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Old 05-08-2012, 05:17   #46
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Re: Sailing between the Swells..

As wind -driven waves travel away from the area in which they were born and raised, they sort themselves out: the longer wavelength ones travel faster, so they all end up travelling in company at the head of the swell train, and the short-arsed ones bring up the rear. So, inevitably, the train stretches out as it moves along.

There is a relationship between height and length, as well.
This explains why the profile becomes much more regular in the process of waves transforming into swells, because 'like' travel with 'like', so the crests and troughs eventually coincide. A tiny bit like menstrual cycles synchronising, perhaps, when many women are living in close proximity...


As daddle says, a big swell will have come from winds which were very strong. To be a swell, they must also have come from some distance away.

Another possibility is that the winds causing them were less strong, but blew from the same direction for a long time.

Locally generated wind waves disappear as soon as the wind drops, whereas once they've had time to become swells, the way the energy is organised allows them to travel, in some cases, many thousands of miles.

The longer the wavelength, the more the 'endurance': such swells will travel further before they diminish.

Nevertheless they do eventually diminish, and a very long, low 'ground swell' is likely to have come from much further away, and to have lost some height on the way.


The fronts of waves are typically steeper than the backs. This is not the case for swells, unless they encounter a contrary current or run into shallow water.
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Old 05-08-2012, 05:34   #47
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Re: Sailing between the Swells..

One more thought on long, low swells:
In extreme cases, such swells can be almost imperceptible but nevertheless contain huge amounts of energy, giving rise to strange phenomena.

I recall once sailing out the local harbour towards the open sea, alternately catching up with for what seemed like several minutes and then dropping behind a boat ahead, so that we went from what looked nearly to be half a mile behind to not much more than a quarter. After a few cycles of this I was beginning to suspect I was hallucinating, but as we got out into the open sea we started seeing huge rafts of kelp (including one with a boat stuck in the middle).

Then we noticed that the ocean was piling up, slowly, in turn, against all the coastal cliffs angled at a certain aspect to the sea.

This 'mounting' was extraordinarily slow, but on a couple of occasions the water eventually overtopped a cliff face, and some of the cliffs were of the order of 30m high.

We inferred, and it was later confirmed, that an extraordinarily powerful tropical cyclone had become 'extra-tropical' and travelled down into the South Pacific nearly as far as 40 South, some thousand miles or more to the east from us.

Despite the visual effects, with our eyes closed we could not feel the swell. However in certain bays within the main harbour, we later heard that the energy refracted and curled in, and the child of a neighbour was pitchpoled in his Opti. This probably doesn't happen that often away from a surf beach (which this was not: generally it's sheltered from any swell)

Another story from around the side of the same bay, before people realised what was happening: a guy with a fancy jetboat (matching mag wheels on the trailer with the towing vehicle, matching paint jobs etc) was backing down the ramp. He got to the water's edge, got out, went back to take the chain off, looked puzzled because he wasn't as close to the sea as he thought - in fact, nowhere near it. He repeated this entire performance a couple of times - and then .... the sea came back in, rather a lot faster than it had gone out, and dumped the whole shooting box off the side of the ramp.

"Next, please!"
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Old 05-08-2012, 05:57   #48
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Re: Sailing between the Swells..

I quickly felt uneasy about my claim two posts back that "The fronts of waves are typically steeper than the backs".

This doesn't seem to make sense when I think about the physics.

I checked and (at least from the scientific study I checked, grippingly titled "Horizontal asymmetry and steepness distributions for wind-driven ocean waves from severe storms") it seems to only hold true for the largest wind driven waves in a given set. The ones which are average height or less are largely symmetrical about a vertical line through the crest, when viewed side on.

I guess this makes sense because in a storm the surface water (contrary to wave theory) moves in the direction of the wind, sometimes at a knot or more, but this slows down quite quickly as you go deeper.
It stands to reason that the larger waves would experience this in a similar way to encountering shallow water, and that his would cause them to steepen on the fronts.

Another possibility: the tallest waves are sticking their tops up into an undiminished windstream, whereas the shorter ones are largely sheltered. The windforce (if the waves were stationary, made from shaving cream) might be expected to lean those top sections forward.

I'm sure most sailors would swear that ALL wind-driven waves are steeper on the fronts, (as I would have, until I thought about it some more) but I think that is an illusion caused by the fact that when we travel into them, horizontal distance is compressed, whereas when we overtake them it is extended, creating the illusion of a flatter slope.

If we ski towards a ramp which suddenly starts moving towards us, we will experience it as shorter and steeper than if it stayed put.
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Old 05-08-2012, 07:44   #49
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Re: Sailing between the Swells..

Once while on watch at night I began to notice this almost imperceptible feeling of going down a very long slow hill and without noticing a change in direction I would feel as tho I was going up hill for long periods of time, being totally confused by this I would be holding on for the big crash that never happened, wanting to see what my brain was lying to me about, I stayed on watch for 8 hours to see the sunrise and these huge totally benign swells and was also alarmed when I saw another sailing vessel near us that was only visible for short periods I took several pictures to show the crew and others only to find that they did not show up when the crew awoke ,when I explained they just looked at me and said OK!RIGHT! and thanked me for the extra sleep
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Old 08-08-2012, 04:41   #50
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Re: Sailing between the Swells..

It is known from photography that if you take pictures of ocean waves and have a long shutter opening, the sea will appear flat i.e. no waves. Probably what happened with your shots.
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