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Old 08-14-2003
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Brian Hancock is on a distinguished road
Understanding Rogue Waves


Although the odds of running into a rogue wave are slim, it's important to know how to avoid and how to handle this oceanic phenomenon.
There are two terms in the sailing lexicon that stab directly at every seafarerís heart; the more tangible of the two is "hurricane-force winds." The other is "rogue wave." Even though the odds of encountering a rogue wave are slim at best, itís a phenomenon that does indeed exist and I for one wouldn't knock those sailors that prefer to stay at the dock instead of throwing their lot to the odds makers. One thing is for sure; if you never venture beyond the marina, you will not encounter a rogue wave. On the other hand, sailors like myself accept the risks and take our chances. So what is this feared phenomenon, what are your odds of encountering one, and what can you do to prepare for and avoid them?

I grew up on the South African coast in an area known for ships vanishing on an all-too-frequent basis. It was part of the lore of the sea and something I came to accept long before I headed across oceans on my own boat. Having a fatalistic bent helps if you are venturing off into the unknown, and for any sailor departing the security of a safe harbor and sailing out to sea is indeed heading off into the unknown. There are simply things out there that remain unexplainable.

I am sure that itís accurate to say that the coast of South Africa has more rogue waves than most areas, and I am equally sure that itís because of the three different, unrelated phenomena that conspire to create them. First there are the prevailing, unrelenting winds kicked up by a never-ending series of low-pressure systems that rotate around Antarctica. North of the lows the wind blows from the west and builds up huge seas that travel unobstructed across the Southern Ocean. The only point of land that sticks down into their path is Cape Horn; however, by the time these waves reach the African coast they have travelled 14,000 miles without obstruction. Rather than big waves, they are gigantic swells with smaller waves forming on their surface.


Though the waves depicted in the movie The Perfect Storm, defy description, they're likely not too much of an overexaggeration regarding the real thing.
Second, traveling down the east coast of South Africa from the opposite direction, the northeast, is the warm Agulhas current, the second swiftest current in the world. Only the Gulf Stream runs faster. At times the Agulhas current runs at as much as five knots. The real kicker is that the large waves rolling in from the west and the fast moving current coming down from the northeast meet right where the third piece of the puzzle comes into play. The continental shelf juts out into the Indian Ocean and as the waves encounter the shallow water. They become steep and dangerous. Add to the mix of big waves, a counter current and shallow water and it becomes a recipe for disaster. On most days of the year yachts and ships pass through the area without any problem. Nevertheless, at times the sea can be wild and dangerous and a breeding ground for these giant and feared phenomenon. When ships are lost or a sailboat sinks, rogue waves are often blamed.

Whether rogue waves exist and whether you will encounter one is predictable only as a statistical probability, the same as the lottery only with a less desirable outcome. There is a percentage of probability, however small, that one wave somewhere will encounter a series of circumstances that propel it from a large disorganized wave, to a gigantic killer. This does not give much comfort to a sailor with a vivid imagination who is constantly looking over his or her shoulder for "the big one." Still, there are things you can do to prepare yourself and lessen your chances of encountering a rogue wave. The most important thing is to understand waves, what causes them, and how they react when encountering currents or shallow water.


Avoiding rogue waves requires a more reliable method than simply identifying wave patterns in multiples of seven.
Wave heights are determined by three things; the amount of wind that is blowing, the fetch or distance that wind travels, and how long the wind has been blowing from a constant direction. It does not take long for the surface friction of the wind on water to bring a ripple that soon turns into a wave, and soon you have a bumpy seaway. Despite the common thought that wave patterns come in multiples of seven, there are many oceanographers that insist that this is simply an old wives' tale.

Bob Guza, a professor of oceanography at Scripps Institute in La Jolla, CA, says: "The grouping of waves is even more random than the sizing of individual waves, and not every wave train is the same height. If the average height is 10 feet, you can expect some waves to be half that tall and other to be twice that tall." It is also not always true that the strongest winds create the largest waves. Once the wind exceeds 50 to 60 knots it begins to break the tops off waves reducing their height, and once it reaches hurricane force, the wind actually flattens out the sea. I have been in a hurricane and remember that the wind and rain flattened the seas out quite markedlyósomething I did not expect, but was extremely grateful to see. That hurricane, by the way, was hurricane Lili. We were approaching San Salvador when it hit. Lili was the second hurricane ever recorded in the North Atlantic in December and it arrived on Christmas day, 1984.

"A consistent wind direction will bring consistent, predictable waves. As soon as the wind changes there is a lag time between the 'old' wave pattern and the new one, and so begins the first opportunity for an odd wave to develop."
A consistent wind direction will bring consistent, predictable waves. As soon as the wind changes direction there is a lag time between the "old" wave pattern and the new one, and so begins the first opportunity for an odd wave to develop. Just as waves turn into breakers when they approach a shoreline, the same thing happens on a larger scale when large swells encounter a continental shelf or water that suddenly becomes shallow. The surface friction slows the wave, especially that deep part of the wave near the ocean floor. The top meanwhile continues on at its usual speed until it outruns the lower half, crests, and then breaks. When a large Southern Ocean swell rolls in toward land the same thing happens, but on a larger scale, and when it suddenly encounters a continental shelf and much shallower water, there is a probability for large, dangerous waves to develop. To add to the problem, the continental shelf slows down the momentum of the leading swells allowing the ones behind to catch up. The wave heights stay the same; itís just that the peaks become closer together and the wave faces become steep and treacherous.

The same thing happens when a counter current meets these wind-generated waves. Instead of the ocean bottom causing friction, or shallow water slowing down the leading waves, the counter current works against the approaching swell in the same manner. The deep water of the current "trips" the swell as soon as the two forms of water meet. The sea becomes the most unpredictable along the edges of the current where the undisturbed waves collide with those that have been affected by the current. When there is a sudden change in wind direction with a new wind blowing perpendicular to the current and the old wind direction, the chance of a rogue wave developing becomes much more likely.


Avoiding areas where these waves can appear is your best plan of action, so always combine an accurate forecast with cautious forethought before heading to sea.
We have all felt the effects of a distant storm. Many times you will notice a swell running in from a strange direction and know that there is wind blowing somewhere, perhaps a thousand miles away. We have also noticed occasional large bumps that seem to come from out of nowhere, and know that a ship is nearby and itís the wake that you can feel. In my opinion, a rogue wave develops when a combination of some or all of these phenomena coincide, and suddenly out of nowhere you have that one-in-a-million occurrence that can spell disaster.

Now that you understand waves and what causes them to become confused and dangerous, your first line of defense as a sailor is to avoid areas where these kinds of waves can develop. For sailors leaving the eastern seaboard of the US, the Gulf Stream should be your greatest concern regarding waves as an obstacle for navigation. Fortunately, the Gulf Stream is a body of water whose current and eddies are well known and can be seen clearly from satellite photos. Before you leave land you will need to get an accurate forecast, and from that you can determine how the wind direction will relate to the direction of the Gulf Stream or one of its eddies. Wind against current, especially if you are sailing into the wind, can lead to a dangerous sea state. You will also need to interpolate how the wind and current will relate to the direction that you are planning to sail. If you are crossing the stream in a wind-against-tide situation, the beam seas can be dangerous and should be avoided. Itís this kind of forethought that can spare you from running into the likelihood of bad seas and the remote possibility of a rogue wave.

"Wind against current can lead to a dangerous sea state. To be safe, you'll need to interpolate how the wind and current will relate to the direction that you are planning to sail."
Itís also important for you to plan your route to avoid shoal areas and on a larger scale, continental shelves or seamounts. Many mariners will give the Cape of Good Hope near Cape Town a wide berth if they are passing by the area, especially if the forecast is for the wind to change direction right where the water gets shallow and where it might be affected by a current. In many places seamounts rise up from the ocean floor reducing the depth from 2,000 feet to a little as 30 feet. I have sailed over Vema Seamount in the South Atlantic and noticed strong eddies and whirlpools coming seemingly out of nowhere. It was a calm day when we passed the area, but I can only imagine how things might have been if the wind had been blowing and some other factors had come into play.

You also need to think about the coastline where you are sailing and how the wind and wave directions will be affected by the land. In this case itís not the shallow water, but rather deep water that is usually accompanied by a high landmass. The backwash off high cliffs is a very real danger and can be felt many miles offshore. If you stop and think for a moment about the number of different scenarios that can develop out at sea, from cross seas to backwash, ships wakes, currents, and shallow water, itís no wonder that on any given day on any part of the vast ocean, a rogue can occur. Itís a matter of averages. For your part, prudent preparation and planning are vital. Be sure that your gear is stowed properly at all times and that your safety gear is ready.

Rogue waves are notorious for arriving without warning and so any prudent mariner will anticipate the worst and be grateful for the best. As Adlard Coles once said, "Reef for the gusts, not the mean." The same can be said for dealing with waves. Expect the 20-footer instead of being lulled by the 10-footers.

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