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Unformatted text preview: 0 ³ « … * ª (Ship’s principal dimensions and related terms)
(À Ship’s principal dimensionsS Usually s a ship requires three dimensions to define its size, these are a length , a breath and a depth. Draft is an important dimension for shipping management, sos it is given here. 1. Length There are various ways of defining the length of a ship. First the length between perpendiculars is the distance measured parallel to the base at the level of the summer load waterline from the after perpendicular to the forward perpendicular. The after perpendicular is taken as the after side of the rudder post where there is such a post, and the forward perpendicular is the vertical line drawn through the intersection of the stem with the summer load waterline. In ships where there is no rudder post the after perpendicular is taken as the line passing through the centre line of the rudder pintles. The perpendiculars and the length between perpendiculars are shown in Figure 1.1. It is obvious from Figure 1.1 that the length between perpendiculars (L.B.P.) does not represent the greatest length of the ship. For many purposes, such as the docking of a ship, it is necessary to know what the greatest length of the ship is. This length is known as the ‘length overall (L.O.A.)’ and is defined simply as the distance from the extreme point at the after end to a similar point at the forward end. In most ships the length overall will exceed by a considerable amount the length between perpendiculars. The excess will include the overhang of the stern and also that of the stem where the stem is raked forward. In modern ships having large bulbous bows the length overall may have measured to the extreme point of the bulb. The length on the waterline (L.W.L.) is often used, particularly when dealing with ship resistance. This is the distance measured on the waterline at which the ship is floating from the intersection of the stern with the waterline to the intersection of the stem with the waterline. This length is not a fixed quantity for a particular ship, as it will depend upon the waterline at which the ship is floating and upon the trim of the ship. 2. Breadth The mid point of the length between perpendiculars is called “amidships’ and the ship is usually broader at this point. The breadth is measured at this position and the breadth most commonly used is called the ‘breadth moulded’. It may be defined simply as the distance from the inside of plating on one side to a similar point on the other side measured at the broadest part of the ship. As is the case in the length between perpendiculars, the breadth moulded does not represent the greatest breadth of the ship. In many ships the breadth extreme is the breadth moulded plus the
1 thickness of the shell plating on each side of the ship. In the days of riveted ships, where the strakes of shell plating were overlapped the breadth extreme was equal to the breadth moulded plus four thicknesses of shell plating, but in the case of modern welded ships the extra breadth consists of two thicknesses of shell plating only. A t er per f pendi cul ar Sum er l oad w er l i ne m at
A t er si de of r udder post or f cent r e l i ne of r udder pi nt l es Aid m shi ps B bous bow ul Lengt h bet w een per pendi cul ar s( L. B P) . Lengt h on w er l i ne ( L. WL) at . Lengt h over al l ( L. O A .) Fi gure 1. 1 Shi p’ s Lengt h
The breadth extreme may be much greater than this in some ships, since it is the distance from the extreme overhang on one side of the ship to a similar point on the other side. This distance would include the overhang of decks, a feature which is sometimes found in passenger ships in order to provide additional deck area. It would be measured over fenders, which are sometimes fitted to ships such as cross channel vessels which have to operate in and out of port under their own power and have fenders provided to protect the sides of the ships when coming alongside quays. 3S Depth
2 For w r d a per pendi cul ar The third principal dimension is depth, which varies along the length of the ship but is usually measured at amidships. This depth is known as the ‘depth moulded’ and is measured from the underside of the plating of the deck at side amidships to the base line. It is shown in Figure 1.2 (a). It is sometimes quoted as a ‘depth moulded to upper deck’ or ‘depth moulded to second deck’, etc. where no deck is specified it can be taken the depth is measured to the uppermost continuous deck. In some modern ships there is a rounded gunwale as shown in Figure 1.2 (b). In such cases the depth moulded is measured from the intersection of the deck line continued with the breadth moulede line. 4S Draught The draught at which a ship floats is simply the distance from the bottom of the ship to the waterline. If the waterline is parallel to the keel the ship is said to be floating on an even keel, but if the waterline is not parallel then the ship is said to be trimmed, If the draught at the after end is greater than that at the fore end the ship is trimmed by the stern and if the converse is the case it is trimmed by the bow or by the head. 3 B read th extrem e D p h mu d d D et ol e( ) Cb amer Breadth Moulded(B) In d o si e f si d p ati n el g B ase l i ne (t op of keel ) F gu 1. 2( a) i re
D eck Ru d o n ed g n al e uw D
Sd ie F gu 1. 2( b) i re New words and expressions principal dimension perpendicular ° n. ªW ¹¿ ˆ adj. length between perpendiculars summer load waterline r udder post p intle length overall
4 n. F n er ed length on the waterline b readth moulded overhang keel t r im bulbous bow camber fender rounded gunwale n. ª¸8 ? n. v. *¸8 ª? n.ª¸ 8 ? v. v t. ¡è 0³ «* …ª Te rms relating to a ship’s hull The main body of a ship is the hull. This is the area between the main deck, the sides and the bottom. It is made up of frames covered with plating. The foremost part is called the bow and the rearmost part is called stern. See Figure 1.3. MI N D C A EK BW O SD IE BT O OT M F gur e 1. 3 i SEN TR The hull is divided up into a number of watertight compartments by decks and bulkheads. Bulkheads are vertical steel walls going across the ship and along. Decks divide the hull horizontally. Those dividing up cargo spaces are known as ’tweendeck. The hull contains the engine room, cargo space and a number of tanks. In dry cargo ships the cargo space is divided into holds, in liquid cargo ships it is divided into tanks. At the fore end of the hull are the fore peak tanks and at the after end are the after peak tanks. They are used for fresh water and water ballast. The space between the holds and the bottom of the hull contains double bottom tanks. These are used for ballast water and fuel. See Figure 1.4. 5 Bul kheads
’ t weendeck ’ t weendeck Bul kheads
’ t weendeck ’ t weendeck Peak t ank Hol d Hol d Engi ne r oom Hol d Hol d Peak t ank Doubl e bot t om t anks Doubl e bot t om t anks Fi gur e 1. 4 New words and expressions the main deck f rame plating bow s tern bulkhead ’ tweendeck t he forepeak tank t he after peak tank ballast water double bottom tank n. ª ¸¨ 1 n. n. n. n. watertight compartment ð 8· ß T e rms of ship tonnage measurement
A problem that recurs frequently in the shipping industry is the need to measure the size of a ship or the size of a fleet of ships. One reason for doing this is to measure the cargo carrying capacity, but there are many other commercial reasons. For example, port authorities will wish to charge large ships higher wharfage fees than small ships, and the same applies to the Panama and Suez Canal Authorities. To meet these needs a whole range of different measurement units have been developed in the shipping industry, each adapted to some particular need. In this section we briefly review the principal units currently in use.
6 1. Gross registered tonnage (grt) One major issue of concern to shipowners, particularly liner companies handling low-density cargo, is the internal volume of the ship, and this is recorded by the gross registered tonnage. The grt is a measure of the total permanently enclosed capacity of the ship and consists of: (1) underdeck tonnage; (2) ’tweendeck tonnage; (3) superstructures; (4) deckhouses and other erections. Certain spaces such as navigational spaces (wheelhouse, chart rooms, etc.), galleys, stairways, light and air spaces are exempted from measurement, in order to encourage their adequate provision. The official gross tonnage of a vessel is calculated by the Government Surveyor when it is first registered. One ton equals 100 cubic feet of internal space. 2. Gross tonnage (gt) The 1969 IMO Tonnage Convention introduced a new simplified standard procedure for calculating gross tonnage, and this is now used in all countries that are signatories to the convention. Instead of going through the laborious process of measuring every open space in the ship, the gross tonnage is calculated from the total volume of all enclosed spaces, measured in cubic metres, using a standard formula. For some ship types, especially those with complex hull forms, the gt and the grt may be significantly different. 3. Net registered tonnage (nrt) Under the existing rules, nrt is supposed to represent the cargo volume capacity of the ship and is obtained by deducting certain non-revenue-earning spaces from the grt. The net registered tonnage is expressed in units of 100 cubic feet. 4. Net tonnage (1969) A formula introduced by the 1969 Tonnage Convention gives net tonnage (1969) as a function of the moulded volume of all the ship’s cargo spaces, with corrections for draughts less than 75 per cent of the ship’s depth and for the number of berthed and unberthed passengers. The net tonnage so calculated cannot be less than 30 per cent of gt. The net tonnage is also dimensionless. 5. Deadweight (dwt) In many trades the principal concern is with measuring the cargo-carrying capacity of a fleet of ships, and for this purpose deadweight tonnage is used. The deadweight of a ship measures the total weight of cargo that the vessel can carry when loaded down to its marks, including the weight of fuel, stores, water ballast, fresh water, crew passengers and baggage. As a rule, the non-cargo items account for about 5 per cent of the total deadweight in mediumsized ships, although the proportion is lower in large vessels. As an example, a 35,000 dwt bulk carrier would probably be able to carry about 33,000 deadweight of cargo. Deadweight can also be measured as the difference between the loaded ship displacement and its lightweight (see below for definition). 6. Lightweight (lwt) A ship’s lightweight is the weight of the vessel as built, including boiler water, lubricating oil and the cooling system water.
7 7. Standard displacement (sd) This is the theoretical but accurate weight of the vessel fully manned and equipped with stores and ammunition but without fuel or reserve feed water. 8. Suez and Panama tonnages For ships transiting the Suez and Panama Canals, different systems of measurement are used to assess the dues payable. All ships have to be specially measured for the assessment of their dues when passing through these areas New words and expressions wharfage superstructure deckhouses n. ö¨ ß · n. n. *? ª( ´ G ross registered tonnage 8 ...
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- Fall '08