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post #11 of 80 Old 12-02-2018
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Re: Why is wind against current a problem?

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.

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

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.

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post #13 of 80 Old 12-02-2018
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Re: Why is wind against current a problem?

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?
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Re: Why is wind against current a problem?

^^^ 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.

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Re: Why is wind against current a problem?

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.
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Re: Why is wind against current a problem?

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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.

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Re: Why is wind against current a problem?

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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.
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Re: Why is wind against current a problem?

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Originally Posted by Barquito View Post
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.
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post #19 of 80 Old 12-03-2018
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Re: Why is wind against current a problem?

Quote:
Originally Posted by Barquito View Post
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.
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Last edited by Arcb; 12-03-2018 at 07:20 AM.
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Re: Why is wind against current a problem?

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.
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