Simplification of few terms
1. The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy.
2. Rudder post is situated near aft end of the ship.
3. Summer load line: is waterline at Summer draft.
4. Bow or Stem is the forward part of the ship
5. Keel is the bottom of the ship
6. Deck means flooring of the ship
7. Draft (or draught) means depth of ship below waterline
After Perpendicular (AP): A perpendicular drawn to the waterline at the point where the after side of the rudder post meets the summer load line. Where no rudder post is fitted it is taken as the center line of the rudder stock.
Forward Perpendicular (FP): A perpendicular drawn to the waterline at the point where the fore-side of the stem meets the summer load line. For most of the hydrostatic calculations, the forward perpendicular is used as the forward reference of the hull.
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.
Lloyd’s Length: Used for obtaining scantlings if the vessel is classed with Lloyd’s Register. It is the same as length between perpendiculars except that it must not be less than 96% and need not be more than 97% of the extreme length on the summer load line. If the ship has an unusual stem or stern arrangement the length is given special consideration.
Register Length: 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 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, SOLAS and MARPOL conventions, and determines the application of requirements of those conventions to a ship. It is defined as 96% of the total length on a waterline at 85% of
Ship Construction.
Molded dimensions are often referred to; these are taken to the inside of plating on a metal ship.
Base Line: A horizontal line drawn at the top of the keel plate. All vertical molded dimensions are measured relative to this line.
Molded Beam (Breadth): Measured at the midship section, this is the maximum molded breadth of the ship.
Molded Draft: Measured from the base line to the summer load line at the midship section.
Molded Depth: Measured from the base line to the heel of the upper deck beam at the ship’s side amidships.
Extreme Beam:(Breadth) 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 that has permanent means of closing all openings, and below which all openings in the ship’s side have watertight closings.
Deck Line: The deck line is a horizontal line marked amidships on each side of the ship. Its upper edge shall normally pass through the point where the continuation outwards of the upper surface of the freeboard deck intersects the outer surface of the shell plating. In case difficulty, position is decided by authorities.
In Simple words, Deck line is line from where freeboard is measured and is drawn at a point on midship where deck plate meets the side shell plate.
The deck line is fixed and marked when the vessel is built in the yard. It is not changed during the life of the ship. The deck line marking is 300 mm long and 25 mm in width.
Sheer: A rise in the height of the deck (curvature or in a straight line) 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 center above the height of deck at side. Straight line camber is used on many large ships to simplify construction.
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 molded beam. Large cargo ships often have no rise of floor.
Half Siding of Keel: The horizontal flat portion of the bottom shell measured to port or starboard of the ship’s longitudinal center line. This is a useful dimension to know when drydocking.
Tumblehome: The inward curvature of the side shell above the summer load line. This is unusual on modern ships.
Flare (Flare of Bows): 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(Rake of Stem): 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’s 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 is discussed, and the gross tonnage is quoted from Lloyd’s Register. Tonnage is a measure of the enclosed internal volume of the vessel (originally computed as 100 cubic feet per ton). This is dealt with in detail in Chapter 30.
Deadweight: This is loading capacity of the vessel.
Air Draft: Air draft is the distance from the surface of the water to the highest point of the ship.
Bilge Radius: the radius of the rounded portion of a vessel’s shell that connects the bottom to the sides.
