[MastUndSchotbruch]

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!

 

[Arcb]

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.

 

[RobGallagher]

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.

 

[roverhi]

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.

 

[midwesterner]

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.

 

[MikeOReilly]

As others have said, it’s 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 there’s enough force the waves can be large and steep. This is what makes crossing the Gulf Stream challenging at times (not that I’ve ever done it — yet).

 

[MarkofSeaLife]

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.

 

[MarkofSeaLife]

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.

 

[SanderO]

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.

 

[Don L]

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.

 

[outbound]

What interests me is the “washing machine “ effect in the stream. You get the bigger, short period waves from the wind against wave effect but when a storm is nearby or has recently passed by a second independent wave train also comes through. This creates two wave trains coming from different directions. Sometimes they cancel but sometimes they add giving you some truly monster waves that catch you unaware. I’ve had occasion to be pooped when the majority of the waves are hitting near the bow. Weird. I’ve never seen that effect in a coastal setting.

 

[SanderO]

Fetch out in the ocean can be hundreds of even thousands of miles. You can even encounter HUGE rolling waves 40 or more feet high and very long and it's like sailing over hills and in valleys. With nothing to stop the waves they can and will meet other wave trains and this can make for confused seas.

 

[Barquito]

I know just enough about physics to know when I am an idiot. I still don't get it. It seems like a frame-of-reference problem. The water in the gulf stream does not know it is going North. Actually if I am not mistaken, it is going East at about 24,000 mph due to the spin of the earth. So why wouldn't the surface of the Gulf stream just react as if the wind is 4 kt faster than if the wind was from the South?

 

[Arcb]

^^^ The wind is the catalyst for the waves, but the wind energy is transfered into the wave. This is demonstrated in the concept of fetch. Waves are bigger after 100 miles of fetch than after 1 mile of fetch because after 100 miles the waves are storing more wind energy because it has been acting on the waters surface for a longer period of time. The issue isnt so much the wind itself colliding with the opposing current so much as the stored energy in the waves colliding with the opposing current.


The wind will have an immediate effect on the waves in terms of contributing to the instability of the waves as they build in size, however, it is the cumulative effect of the winds energy over time, interacting with the energy in the opposing current that is the bigger issue.

 

[ReefMagnet]

Well I'm no physicist, but here's my take.

If the wind is against a 4 knot current, the difference in apparent wind speed to the water isn't 4 knots, it's 8 knots. Because the wind running with the tide has it's influence on the water subtracted by 4 knots, too.

Now if we have, say, 20 knots of wind and there is an apparent 8 knot difference in the speed, the different apparent wind on the water is 16 knots vs 24 knots. Because the effective speed of the wind has increased by 50%, the energy delivered by the wind to drive the wave has increased by over a factor of two.

Then throw in the frictional effects of the wave meeting the apparent resistance of a 16 kn current, vs a 24 kn current and then include the shaping of the wave by the friction of the air that moves past it as well and BINGO! There's your difference with wind opposing current.

As for the spinning of the Earth. That's the Coriolis effect and it does influence wind flows but it's effect on the water is akin to the effect on a tennis ball when thrown up in the air inside a vehicle moving at constant speed and direction which is 2/10's of almost nothing.

 

[Barquito]

If the wind is against a 4 knot current, the difference in apparent wind speed to the water isn't 4 knots, it's 8 knots. Because the wind running with the tide has it's influence on the water subtracted by 4 knots, too.

In my example a mean to say there is a NET 4 kt difference in apparent wind speed over the surface of the water. The Gulf stream is about 2kt. So, with my numbers a 20 kt wind out of the north would be 22 kt over the surface of the water. If the wind was out of the south, then this would be 18 kt over the surface of the water. However, the difference between 22 kt and 18 kt does not account for how nasty the water would be if out of the north. Thus my problem with being stuck thinking of this as a frame of reference scenario.

 

[ReefMagnet]

In my example a mean to say there is a NET 4 kt difference in apparent wind speed over the surface of the water. The Gulf stream is about 2kt. So, with my numbers a 20 kt wind out of the north would be 22 kt over the surface of the water. If the wind was out of the south, then this would be 18 kt over the surface of the water. However, the difference between 22 kt and 18 kt does not account for how nasty the water would be if out of the north. Thus my problem with being stuck thinking of this as a frame of reference scenario.

Even at 2 knots, the energy the wind can impart on the water still increases by 50% in the 20 kn scenario. Regarding frame of reference, hitting waves with vertical faces is a lot more "interesting" than hitting waves of the same height with a more gradual "angle of approach". Also, in many locations fast currents are generated by the shape of the sea floor and/or by waters being constrained by land which does other interesting things to wave shape when the effects of refraction and diffraction of waves is considered.

 

[midwesterner]

In my example a mean to say there is a NET 4 kt difference in apparent wind speed over the surface of the water. The Gulf stream is about 2kt. So, with my numbers a 20 kt wind out of the north would be 22 kt over the surface of the water. If the wind was out of the south, then this would be 18 kt over the surface of the water. However, the difference between 22 kt and 18 kt does not account for how nasty the water would be if out of the north. Thus my problem with being stuck thinking of this as a frame of reference scenario.

I think the physics is just more complex than simply measuring the force of one type of energy against the force of another. You may just have to trust the word of people who have sailed in it.

There have been enough accounts on here, by experienced sailors who have sailed the Gulf Stream when a wind is blowing counter to it, and they have reported very rough seas. For whatever the reason, it is real.

One thing that occurs to me, is that current runs some depth down in the water: What, one fathom deep, a fathom and a half deep, two fathoms deep? I don't know, scuba divers could probably give us a better idea. Current can involve a very large volume of water to some significant depth.

Wind, on the other hand, affects mostly the surface of the water.

It seems to me like if you were to try to push your bed across the room by pushing down, and against pillows on a bedspread. The bed is not going to move, and you're going end up with a pile of pillows and bedspread at the other end of the bed.

I know that I've kayaked and canoed on the Missouri river when the Corps had the river up near the banks with a 15 knot current in the channel,when strong winds began to blow up river. Suddenly I thought I was out at sea in a storm, with waves and swells. I could see that I was going backwards. I had to paddle as hard as I could just to make headway down river.

The Corps keeps the channel 15 to 20 feet deep. That's 20 foot deep water flowing at 15 knots, and some strong winds blowing, maybe the top foot of the water surface. I can't explain all the physics of it, but I've seen it completely change the character of a river I've been floating on most of my life, into something I didn't recognize.

 

[Arcb]

In my example a mean to say there is a NET 4 kt difference in apparent wind speed over the surface of the water. The Gulf stream is about 2kt. So, with my numbers a 20 kt wind out of the north would be 22 kt over the surface of the water. If the wind was out of the south, then this would be 18 kt over the surface of the water. However, the difference between 22 kt and 18 kt does not account for how nasty the water would be if out of the north. Thus my problem with being stuck thinking of this as a frame of reference scenario.

I asked Google, but couldn't really find a decent online resource. Then I checked my text books and even they had very little information on the subject. Reed's Maritime Meteorology (which you can find free PDF copies of on the net) does have about 9 pages on wave and swell formation. In the wave chapter there is 1 paragraph on contrary wave and current directions. It doesn't explain why the effect happens, but it does confirm that waves moving opposite to a current are shorter (horizontally, not vertically) and steeper than those not in a contrary current. Of interest is, if the effect was a simple matter of increased relative wind the waves would be bigger, but not shorter (horizontally) and steeper.

If you want to confirm the effect for yourself without sailing in these conditions, it's pretty easy to do. Spend some time hanging out on a good weather site like Windy.com on the wave height page. Look for a wind blowing contrary to a current. Gulf stream is easy. Then start clicking on wave details both inside and outside the current and you will observe the waves inside the contrary current pretty consistently have shorter wave periods than those not in the current. This indicates shorter (horizontally), steeper waves. When passage planning folks generally pay attention not just to wave height and wind direction, but also wave period as relatively shorter periods indicate a rougher ride.

Or better yet, go surfing. You will learn more about wave shape in a day of surfing than you will in years of keel boat sailing.

 

[capecodda]

Arcb's explanation is the clearest one I've ever heard. Thanks for this.

We are in Vineyard Sound almost every day in the summer. The usual wind is SW. The current runs about 2 kts with the wind then against it with the tide. The effect is dramatic. Wind and tide together, it flattens out. Against, it's a mess. The other place you see this in my neighborhood is the Buzzard's Bay side of the CC Canal. In both cases, you get waves where the period in seconds and height are nearly equal. Square waves. Washing machine.

We get additional effects due to rips in Vineyard Sound where there is a relatively quick depth change. These cause a wave to occur at the shallow spot and then a disturbance to continue down current of the rip. More like what you see in a river.

It's one confusing body of water this way, but it's missing the big ocean swell as we are protected by the Vineyard, the Elizabeths and Nantucket. So we don't see much in terms of long period swells. Surfers don't come here, they go to the outer cape.