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Saturday, January 27, 2018

How Nasty is a Fifty Foot Wave?

Note: to watch the video click on the watch on Youtube link!

     I experienced 30-footers on an LST crossing the North Atlantic and we marveled at the beating a couple of nearby smaller destroyers were taking. When our ship's screws came out of the water the whole ship banged and shuddered and shook and the rolls were nasty, but watching what those destroyers were going through was down right scary. It looked like they were going to tip over and we watched as the hull was practically submerged when they plowed into a wave. That was nothing compared to the really bad boys!
     The biggest, baddest waves don't begin that way. Winds at sea generate waves that average ten feet high and during storms, 30-footers are common. But what creates waves the size of buildings, including the ones big-wave surfers covet and coastal dwellers fear?
     In a word, land. A wave approaching a shoreline meets shallower and shallower water, slowing the wave’s leading edge. Now much of the energy that had been propelling the wave forward has nowhere to go but up, so the wave grows taller and taller. Unlike the waves seen at the beach, tsunami waves don’t break because they don’t get steep enough. Energy distributed throughout the water column and wavelengths extending a hundred miles give them frightening stability. They arrive as towering, surging masses.

25 feet Teahupo’o, Tahiti’s waves are modest in height but surfers call the thick lips the world’s “heaviest.”
29 feet As the tide comes in on Hangzhou, China, a wave called the Silver Dragon travels up the Qiantang River, opposite the direction of the river’s flow. This tidal bore is largest in September.
30 feet The Banzai Pipeline in Oahu, Hawaii, is the most dangerous surf wave. It tosses boarders directly into a shallow reef. At least ten people are believed to have died there.
50 feet The Indian Ocean tsunami ten years ago traveled at speeds reaching 500 miles per hour and barged up to a mile inland. It killed some 200,000 people, making it the deadliest wave known.
78 feet Garrett McNamara holds the record for the largest wave ever surfed, set in 2011 in Nazare, Portugal. He claimed to have surfed a 100-footer also at Nazare, but the height wasn't ever confirmed.
84 feet Until 1995, most scientists dismissed sudden, unexpected swells known as rogue waves as maritime myth. But on New Year’s Day of that year, a monitoring platform off Norway’s coast recorded a single 84-foot wave surrounded by 20-footers.  The simplest explanation for these monsters is that two or more waves meet and align in such a way that their crests combine into one much larger crest.
100 feet An earthquake followed by a landslide in 1958 in Alaska’s Lituya Bay generated a wave 100 feet high, the tallest tsunami ever documented. When the wave ran ashore, it snapped trees 1,700 feet upslope. Five deaths were recorded, but property damage was minimal because there were few cities or towns nearby.

     Rogue Waves: When a ship encounters high waves high amplitude pitching and heaving combined, produces an effect that sends the bow out of the water. As the wave passes aftward, the bow falls onto the surface (make that slams the surface), with high acceleration resulting in tremendous forces in the forward structure of the ship. Due to high forces in the forward structure, the hull at the bow section is often prone to cracks that can develop over the entire depth of the bow section. The result can be buckling of these plates. When forward structures have been subjected to large number of cycles of freak waves or slamming forces over a long period of time, the structure undergoes fatigue which could lead to complete rupture of bow section.
     So, how do ships survive a harrowing storm that can batter even a large, sturdy vessel? Most modern cargo ships are designed to tough out all but the heaviest weather, but hurricanes are the largest and among the most dangerous storms on the ocean, and no crew wants to find itself in the midst of one.
     Ovously, avoiding them is best and that requires good weather information. A century ago, weather updates at sea were limited to Morse code messages, but since the 1980s, weather updates are available right on the ship's bridge. 
     US cargo ships are required to carry a Navigational Telex machine, a radio receiver that picks up radio signals and converts them into a text printout. Another system called Weatherfax uses higher frequency radio waves to send black-and-white images to shipboard fax machines. Today, captains can also receive weather maps, satellite images, and other information. Some vessels have more high-tech tools aboard, like onboard computer systems that help plan routes based on weather forecasts.
     The most dangerous ship in a hurricane is an empty one because the weight of cargo helps stabilize the ship against the waves. Ballast provides a little stabilizing weight when ships are empty, but it may not be enough. When a ship's ballast water is way down in the bottom of the ship, the ship has a very wicked roll to it. One sea captain report seeing ships go from a thirty degree roll to a thirty degree roll in the other direction in 3-4 seconds. Not only are such rolls tough on the crew, they are bad for a ship.  Modern cargo ships are constructed of thick steel, but if the waves are large enough and their battering lasts long enough, the pounding of those impacts can still break a ship apart.
     At a modern ship speed of 14 knots, they should be able to outrun a hurricane, but they do get caught in one they try to steer for what is called the "low side" or "clean side" of the storm...usually the side counterclockwise from its leading edge.
     In the teeth of the storm, a ship's survival depends on sea room and steering-way. Sea room means that the ship is a safe distance from anything it might crash into which why cargo ships try to stay well offshore.
     Steering-way means that the ship is moving forward with enough power to steer rather than just getting pushed around by waves and wind. The ship must keep its bow pointing into the waves to plow through them since a massive wave striking the ship's side could roll the ship over. It requires forward momentum to counter wind and waves trying to turn the ship.

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