Deadman

Where heavy loads are involved, union purchase is obviously not the answer. The winch arrangement of certain vessels enables two parallel derricks to be linked together with two adjacent cargo winches in a system termed ‘swinging derricks‘. Swinging derricks maintain the speed of operation of union purchase, but enable liftings up to the maximum capacity of the smallest derrick or cargo winch involved, using in place of a third cargo winch a ‘deadman‘ – a suspended deadweight on one line (e.g. a mass of old wire) the purpose of which is simply to provide tension. One winch is used to ‘swing’ the boom from over the hatchway to the quayside, a second winch being used to ‘swing’ the boom back to its original position. A variation is the ‘self-swinging derrick‘, or ‘crane-derrick‘, a single derrick system that works in the same fashion as a crane, by using only its own immediately associated winches and therefore does not interfere with cargo-handling at adjacent hatchways. Such a derrick is normally to be found in isolation at a hatchway and, just like a crane, is capable of extremely fast operation by only one, skilled driver utilising a joystick control. A commercial example of a self-swinging derrick is the ‘Velle’ type. Typically cargo derricks lift between 5 and 15 tonnes swl, but it is not unusual for conventional-type derricks to be adapted for lifts of up to 50 tonnes. Some general-cargo ships are equipped with ‘Stulcken derricks‘ (see Appendix 1:2) which, in some cases, can safely lift weights of up to 450 tonnes, having the added advantage of serving two hatchways immediately fore and aft of the location of its samson posts.