Chains hold strong on 30,000-tonne ship

3 June 2024

It’s the cable or chain, not the anchor, that counts. 

HMAS Adelaide’s anchor, coming in at a hefty 20 tonnes, is small change compared to the almost 30,000-tonne ship – which is not carrying helicopters, trucks and tanks. 

But despite the anchor’s weight, large ships like the landing helicopter dock are kept in place by the weight of the cable lying across the ocean floor. 

Although the anchor does dig into the seabed, HMAS Adelaide boatswain Petty Officer Thomas Archer said the chain did most of the heavy lifting. 

The crew calculate the ocean depth and wind strength before boatswains operate the ‘guillotine’ to release the chain. 

“We count the cable as it’s veering, or released. They’ll say five shackles on deck – the length of a shackle is 90 feet – and as it comes out we’ll call one shackle on deck and so on,” Petty Officer Archer said. 

“Boatswains deal with everything: ropes, boats and guns – we’re one of the oldest jobs in the Navy.”

When the ship’s at anchor, boatswains check the cable’s angle about every half hour to see how the ocean or wind is moving the ship. 

“If the cable starts picking up, it tells us the wind is too great and we may need to drive back up and let more cable out otherwise we risk dragging the anchor,” Petty Officer Archer said. 

A slack chain provides much greater holding power than if it’s taught. It’s called a catenary curve in geometry and acts as a shock absorber between anchor and ship. When the wind dies down the chain returns to rest in its natural curve. 

Before weighing anchor and leaving, the ship is positioned above the slack chain and a windlass pulls the anchor directly up. 

“We don’t want to pull in the cable from far away because the anchor could get stuck,” Petty Officer Archer said.  

“In a perfect world we can drive the ship directly over the anchor and pull it straight up.” 

If the crew do need to ditch the anchor, the cable can be broken. 

At regular intervals, a lugless joining shackle connects links together. 

On smaller ships the chain can even be used as a tow line.

This is one piece of equipment that’s stood the test of time.



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