Ship Tonnage and Measurement Definitions

Displacement Tonnage

Displacement tonnage is the total weight of the volume of water a ship displaces when it is sitting in the water.

Standard Displacement Tonnage

Standard displacement tonnage is basically the same thing as displacement tonnage, but you must subtract the weight of any fuel and potable water carried on board the ship.

Deadweight Tonnage

Deadweight tonnage is the weight of all the cargo, fuel, dry provisions, supplies, etc. carried on board the ship.

Lightweight Tonnage

Lightweight tonnage is described as the weight of the ship when it was built in the shipyard, including all framing, machinery, decking, etc.

Gross Registered Tonnage

Gross registered tonnage is a measurement of the volume of all enclosed spaces on a ship, with 100 cubic feet equal to one ton.

Net Registered Tonnage

Net registered tonnage is the measurement of the volume of actual cargo storage areas when dividing the cubic volume in feet by 100 to get your tonnage.

Length Overall

Length overall is the distance measured between the fore and aft extreme points of the ship.

Length Between Perpendiculars

Length between perpendiculars is the distance measured along the loaded waterline between the aft and fore perpendiculars.

Aft Perpendicular

Aft perpendicular is a vertical imaginary line taken at the aft side of the rudder post.

Fore Perpendicular

Fore perpendicular is a vertical imaginary line that intersects the forward side of the stem with the loaded water line.

Registered Length

Registered length is the length from the fore part of the stem to the aft side of the stern post.

Light Waterline

Light waterline is the waterline of a ship carrying only her regular inventory, but no cargo.

Deep Waterline

Deep waterline is the waterline at maximum loaded draught in seawater.

Beam

Beam is the width of the hull measured from starboard to port at the outside of the frames but inside the shell plating.

Breadth Overall

Breadth overall is the maximum width measured from the outer hull on the starboard side to the outer hull on the port side.

Draught Forward

Draught forward is the vertical distance between the summer load line and the underside of the keel measured at the forward perpendicular.

Draught at the Stern

Draught at the stern is the vertical distance between the summer load line and the underside of the keel measured at the aft perpendicular.

Depth

Depth is the distance from the deck amidships to the bottom of the keel.

Freeboard

Freeboard is the vertical distance between the waterline and the deck line.

Air Draught

Air draught is the vertical distance between the summer waterline and the highest fixed point of the ship.

Down and Trimmed by the Head

Down and trimmed by the head is when the forward draught is larger than at the stern.