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SHIP CONSTRUCTION

MASTS & 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 preventers 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 un-stayed.

MAST CONSTRUCTION AND STIFFENING Tubular steel sections are commonly used in mast and post construction, the sections being rolled in short lengths and welded in the shipyard. The short lengths may be tapered and are of different plate thickness to allow for the greater stresses experienced at the base of the mast. Where connections are made for fittings such as the gooseneck and a masthead span swivel, doubling or welded reinforcing pads may be provided. To obtain the necessary mast scantlings, excessive doubling or internal stiffeners are rarely found in modern practice, except where a heavier derrick than that for which the mast was originally designed is carried. Higher tensile steels are often used to advantage in mast construction, giving less weight high up in the ship and dispensing with the need for any form of support, without excessive scantlings.

Cross-trees, mast tables, etc., may be fabricated from welded steel plates and sections.

Derrick booms are as a rule welded lengths of seamless tubular steel. The middle length may have a greater diameter to allow for the bending moment, to which the boom is subject in addition to the axial thrust.

At the base of the mast adequate rigidity must be provided, the amount of additional structural stiffening increasing with the size of derricks carried by the mast. Many cargo ships have mast houses into which the masts are built, the house being suitably strengthened. These houses need not be designed to support the mast, the structure being of light scantlings, and the support provided by stiffening in the tweens. Where the house is strengthened the masts or posts generally land on the upper deck, but where heavy derricks are installed the mast may then land on the upper tween deck. Since the derricks and mast are as a rule midway between holds they land over the hold transverse bulkheads which lend further support.

Heavy derrick masts will require extensive stiffening arrangements in the mast house, and also in the tweens, with support for the transverse bulkhead so that the loads are transmitted through the structure to the ship’s bottom. Partial longitudinal and transverse bulkheads with deck girders may provide the mast house stiffening. Stiffened plate webs at the ship’s centre line in the tweens, and heavier stiffeners on the transverse bulkhead in the hold then provide the additional strengthening below decks (see Figure 57). Heavy insert plates are fitted in way of the mast at the various decks.

 

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