Here are the purposes of the following principal dimensions of a ship:
(a) LOA: Length Overall (LOA) is the length measured between the extreme ends of the ship from forward to aft. It is used to determine the overall size of the ship and its berthing requirements. It also affects the ship’s resistance, speed, and maneuverability.
(b) LBP: Length between Perpendiculars (LBP) is the length measured between the forward perpendicular (FP) and the aft perpendicular (AP) of the ship. It is used in stability calculations, hydrostatics, and tonnage measurements. It also influences the ship’s wave-making resistance and propulsion efficiency.
(c) Draught: Draught is the vertical distance measured along the shipside from the bottom of the keel to the waterline. It indicates how deep the ship is submerged in water and how much clearance it has below the waterline. It affects the ship’s displacement, buoyancy, stability, and resistance. It also determines the maximum depth of water that the ship can safely navigate.¹
(d) Depth: Depth is the maximum depth measured along the centreline of the ship from the bottom of the keel to the top of the uppermost deck. It indicates how high the ship is above the waterline and how much freeboard it has. It affects the ship’s strength, stability, and seaworthiness. It also determines the maximum height of cargo that can be loaded on the ship.
(e) Breadth: Breadth is the maximum breadth measured at midship between the inner sides of ship side plating. It indicates how wide the ship is and how much cargo space it has. It affects the ship’s stability, resistance, and maneuverability. It also determines the maximum width of waterway that the ship can pass through.
(f) AP: Aft Perpendicular (AP) is a vertical line passing through the intersection of the summer load waterline (SLWL) and aft side of the rudder post or the center of the rudder stock, if rudder post is not fitted. It marks one end of the LBP and defines where to measure draught at aft.
(g) FP: Forward Perpendicular (FP) is a vertical line passing through the intersection of the SLWL and forward side of the stem of the ship. It marks one end of LBP and defines where to measure draught at forward.
(h) LWL: Length at Waterline (LWL) is the length measured at an existing waterline, from forward to aft ends of
the ship. It varies with draught and trim of the ship. It affects the ship’s displacement, wetted surface area, and resistance.
Source: Conversation with Bing, 9/10/2023
(1) Principle Dimensions of a ship – Marine engineers knowledge. https://www.marineengineersknowledge.com/2020/01/principle-dimensions-of-ship.html.
(2) Basic Naval Architecture and Ship Dimensions Explained. https://www.brighthubengineering.com/naval-architecture/26220-what-are-the-basic-dimensions-of-a-ship/.
(3) DESIGN CHARTS FOR CONTAINER VESSELS ABSTRACT. https://aast.edu/pheed/staffadminview/pdf_retreive.php?url=6995_238_1_new%20design%20charts.pdf&stafftype=staffpdf.
(4) ISO 7462:1985(en), Shipbuilding ? Principal ship dimensions …. https://www.iso.org/obp/ui/.