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Swallowers of Ships, Eastern Cape and Kwa-Zulu Natal, South Africa.
Giant waves, some 100 feet and more in height, have been reported by seamen for centuries. Until the 20th century such reports were usually discounted as tall tales. Only in recent decades have oceanographers seen evidence that gigantic waves occur in the open sea.
In the 60’s one such wave damaged the superstructure of the 44000 ton Italian liner Michelangelo 80 feet above the waterline. And in the winter of 1942 the 81273-ton Queen Mary, with 15000 American soldiers on board, laboured though 20-30 ft waves towards England, 700 miles to the east. Suddenly the sea seemed to drop away and she was drawn into the deep trough of a wave coming at her broadside. Looking up, men on the bridge could not believe their eyes. Although high above the waterline, the mountainous wave was so high they could not see its top. As the mass of water bore down on them, the crest tore away in an avalanche of sea. The upper decks were under water and the great liner listed to one side, within inches of capsizing. The Daily Mail newspaper reported that “…those who had sailed in her since first she took to sea were convinced she would never right herself. Her safety depended on no more than 5 degrees. Had she gone those inches farther to port, the Queen Mary would have been no more”. Many other ships that have encountered such waves have not been so lucky. And they have been most unlucky off the coast of South Africa.
The Cape Peninsula has been graced with several names over the centuries. It was called the Cape of Good Hope by the Dutch. It was later called “The Fairest Cape in all the World” by Sir Francis Drake. However, winds that ferociously lash the coast on occasion make seafaring hazardous, and it was called the Cape of Storms by the Portuguese. Its reputation for being the most hazardous passage on earth other than Cape Horn is well deserved – as evidenced by the many shipwrecks along our coast.
Less well-known however is the mysterious disappearance of several ships, without trace, under calm conditions off our coast. In July 1909 the liner SS Waratah sailed from Durban for Cape Town. She overtook the steamer Clan MacIntyre and was last seen through the rain by troops on manoeuvres, after which she disappeared. A protracted search by the Union-Castle freighter Sabine and another vessel – the Wakefield – found no trace of the liner or its 211 passengers.
On September 25 1973 the 258000-ton tanker Svealand was steaming in a south-westerly direction, at reduced speed, 23 miles east of Hood Point near East London. She encountered 30-40 ft waves and gale-force winds and reported damage and casualties when she plunged into a long deep “hole in the sea” which appeared ahead of the ship.
On May 17 1974 the Norwegian supertanker Wilstar was moving at reduced speed off Durban in big but manageable waves coming in sequences of seven crests. Suddenly the seventh was a giant. It submerged the main deck, ripped off the bow and tore away thick hull plates. Steel beams heavier than railroad tracks were snapped off. However, while it suffered damage, the Wilstar did not go down.
G K Mallory, an oceanographer at the University of Cape Town, investigated 10 such incidents and identified the conditions that cause such waves, the main cause of which are storm winds. During the austral winter (May through October), cyclones pass near Marion Island, creating a south-westerly fetch of over 1200 miles behind the storm front. The swells generated by such winds reach maximum height by the time they reach South Africa travelling north-eastwards, parallel to the coast. Note that such swell-trains can be generated by more than one source, each having its own wavelength and frequency. These episodic waves, known as Cape Rollers by seamen who sail around the Cape, are 50 feet or more from trough to crest. It is also worth noting that the swells are fully developed even though coastal winds may be light.
The core of the swift (4 to 5 kt) Agulhas Current, which travels down the east coast of Africa, is found outside of the continental shelf but within 100 nautical miles of the shoreline. This current runs “head on” into the swells between Richards Bay and East London, which shortens their wavelength, raises their height and steepens their slope. Occasionally such waves interact, converting them into giants that represent a real danger to shipping. Brian Igpen firstname.lastname@example.org who writes a column in the Argus called Port Pourri refers to them as Mallory Waves.
Mallory’s investigation revealed that, in 10 incidents, none occurred shoreward of the 100 fathom line. In addition, the effect of the Agulhas Current is much weaker further out to sea where the velocity of the current is lower and the sea much deeper. Accordingly it was recommended that, since such waves only form on the 100 fathom line, ships travelling south should be in water deeper than 100 fathoms, and ships travelling northwards should be in shallower water. Whether this is correct remains to be seen. However, since these recommendations were implemented, no ships have been lost. It would appear that few are willing to test the validity of this theory. Alan McIver AVKiller
2473/2%Last update: 2014-05-14 14:08
Author: Alan McIver
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