Page 288 - Ship Construction.DJ Eyres 6Ed

Ch24-H8070.fm Page 277 Wednesday, October 18, 2006 7:00 AM

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Lifting Arrangements

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The past three decades have seen a considerable reduction in the number
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
of shipyard built and rigged derricks as shipboard lifting devices and their
replacement with bought in cranes. The crane being less labour intensive,
faster operating, more easily controlled and taking up less deck area. They
do however require a higher degree of onboard maintenance.
When ordering a new ship the shipowner normally specifies the number,
safe working load, position, and any special features of the cargo lifting
devices to be fitted. Where derricks are specified the shipbuilder or an out-
side specialist consultant is then responsible for the detailed design of each
derrick rig, together with the various fittings at the derricks and mast.
Where patent derricks and masts are fitted the patentee may supply the
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drawings, etc., to the shipyard, who then build and erect these rigs. Masts,
except some patent types, are the responsibility of the shipbuilders; blocks,
wire, and usually derrick booms being supplied by an outside manufacturer
to the shipyard’s specification.

Masts and Sampson Posts

Masts on a general cargo ship may fulfil a number of functions but their
prime use in modern ships is to carry and support the derricks used for cargo
handling. Single masts are often fitted, but many ships now have various
forms of bipod mast which are often more suitable for supporting derricks,
although some types can restrict the view from the bridge. Sampson posts are
often fitted at the ends of houses and may be found at the other hatches also.
The strength of masts and sampson posts is indicated by the classification
societies. As a result of the span loads and derrick boom thrusts, a single
mast or post may be considered similar to a built-in cantilever with axial
and bending loads. Some torque may also be allowed for where the post
has a cross-tree arrangement to an adjacent post. Where shrouds and pre-
venters are fitted these must be allowed for, which makes the calculations
somewhat more difficult. In modern ships there is a tendency to simplify
the rigging which can restrict cargo handling. Shrouds are often dispensed
with and preventers may only be rigged when heavy derricks are used. Each
mast or post has adequate scantlings so that they may remain unstayed.