The glossary.
Plain words for heavy metal.
50 terms you will meet on this site, from the measure of a ship's volume to the moment a hull first floats.
- air draught→
- The height from the waterline to the highest point of the ship. Air draught, not length, is what matters when a bridge or cable crosses overhead during a transit.
- azipod→
- An electric propulsion motor mounted in an underwater pod that can rotate 360 degrees horizontally. It eliminates traditional shaftlines and rudders, improving maneuverability on modern mega-cruise ships.
- ballast water
- Water taken aboard into dedicated tanks when a vessel is carrying little or no cargo, ensuring adequate stability, propeller immersion and safe handling in open waters.
- beam→
- The width of a hull at its widest point, usually at midship. Beam sets which locks, dry docks and canals a vessel can enter.
- bow thruster→
- A lateral propulsion unit fitted in a transverse tunnel below the waterline at the bow. It provides sideways thrust, giving massive vessels precise handling during docking.
- build year
- The year the vessel was completed at the shipyard. Delivery to the owner usually follows sea trials in the same or the following year.
- bulbous bow→
- A protruding underwater bulb at the front of a hull that modifies water flow, cancelling out bow waves to cut drag and boost fuel efficiency at cruising speeds.
- bunkering
- The process of supplying fuel oil, liquefied natural gas (LNG) or alternative marine fuels to a ship dedicated storage tanks for voyage propulsion.
- classification society
- An independent body that verifies a vessel is designed and built to published rules: Lloyd's Register, Bureau Veritas and DNV are examples. A class certificate is not the same as a flag-state certificate.
- cold ironing
- Supplying shoreside electrical power to a berthed vessel so auxiliary diesel engines can be shut down completely while docked, cutting local port emissions to zero.
- container slot
- One TEU-sized position aboard a containership. A ship of 24,000 TEU offers roughly 24,000 slots, though cargo stacking and weight limits affect real utilisation.
- deadweight tonnage (DWT)
- The mass a vessel can safely carry: cargo, fuel, stores and everyone aboard: in metric tonnes. DWT measures carrying capacity, whereas gross tonnage measures volume.
- deadweight vs displacement→
- Displacement measures the total mass of the ship and water moved aside, whereas deadweight measures only the variable load: cargo, fuel, water, crew and provisions.
- delivery
- The contractual handover of a completed ship from the yard to its owner, normally after acceptance sea trials. A delivered ship may still spend days or weeks preparing for its first commercial voyage.
- draft survey
- A calculation method used on bulk carriers and tankers where water displacement is measured before and after cargo operations to establish the exact cargo weight transferred.
- draught
- The vertical depth of a hull below the waterline, usually quoted for the stern where it is deepest. Draught decides which channels, ports and canals a ship can use: the Panama limit is a beam and length rule, not primarily a draught one.
- dual-fuel→
- Engines designed to run on conventional fuel or on an alternative such as LNG, methanol or ammonia. Many recent cruise and container newbuilds are dual-fuel to cut emissions in port and on the main voyage.
- FEU
- Forty-foot equivalent unit: the measure for 40-foot containers, which occupy twice the space of a TEU.
- flag state
- The country whose flag a vessel legally flies. The flag state regulates registry, crewing and many safety obligations; a merchant may register under a different jurisdiction than its owner's home country.
- Flettner rotor
- A tall, spinning vertical cylinder installed on deck that extracts thrust from crosswinds using the aerodynamic Magnus effect, lowering propulsion fuel burn.
- FLNG→
- Floating liquefied natural gas: a moored facility that filters, liquefies and stores natural gas offshore. Facilities such as Prelude FLNG hold hulls larger than any ship, but they are not ships.
- float-out→
- Moving a hull out of its building dock or into the water during construction. A float-out is a construction milestone: the ship is intact, but still months from delivery.
- freeboard
- The vertical distance measured from the waterline up to the lowest point of the weather deck, providing the reserve buoyancy needed to resist sinking in heavy seas.
- gross register tonnage (GRT)→
- An obsolete historical volume measure where 100 cubic feet equals one ton. Modern ships use gross tonnage (GT) under the 1969 convention; the two figures are not interchangeable.
- gross tonnage (GT)
- A dimensionless volume measure of a ship's enclosed spaces, calculated under the International Convention on Tonnage Measurement (1969). GT is the number quoted on cruise-ship comparisons, since cabin space adds volume faster than other business metrics.
- hull number
- The yard serial number assigned to a hull when its keel is laid. Sister ships share a design but carry different hull numbers.
- IMO number
- A unique seven-digit identifier issued to a ship by the International Maritime Organization. The number stays with the hull even after a rename or a change of flag.
- keel laying
- The formal start of steel construction at a yard: the moment a new hull begins to exist.
- knot
- The universal nautical unit of speed, equal to one nautical mile per hour (approximately 1.852 kilometres per hour or 1.15 miles per hour).
- length overall (LOA)→
- The distance between the extreme fore and aft points of a ship's hull. LOA is the figure used in most longest-ship comparisons.
- LNG carrier→
- A tanker built to carry liquefied natural gas in heavily insulated tanks near −163 °C. These ships are usually sized by volumetric capacity in cubic metres rather than by length alone.
- Malaccamax→
- The practical maximum size for the Strait of Malacca, which is draught-limited at roughly 20–21 metres rather than limited by length or beam. There is no single published length and beam envelope.
- metacentric height (GM)
- A primary metric in naval architecture that measures initial static stability against rolling. A larger GM produces a stiffer ship with rapid recovery rolls.
- midship section
- The cross-section of a hull halfway between the forward and aft perpendiculars, establishing the vessel maximum submerged width and depth profile.
- neopanamax→
- Ships sized for the expanded Panama locks, opened in 2016: a stated envelope of about 366 metres long, 49 metres wide and 15.2 metres draught.
- panamax→
- The stated envelope of the original Panama locks (1914): about 294 metres long, 32.3 metres wide and 12 metres draught. Any ship above these limits crosses by the newer locks or not at all.
- passenger capacity→
- A cruise ship's berth capacity: the number of guests it can actually accommodate: which is lower than the crowd aboard on a sold-out voyage. P&O's Iona carries more than 5,000 people.
- Plimsoll line
- A statutory reference mark painted on a hull midships indicating the maximum safe legal draft to which a vessel may load across different seasons and water densities.
- scrubber
- An exhaust-gas cleaning system fitted in engine exhaust funnels to wash out sulfur oxides (SOx) and soot particles before gases reach the atmosphere.
- sea trials
- The post-fitting-out tests in which a hull runs at speed ahead of delivery: the moment builders prove the engines, steering and electronics in the open sea.
- sister ships→
- Vessels built to the same design at the same yard or group. They share a class, but details such as catering and finishes may differ, making strict 'identical' a stretch.
- squat effect→
- The hydrodynamic phenomenon in shallow or narrow fairways where rushing water beneath the hull drops local pressure, drawing the hull downward and increasing its effective draft.
- Suezmax→
- Practical guidance around 20 metres draught for the Suez Canal, which imposes no parallel-lock length or beam limit as the world's canals do. The largest long-haul tankers are Suezmax-fitted.
- TEU→
- Twenty-foot equivalent unit: the standard box size that container flows are counted in. A 20,000-TEU containership carries roughly 20,000 containers, typically a mix of 20- and 40-foot units.
- trim
- The difference between the draft measured forward and the draft measured aft. A ship is said to be trimmed by the stern when its stern sits deeper in the water.
- two-stroke marine diesel
- A massive, low-speed internal combustion engine that turns the propeller at around 60 to 100 RPM without gearboxes, providing maximum thermal efficiency for cargo giants.
- ULCV→
- Ultra Large Container Vessel: containerships carrying roughly 14,500 TEU or more, including the 24,000 TEU mega-max series running between East Asia and Europe.
- VLCC→
- Very Large Crude Carrier: supertankers typically ranging between 200,000 and 320,000 deadweight tonnes, capable of transporting two million barrels of crude oil per voyage.
- waterline length (LWL)→
- The horizontal length of a hull measured strictly where it meets the water surface, distinct from length overall (LOA) which includes overhangs.
- yard number→
- Another name for the hull number: the serial a yard uses while a ship is on its building berth.
A
B
C
D
F
G
H
I
K
L
M
N
P
S
T
U
V
W
Y
How the definitions relate
Two distinctions matter most. First, gross tonnage is volume, while deadweight tonnage is carrying capacity. The two answer different questions. Second, a longest ship is decided on hull length, but most real-world limits, such as canals, ports and bridges, are drawn against beam, draught and air draught. The canal-fit tool and the methodology explain both numbers per vessel.
Definitions are written for this site's plain-language tone, checked against standard maritime reference usage, and are not legal or regulatory definitions.