Portcities Southampton
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The heavyweights

The heavyweights

Cargoes become heavier

In the years between the two world wars, operators of cargo liners had increasing difficulty accommodating large loads on conventional ships.  Railway locomotives were often exported from Europe and the USA, and as they grew in size they became difficult to lift on and stow on ordinary ships.  Several owners adapted their cargo liner designs to carry this sort of cargo.  They strengthened their masts, provided extra support wires, and added massive derricks capable of lifting over 100 tons.  On some ships, several derricks could work in unison to increase the total lift.  These heavy-lift ships had large hatches and holds to take these huge items of cargo, but often they were so large that they could only be accommodated on the deck, which was specially strengthened.   

In the years after the Second World War, such heavy lifts became even more common, as indivisible items such as transformers, reactors and equipment for oil refineries increasingly needed to be carried.  Most cargo liners therefore had at least one heavy lift derrick.  In the 1960s, several vessels were fitted with the German-designed Stülcken derrick, working between the two arms of a V-shaped mast, so that it could serve two holds.  With this derrick, loads of up to 300 tons could be handled.

Driving on and floating on

But loads became increasingly heavy, too large for conventional ships.  Some heavy lift ships were designed so that heavy loads could simply be driven on to their decks, loaded on multi-wheeled trailers or crawlers.  In this way loads weighing hundreds of tons, such as massive container cranes, could be moved. 

Perhaps the most novel concept is the dock ship.  The dock ship’s hull has a well at the centre, and has tanks which can be flooded so that this well is underwater.  The item to be carried is floated out over the well, and the water pumped out of the dock ship’s tanks, which rises and lifts the cargo.  The item to be carried must be watertight and capable of floating or is loaded on a barge.  Whole ships have been conveyed long distances in this way.