Modern dry cargo ship designs and facilities for cargo
Basic knowledge of general cargo ships
Modern dry cargo ship designs maximize hold space . A typical mid-size ship may have five or six holds; three or four forward of the machinery space and superstructure, and one or two aft. The machinery spaces and superstructure are usually located about threequarters aft. Older designs typically have three holds forward of the superstructure and two aft. Holds aft of the accommodation and machinery spaces improve the trim of the vessel when partially loaded, and provide the ship with sufficient draft aft for stability and propeller immersion. Small freighters often have machinery and accommodation spaces aft of all cargo holds. Deadweight of modern general cargo liners ranges from 9,000 to 25,000 tons; speeds range from 17 to 22 knots. Tramps are typically smaller and slower, with speeds ranging from 12 to 18 knots.
The speed-to-length ratio is generally 0.87 or less as higher ratios are usually not economical. Laden drafts are as deep as channels to the intended terminal ports allow, typically in the 26- to 29-foot range. Hull depth is selected to provide the desired draft and satisfy statutory freeboard requirements. Depth of the double bottom is kept low to maximize cargo space.
Fig:General cargo ships midship cross section
One or more ’tween decks may be fitted to facilitate flexibility in cargo loading and unloading, cargo segregation, and to improve stability. There may be watertight doors in the bulkheads on the ’tween decks levels. Denser cargoes are carried in the lower holds with high stowage factor products normally stowed in the ’tween decks. Refrigerated spaces may be built into the ’tween decks.
Tramps are designed to carry a wide variety of commodities while liners may be designed for a specific trade. Ship designs for a specific trade strive for "full and down" operation; the ship’s freeboard is down to her loadline with cargo cubic fully occupied.
For a given trade, hold spaces are usually designed so that the ratio of bale cubic to deadweight is 10 to 15 percent greater than the overall stowage factor of the goods carried to allow for more rapid cargo handling and broken stowage – the spaces between and around cargo units, including dunnage, and spaces not available for cargo stowage because of physical obstructions or ventilation and access requirements.
Holds are sized and provided with cargo gear to limit the amount of cargo cubic per stevedore gang to about 60,000 cubic feet; holds in the midbody are therefore usually shorter than those nearer the ends of the ship. The conflict between the desire to shorten holds and the length required by cargo gear and hatches sometimes dictates the assignment of midships spaces to machinery or to fuel, cargo, or ballast deep tanks rather than holds.
Hatches are as large as possible without compromising hull strength (the main or second deck is normally the strength deck) to reduce the requirement for horizontal movement of cargo within the holds. Hatches served by two sets of cargo gear generally measure 20 by 30 feet or larger. Hatches on older ships are generally smaller than those on newer ships. Hatches are surrounded by coamings to reduce the risk of flooding in heavy seas. Covers are usually constructed of steel (or wood on older vessels). The main deck plating between hatches is not effective in providing longitudinal strength, and is sized to carry fairly light local loads. The deck plating outboard the hatches is therefore much heavier, often exceeding five-eighths inch in thickness.
Cargo gear is designed for speed and flexibility for handling breakbulk, palletized, or container cargo. Various combinations of derricks, winches, and deck cranes are used for the handling of cargo. Cranes are fitted on many vessels to reduce manpower requirements. Some ships have special heavy-lift derricks that may serve one or more holds. Booms are rigged for either yard and stay (burton) or swinging-boom operation.
Various combinations of derricks, winches and deck cranes are used for the Handling of cargo.
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