- Most common in use on general cargo vessel
- Often referred to as “married gear”
- combination of two derricks :
- one plumbed over hatch and one ashore
- coupled or joined by a union hook
- worked in conjunction with each other
- have a preventer guy on its outboard side in addition to other two guys, because of the heavy cross pull on the derricks when the weight is being transferred. Also known as standing guy, they are set up tighter than the slewing guy.
- Each cargo boom is joined to vertical mast or post by a swivel fitting known as a gooseneck
- up and down or luffing movement of the boom is carried out by a topping lift/span tackle
- horizontal or athwartship movement -controlled by slewing guy
- two booms are linked by schooner guy
- A cargo wire or runner, form each boom is joined by a three-way swivel which is known as a union hook
- Operations – cargo lifted by one derrick to the necessary height, and then slackened on one runner and hauling on the other, eased over until it is under the other derrick when it can be lowered on the hold or quay.
Advantages
- fastest method for discharging break bulk, non -unitized general cargo
Disadvantages
- used for light loads (1.5 to 2 tonnes per load)
- winch men must be highly skilled and experienced
- cannot be used for “spot loading”
- repositioning the derrick is time consuming
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