Navy declares victory in lengthy battle to raise sunken tug
Bungs, welds, snatch blocks and the oldest vessel afloat in SA waters were all used to raise a sunken tug in Simon’s Town naval dockyard.
South African Navy spokesman Commander Greyling van den Berg said the salvage mission also relied on “several days and nights of ingenuity, tenacity and sheer hard work”.
The decommissioned tug De Mist, which was tied up in the harbour, sank on November 10 after listing in a strong wind.
“Although [SA Navy divers] valiantly attempted to save the tug, the wind and sea conditions made it very difficult,” said Van den Berg. “The wind was blowing the tug onto its pneumatic fender [the large inflatable rubber that prevents a vessel bumping against the quay].
“However, due to the weight on the fender it burst, causing the tug’s gunwale [the raised side of the vessel] to dip underwater. Once this happened the tug sank within minutes.”
The tug settled on its port side in 10m of water, and Van den Berg said the objective of the salvage operation was to raise the vessel and tow it to the synchrolift 50m away.
“Shallow water salvages are known to be extremely difficult because of the lack of air expansion and buoyancy at this depth,” he said.
“The main challenge experienced ... was trying to make the 39-year-old tug airtight and thus buoyant. The divers used underwater welding equipment to seal off holes in order to pump her full of air, but as soon as leaks were sealed, other leaks bubbled to the surface.
“The Armscor dockyard personnel were constantly on hand to provide bungs and other mechanisms for leak stopping and shoring.”
On November 28, divers raised the aft of the tug and rested its keel on the bedrock. Then they did the same on the forward side.
The Mooring Lighter - built by dockyard apprentices in 1902, and the oldest vessel still afloat in SA waters - was used to create further lift, said Van den Berg.
“Because of her age, the Mooring Lighter does not have a generator and cannot operate her capstan. In order to mitigate this, the winch of the tug Umalusi was used.
“The docking plate under the tug kept on getting stuck in the sand during the lifting process, adding to complications. Throughout the lifting process, divers continued sealing off leaks and filling fuel, freshwater and ballast tanks with air.
“On December 3, the naval riggers and the naval harbour master devised a way of using snatch blocks to create further buoyancy. The aim was to raise the tug high enough to clear the platform of the synchrolift.
“This was achieved on Wednesday [but] … airlocks in the tug created balance issues, which were finally overcome.”
Van den Berg called the operation “an amazing display of teamwork and resourcefulness by the dedicated team of professionals who worked tirelessly to solve numerous mathematical, physical and engineering problems”.
He added: “This exercise also provided invaluable practical salvage experience for the navy divers, at no cost to the organisation.”