Aug

5

 Rogue waves can be defined in many ways, yet the one I prefer is "a wave of extreme severity that appears unexpected even to an expert". Given that definition, rogue waves do exist, yet there is no evidence that they would be globally more frequent than conventional (non-linear) waves theories predict, they just don't happen where and when — i.e. in the most extreme sea states — one would expect them. There is no evidence either that the "modulational instability" theory that my colleague Prof. Akhmediev puts forward to explain them would not apply: the theory was validated in wave tanks, optical fibers and plasmas. It is just impossible to know whether the necessary boundary conditions are satisfied in nature. Several points may be noteworthy to the Specs:

1. A recent article shows that rogue events can be empirically predictable, but that for ocean waves the delay would be of the order of the time needed to shout "Buddies, grab something and hold on to it!"

2. "Normal" extremes are at least as frequent as "special" ones, and all indicators based on breather theories such as Akhmediev's have false alarm rates of at least 90%, perhaps 99%, and still fail to warn of about 10% of actual rogue waves.

3. Experienced sailors deny having met "rogue waves". They say that they encountered waves that were rogue, they capsized or broke some ribs (Roger Taylor, Isabelle Autissier), but when you discuss it directly with them, nothing that they were not expecting and had not prepared their ship for.

4. Only a very small percentage of fatalities occur well off the coast (Nikolkina & Didenkulova), most of them happen at the coast or in shallow waters where victims feel wrongly on safe ground or in safe waters.

William Weaver comments: 

Your fourth point is similar to many car accidents happening within a small area from home. People make more mistakes when they feel comfortable. I'm not sure how this changes when accounting for activity though. For example, you could normalize accidents per mile driven and then compare close to home versus far from home, or do a similar normalization for the study mentioned with at sea fatalities, which I have not read. It might be helpful to measure after splitting into bins for types of vessel (cargo ships might have more miles, farther from home travel and less fatalities), and by type of mariner, which might be self defeating as it is more likely less experienced seamen would stay close to shore ( which is not the case for drivers staying close to home).

It seems like a data set prone to torture someone. But do we make more mistakes when we feel safe? The opposite might be true too. My observation is the difference between good and great traders is often the number of mistakes they make. Bring back the checklist posts?

Jeff Rollert writes: 

I see some sample issues. Most sailors (N) do not venture more than 20 miles from the coast. The ones that do are orders of magnitude better prepared (boat) with experienced crew. So I suggest the distribution is bimodal.

Also, there is more than one kind of "rogue" wave. One is an overtaking wave, which is when two waves combine; the second, and more deadly, is when the wave comes from a direction that is unexpected. In my experience, these are the most deadly, as they roll (or broach) the boat. They are also the hardest on the structure of the boat and hit the weakest points harder (deck access from the stern, control or bridge room windows and electrical systems). For fisherman, they dread losing engine power, as the boat becomes exposed in the same way, as fishing boats have the windage at the bow, which turns the boat and presents a breaking wave to the stern.

Lastly, looking at single vs combined storm fronts also messes with their structure.

IMHO, they oversimplified their model.

Jim Sogi writes: 

Surfers expect at least 1 wave each day that will be 3-5x the smallest wave. It's called the wave of the day. In a random sequence this would be one of the far tails. They tend to come with the incoming tide as there is an extra push from the moon.

I think the rogue waves in the ocean might be 20x or more if there were a number of storms with crossing wave trains which could combine. It's the cross chop that creates these large events through random combinations. Rogue is often and mistakenly used as unexpected. Sailing through large parallel swells on a calm surface is quite easy. Sailing through a cross chop is very tiring and rough. Refraction and reflection from shores often make coastal sailing rougher than in the mid ocean.

What always surprises me is how calm the ocean tends to be. You would think it would be rougher given how big it is. 


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