The hull form of a ship may be defined by a number of dimensions and terms which are often referred to during and after building the vessel. An explanation of the principal terms is given below:
After Perpendicular (AP): A perpendicular drawn to the waterline at the point where the aft side of the rudder post meets the summer load line. Where no rudder post is fitted it is taken as the centre line of the rudder stock.
Forward Perpendicular (FP): A perpendicular drawn to the waterline at the point where the foreside of the stem meets the summer load line.
Length Between Perpendiculars (LBP): The length between the forward and aft perpendiculars measured along the summer load line.
Amidships: A point midway between the after and forward perpendiculars.
Length Overall (LOA): Length of vessel taken over all extremities.
Register Length: is the length of ship measured from the fore-side of the head of the stem to the aft side of the head of the stern post or, in the case of a ship not having a stern post, to the fore-side of the rudder stock. If the ship does not have a stern post or a rudder stock, the after terminal is taken to be the aftermost part of the transom or stern of the ship. This length is the official length in the register of ships maintained by the flag state and appears on official documents relating to ownership and other matters concerning the business of the ship. Another important length measurement is what might be referred to as the IMO Length. This length is found in various international conventions such as the Load Line, Tonnage and SOLAS conventions and determines the application of requirements of those conventions to a ship. It is defined as 96 per cent of the total length on a waterline at 85 per cent of the least moulded depth measured from the top of keel, or the length from the fore-side of stem to the axis of rudder stock on that waterline, if that is greater. In ships designed with a rake of keel the waterline on which this length is measured is taken parallel to the design waterline. Moulded dimensions are often referred to; these are taken to the inside of plating on a steel ship.
Base Line: A horizontal line drawn at the top of the keel plate. All vertical moulded dimensions are measured relative to this line.
Moulded Beam: Measured at the midship section is the maximum moulded breadth of the ship.
Moulded Draft: Measured from the base line to the summer load line at the midship section.
Moulded Depth: Measured from the base line to the heel of the upper deck beam at the ship’s side amidships.
(Note: Moulded dimensions are referred to measurement taken to the inside of plating on a steel ship).
Extreme Beam: The maximum beam taken over all extremities.
Extreme Draft: Taken from the lowest point of keel to the summer load line. Draft marks represent extreme drafts.
Extreme Depth: Depth of vessel at ship’s side from upper deck to lowest point of keel.
Half Breadth: Since a ship’s hull is symmetrical about the longitudinal centre line, often only the half beam or half breadth at any section is given.
Freeboard: The vertical distance measured at the ship’s side between the summer load line (or service draft) and the freeboard deck. The freeboard deck is normally the uppermost complete deck exposed to weather and sea which has permanent means of closing all openings, and below which all openings in the ship’s side have watertight closings.
Sheer: Curvature of decks in the longitudinal direction. Measured as the height of deck at side at any point above the height of deck at side amidships.
Camber (or Round of Beam): Curvature of decks in the transverse direction. Measured as the height of deck at centre above the height of deck at side.
Rise of Floor (or Deadrise): The rise of the bottom shell plating line above the base line. This rise is measured at the line of moulded beam.
Half Siding of Keel: The horizontal flat portion of the bottom shell measured to port or starboard of the ship’s longitudinal centre line. This is a useful dimension to know when dry-docking.
Tumblehome: The inward curvature of the side shell above the summer load line.
Flare: The outward curvature of the side shell above the waterline. It promotes dryness and is therefore associated with the fore end of ship.
Stem Rake: Inclination of the stem line from the vertical.
Keel Rake: Inclination of the keel line from the horizontal. Trawlers and tugs often have keels raked aft to give greater depth aft where the propeller diameter is proportionately larger in this type of vessel. Small craft occasionally have forward rake of keel to bring propellers above the line of keel.
Tween Deck Height: Vertical distance between adjacent decks measured from the tops of deck beams at ship side.
Parallel Middle Body: The length over which the midship section remains constant in area and shape.
Entrance: The immersed body of the vessel forward of the parallel middle body.
Run: The immersed body of the vessel aft of the parallel middle body.
Tonnage: This is often referred to when the size of the vessel and the gross tonnage is quoted from ships’ Register. Tonnage is a measure of the enclosed internal volume of the vessel (originally computed as 100 cubic feet per ton).
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