Rogue Waves

[GordMay]

A couple of interesting articles about
ROGUE WAVES

Ship-sinking monster waves revealed by ESA satellites
Full story: http://www.esa.int/esaCP/SEMOKQL26WD_index_0.html

”Once dismissed as a nautical myth, freakish ocean waves that rise as tall as ten-storey apartment blocks have been accepted as a leading cause of large ship sinkings. Results from ESA's ERS satellites helped establish the widespread existence of these 'rogue' waves and are now being used to study their origins.

Severe weather has sunk more than 200 supertankers and container ships exceeding 200 metres in length during the last two decades. Rogue waves are believed to be the major cause in many such cases...”

Freak waves, rogue waves, extreme waves and ocean wave climate
by Kristian B. Dysthe, et al ...
Full article: http://www.math.uio.no/~karstent/waves/index_en.html

”How high is the highest wave? Which properties does it have? How often and under what circumstances do extreme waves occur? We have let these questions be the basis for a continuous research effort since 1995. On these pages we try to give a summary of the topic and of our activities. The pages are under construction ...

It is well known that extreme waves often occur in areas were waves propagate into a strong opposing current. A well known example where many large ships have encountered difficulties is the Agulhas current outside South Africa. The strong current going south meets strong swell from storms in the Antarctic Ocean....”

[Alan Wheeler]

I watched a TV documentary about a year ago on the freak wave thing. Scary as hell. Being a sound engineer and playing with toys at the fore front of our industry,(yeah yeah, call me a nerd) I understood exactly what was being described. I kept saying to my wife all through the programe (much to her annoyance) about how a wave like this would occur. Toward the end of the Doco, Some science guy explained why they think they occur and it was just what I was telling my wife.
Now, I am no oceanic Expert, so take this with a grain of salt, but this is how I see it.

These really big freaky monsters are slightly different to waves created against currents. Against currents, the waves a reasonably predictable as to direction, and their shape will tend to be different. Steep face, not moving any faster than the surrounding seas and infact, they may not travel very fast at all. They can have a lot longer life expectancy, due to the current providing much of the waves energy.
The freak waves that have claimend shipping outside of current streams, are different again. They are usually short duration. Very unpridictable in direction and nearly always will move in a direction differing the normal swell. They are extremely steep faced and move with tremendouse speed and energy. They are believed to occur when waves eitherside collapse expending their energy into a wave in the middle. However, the collapse is at a different attitude to the normal wave direction and this new wave will stand high and move in a different direction, sometimes gathering even more energy from other waves around it. Thus a wave being created that can stand to heights of 30M+ when the sea around is at 12M.
However, having been on a ship in 8M sea's, I have no intention of ever wanting to be in anything like that again, especially in a 46ft yacht. I have been in 4M in the yacht and that was about my limit. No not the yachts limit, my limit.

[BC Mike]

It is interesting that the waves are dismissed as nautical myth until we have an exspensive scientific way of finding them. Phil Weld, David Lewis and many others used the words rogue wave in there writings. Frankly I take heed of the first hand accounts of reliable witnesses and need not wait for science to discover them. They have obviously been out there ( the waves ) forever, as in long before they make it to my TV screen. BC Mike C

[Alan Wheeler]

Yes I agree. Many very experianced Captains had their stories dismissed as "Tall Sea yarns" for many years, untill the day the guy on that North Sea oilrig had measured and had actually hard data to support that the wave existed. Unitll that point, Science stated that a wave of that magnatude couldn't exist and if certain events took place that could support one, it would be on a scale of 1 wave in 10,000yrs. Then a Tanker was severly crippled off the bottom of Africa, and scientist said, OK, so a big wave can occur more often, but hey, it still can't be as big as the Sailors are telling us and it is because certain weather conditions against tides cause the problem. Easy to solve, don't go there. Stay further off coasts. Then two ships in the deep Southern ocean were hit. Each was a 1000NM apart and the incidence was at different times. And the thing that hit the scientists, were that niether vessels were in any current streams. One ship that survived had evedince of how high the wave was and science for the first time had to go "ummm, maybe we are wrong.
A maths guy came up with a theory and he could predict a wave happening, but couldn't support his theory. Untill the original guy on the Oil rig came along with hard Data. The Oil rig guy placed special Laser measuring gear on the platform and recorded Wave events during a storm. Another massive 30M wave hit and they had hard measured data of what was taking place around the wave. Placed against the Maths theory, they filled in the blanks and could scientificaly show how the waves form, via absorbing the energy of two other waves around it. So it was at that point that they got the funding to put the Satillite with the SAR radar in space and take a look at the ocean surface. Now the scientist thought they might sea on average, one wave per year. What stoped everyone dead in their racks was that they measured some figure like 30 of them in the first few months.
How does it affect us as yachties?? Well, a yacht has a good chance of going of the tops of these things. Pitchpolling will be the biggest danger and some of the experianced guy's here are better to take this further, as I am swimming in the deep end now.
As for ships, this is where the trouble lies. They don't go up over the wave. A ship is designed to take a crush stress of around 15T/sqM. These waves have been estimated to produce forces of in excess of 100T/sqM. So for designers, the pressure is now being placed on them by the insurance companies to build stronger vessels. But stronger vessels mean more cost. So for them the dury is out on how to proceed.

[tauras]

Gordon, Jeff H, Irwinsailer: My dad had an encounter with a rogue wave out over the grand banks a long time. If given time to react, what is the best way to approach such a wave?
Thanks
Tauras

[Jeff H]

With prayer and a good insurance policy? Damned if I know.

A fellow who single-handed a sistership to my boat from South Africa to the Carribbean described running before waves that were 40-50 feet tall down in the south Atlantic. He said that he was at 16 knots long enough for it to show up on the GPS. I guess if you have enough searoom and the waves are not breaking surfing would give you a pretty good chance on a fast enough boat.

Jeff

[Alan Wheeler]

I don't think there is any good ways to approach them. They come at you like a freight train and from stories I have heard, they sound like a freight train. It is also important to understand, there is a significant difference between just a huge sea and these freak monsters. These freak waves stand up very tall and extremely steep faced and backed. Bow on you may go up the face, but down the other side I don't know. Stern on, I would imagine you will pitch pole. Side on, which is what usually happens when meeting these things, and you will be rolled. We are talking of Sea's that are 40ft, with a freak wave that can measure 100ft and more. One story I heard has estimated a wave of 120ft.

[delmarrey]

For sailboats, I think a roll over (broach) would be your best bet. a properly built sailboat, and powerboat for that matter, should be able to take a roll over with minimal damage. You might loose your mast but you would still be a float once the thing passed.

It would probably be too fast to surf, a Hell of a ride!

Head on, and it may slam you down on your deck and break up.

Or a submarine with a keel and mast, which I'd like to give a try, but don't have the $upport.

So, I vote for the rool over!....................._/)

[Bob]

Freak waves are a fairly frequent occurence along the South African coast at certain times of the year. These are caused by a strong Southwesterly blowing against the south flowing Agulhas Current beyond the continental shelf.

NOT a time to be at sea - move closer to land and shallower water promptly!

These events are better described at ABNORMAL WAVES

Bob