Glossary & Vessel Guide

A unitless measure of a vessel's total internal volume, calculated from the moulded volume of all enclosed spaces using a formula defined in the International Convention on Tonnage Measurement of Ships (1969). GT replaced the older Gross Register Tonnage (GRT) system.

GT is not a measure of weight. A vessel with 160,000 GT has 160,000 units of internal volume, not 160,000 tonnes of mass. It is the primary basis for port dues, registration fees, and safety regulations worldwide.

A measure of the useful cargo-carrying capacity of a vessel, derived from GT by subtracting the volume of non-cargo spaces (engine rooms, crew quarters, navigation areas). Also defined by the 1969 Tonnage Convention.

NT is always less than GT. It represents the revenue-earning capacity of the ship and is the basis for many canal toll systems globally.

A tonnage measurement system specific to the Suez Canal, calculated using the Suez Canal Authority's own rules (based on the Constantinople Convention of 1873, updated periodically). SCNT measures are typically higher than international NT for the same vessel because the Suez calculation method includes some spaces excluded by the 1969 Convention.

SCNT has been the global gold standard for canal toll billing for over 150 years. Vessels transiting the Suez Canal carry an SCNT certificate issued by a recognized measurement authority. The Panama Canal uses its own tonnage system (PC/UMS) based on similar principles.

The total weight a vessel can carry, measured in metric tonnes. DWT includes cargo, fuel, freshwater, ballast water, provisions, crew, and passengers. It is the difference between the vessel's displacement when fully loaded and its lightweight (empty hull + machinery).

DWT is the standard measure for bulk carrier capacity. A Capesize bulk carrier typically has 150,000 to 200,000 DWT; a VLCC (Very Large Crude Carrier) has 200,000 to 320,000 DWT; a ULCC (Ultra Large Crude Carrier) exceeds 320,000 DWT.

The standard unit for measuring container capacity. One TEU is the volume of a standard 20-foot intermodal container (20 ft long, 8 ft wide, 8.5 ft tall). A standard 40-foot container equals 2 TEU. Container ship capacity is always stated in TEU.

Modern Ultra Large Container Vessels (ULCVs) carry up to 24,000 TEU. A typical feeder vessel carries 1,000 to 3,000 TEU. The world's container fleet moves over 800 million TEU annually.

The total weight of water displaced by the vessel's hull, equal to the vessel's total mass including hull, machinery, fuel, cargo, and everything on board. Displacement is measured in metric tonnes or long tons.

Displacement is the standard tonnage measure for warships, naval auxiliaries, and special floating units (dredgers, floating drydocks, offshore construction vessels) because these vessel types do not have conventional cargo spaces that GT/NT can meaningfully measure.

The maximum length of a vessel's hull measured from the foremost point of the bow to the aftermost point of the stern, in metres. LOA includes all permanent structural elements but typically excludes removable fittings like bowsprits or davits.

LOA is critical for berth allocation, canal transit scheduling, and lock-chamber clearance. A Suezmax tanker has LOA around 274 m; a VLCC around 330 m; a Q-Max LNG carrier around 345 m.

The maximum width of a vessel's hull, measured at the widest point, in metres. Beam determines whether a vessel can fit through locks (relevant at Panama) and how much channel width it occupies in a strait or canal.

A Panamax vessel has a beam of 32.31 m (the old locks limit). A Neo-Panamax vessel can be up to 51.25 m. VLCCs and ULCCs have beams of 58 to 68 m.

The vertical distance between the waterline and the lowest point of the hull (keel), in metres. Draft varies with cargo load: a fully laden tanker sits deeper in the water than the same tanker in ballast. Maximum summer draft is the deepest a vessel can legally sit.

Draft is the binding constraint for shallow-water chokepoints and canal depth limits. The Suez Canal's maximum permitted draft is 66 feet (20.1 m) in the newer channel. The Strait of Hormuz itself has no draft restriction (deep natural water), but approach channels at Gulf ports do.

Laden means a vessel is carrying cargo. Ballast means a vessel is transiting without commercial cargo, carrying only seawater in its ballast tanks to maintain stability and safe trim. Ballast voyages are common for tankers and bulk carriers returning to load ports after delivering cargo.

Ballast transits impose lower environmental and infrastructure demands than laden transits. A ballast tanker has shallower draft, lower collision risk (no hazardous cargo), and reduced navigational complexity.

A unique seven-digit identification number assigned to seagoing vessels by the International Maritime Organization (a UN agency based in London). The IMO number is permanently associated with the hull and does not change even if the vessel is renamed, re-flagged, or sold. It functions like a serial number for ships.

IMO numbers are required under SOLAS (Safety of Life at Sea) for all vessels of 300 GT and above engaged in international voyages. They are the primary key for vessel databases, AIS tracking, and regulatory compliance.

The country in which a vessel is registered and whose flag it flies. The flag state has jurisdiction over the vessel's regulatory compliance, safety standards, and crew conditions. Common flag states include Panama, Liberia, Marshall Islands, Hong Kong, Singapore, and the Bahamas.

Many vessels are registered under "open registries" (flags of convenience) in countries different from their owner's nationality. This is legal and standard practice: over 70% of global tonnage is registered under a flag state different from the beneficial owner's country.

A radio transponder system that automatically broadcasts a vessel's identity, position, speed, course, and destination. AIS is mandatory for all vessels of 300 GT and above on international voyages. Signals are received by other ships, shore stations, and satellites.

AIS data powers the vessel tracking on our Live Traffic page. During the 2026 Hormuz crisis, some vessels have turned off their AIS transponders ("going dark") to avoid detection, which is a violation of SOLAS regulations.

A shore-based system that monitors and manages maritime traffic in a defined area, similar to air traffic control for aircraft. VTS operators track vessel positions via AIS, radar, and VHF radio, providing traffic advisories, routing instructions, and collision avoidance guidance.

VTS is standard at major ports and canal approaches. The Suez Canal Authority operates one of the world's most sophisticated VTS systems. A Hormuz toll authority would fund VTS coverage across the strait's traffic separation scheme.

A mutual insurance association that provides liability coverage to shipowners. P&I clubs cover third-party claims including cargo damage, pollution, crew injury, wreck removal, and collision liability. The 12 members of the International Group of P&I Clubs insure approximately 90% of world tonnage.

P&I coverage is separate from hull and machinery insurance and from war-risk insurance. During the 2026 Hormuz crisis, P&I clubs have imposed special conditions and reporting requirements for vessels transiting the strait.

A separate insurance policy or premium that covers losses arising from war, civil war, revolution, piracy, terrorism, mines, torpedoes, and related perils. War-risk cover is excluded from standard hull and cargo policies and must be purchased separately.

War-risk premiums are quoted as a percentage of the vessel's insured hull value per transit. Before the 2026 crisis, Hormuz war-risk premiums were 0.15% to 0.25% of hull value. At the peak of the crisis, they exceeded 5% (a 25-fold increase). Some underwriters withdrew cover entirely.

Maritime cabotage laws restrict the carriage of goods or passengers between ports within the same country to vessels flagged in that country. The US Jones Act is the best-known example, requiring all goods shipped between US ports to be carried on US-built, US-owned, US-crewed, US-flagged vessels.

Cabotage is relevant to the Hormuz context because Gulf states with cabotage restrictions may face additional constraints when rerouting cargo during strait disruptions. The Trump administration suspended the Jones Act in April 2026 to allow foreign-flagged tankers to move fuel between US ports during the oil crisis.

A narrow passage in a waterway through which a disproportionate share of maritime traffic must pass, creating a concentration of strategic and economic vulnerability. The world has approximately 24 recognized maritime chokepoints.

The five most critical are the Strait of Hormuz (39% of seaborne crude), the Strait of Malacca (24% of all seaborne trade), the Suez Canal (10% of seaborne trade), the Panama Canal (2.5% of seaborne trade, but 40% of US container imports), and the Turkish Straits (20% of global wheat exports). Each is exposed to a different mix of geopolitical, climatic, and operational risks.