Ships’ Cranes or Cargo Gear

Cranes (Derricks) can be used individually within their tried, tested and certificated capacity ie: within what is described as their safe working load (swl) – for example, “10 tonnes SWL”. This loading is subject to regular and stringent testing under which all the derrick equipment – the supporting post (a vertical pillar, called a samson-post where heavy-lift derricks are involved), boom (the derrick “arm”), blocks (pulleys), tackle (wires) and winches (motors supplying power to pull in or to release Wires) – are inspected at least annually by a reputable, outside body, appropriate documentation being thereafter kept on board and made available by the ship’s master upon request.
The capacity of ship’s derricks to handle work is governed not only by the number and location of derricks, but equally by the position of associated winches used to operate the lifting, swinging and lowering of the boom and its wires. It is quite possible that three winches be required to operate some individual derricks, thereby reducing available motive-power needed to operate other derricks at adjacent hatchways. At the worst, this may mean that a five-hatch ship will be at times limited to working only two hatches – Nos. 2 and 4 – caused by the demand on available winches, whereas the breakdown of a single winch could mean the temporary inability to move cargo at one or more hatchways. Such arrangements would not, in general, be considered adequate. With a derrick rig, two methods of working predominate. In the first of these systems, two derricks are set-up in what is known as union-purchase. With this very fast and well-understood method of cargo handling one derrick is topped (or luffed) up and swung out (slewed) to plumb a selected point over the quay, the other a suitable point in the hold being worked. The guys will then be tightened up to hold both derricks in position and their runners (the wire that is fastened to the hook and actually does the lift- ing) are joined together at the cargo hook. It can be seen that one winch driver lifts the load clear of the hatch while the other pulls it over the side and lowers it onto the quay, the midship driver then paying out his wire. This method enables faster handling than with a single, conventional derrick. Furthermore, union-purchase reduces the safe working load of the derricks by more than half. Thus 2 x 10 tonnes swl derricks/winches linked by union-purchase would have a reduced lifting capacity of, say, I x 4.25 tonnes swl, perhaps limited still further by the strength of the tackle used. The system remains, nevertheless, ideal for light but voluminous cargo such as bagged goods, enabling all hatchways to be worked simultaneously. Where heavy items are to be lifted, union-purchase is obviously not the answer. Provided the ship is fitted with small, low-capacity slewing winches of, say, I tonne swl, there is a further alternative means of linking two parallel derricks together with just two adjacent cargo winches – this system being termed swinging derricks. Swinging derricks maintain the speed of operation provided by union-purchase, but enable liftings up to the maximum capacity of the smallest derrick/winch involved, using in place of a third cargo winch what is called a deadman – a suspended deadweight on one line; eg: a mass of old wire hanging free to provide tension. A winch is used simply to swing the boom from over the hatchway to the quayside, a second being utilised for the boom’s return journey. An alternative is to slew (swing) the derrick by hand tackle. A self-swinging derrick or crane-derrick is a single derrick, acting in a fashion similar to that of a crane, but using only its own, immediately associated winches, and thus not interfering with cargo handling at adjacent hatchways. Such a derrick is usually located singly at a hatchway and, like a crane, is capable of extremely fast operation by only one, skilled driver using joystick control. Heavy-lift derricks – eg: Stulckens, have their own immensely strong tackle, in some cases the boom being able to work two adjacent hatches.