind interaction has been physically studied and measured in "towing tank" simulations and the behavior described in formulas.

Rapidly flowing rivers especially the narrow rapids with variegated bottoms will show confused seas and waves just as when two currents moving in different directions encounter one another.

pay attention... someone's life depends on it
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post #10 of 80 Old 12-02-2018
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Re: Why is wind against current a problem?

Quote:
Originally Posted by MastUndSchotbruch View Post

Again, I am not doubting that the effect is real, but can someone explain what is going on?

Thank you!
Think about the mass of the water moving along the stream at 2 knots, then hitting a wall. All that flow energy has to go somewhere, so it goes up into potential energy. ..................... Now replace the wall with a mass of air blowing into the stream having opposing energy of its own.

Add to this the thermal energy differences.

Don't blow air up my rear, be useful and blow it at the sails!
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