Gross tonnage is the number that shapes almost every fee a vessel pays in port, yet crews and shore staff often mix it up with net tonnage and deadweight.
All three measure something different about a ship, and confusing them leads to wrong estimates for port and harbour services in Curaçao or any other terminal. Getting the definitions straight is the first step to reading a port bill correctly.
Gross tonnage measures the whole ship
Gross tonnage (GT) is a measure of a vessel’s total internal volume, not its weight. It comes from a formula in the International Convention on Tonnage Measurement of Ships, which converts the enclosed space of the hull and superstructure into a single dimensionless figure. Every enclosed space counts, from cargo holds to accommodation decks to the engine room. Because GT reflects size rather than mass, it stays fixed for the life of the vessel unless the ship undergoes structural modification.
Port authorities, classification societies and flag states rely on GT as the baseline for almost every regulatory threshold. Manning requirements, safety certification and many international conventions all key off this figure, which is why it appears on the vessel’s tonnage certificate before any other measurement.
Net tonnage and deadweight tell a different story
Net tonnage (NT) is derived from GT but subtracts spaces that do not generate revenue, such as crew quarters, machinery spaces and navigation areas. What remains is meant to represent the ship’s earning capacity, the volume actually available for cargo or passengers. NT is usually the smaller number and is the figure many canal authorities and some port tariffs use instead of GT.
Deadweight tonnage (DWT) is unrelated to either. It measures weight, not volume, and represents everything a vessel can carry when loaded to its maximum draft: cargo, fuel, fresh water, provisions and crew. DWT changes with a ship’s design and loading condition rather than staying fixed like GT.
Charterers quote DWT constantly because it tells them how much cargo actually fits, but it rarely enters into port fee calculations the way GT does.
Anyone working through pilotage charges for an incoming vessel needs to know which of these three figures the tariff schedule actually references, since using the wrong one produces a cost estimate that misses the real number by a wide margin.
The same distinction matters when preparing agency fees and disbursement accounts ahead of a call, and it resurfaces again once you look at what a port call involves from arrival to departure.
How Curaçao calculates port dues from gross tonnage
Curaçao, like most Caribbean and Dutch Caribbean ports, bases its main port dues on gross tonnage rather than net tonnage or deadweight.
The published tariff sets a rate per GT unit, sometimes on a sliding scale that decreases slightly as tonnage rises, and that figure forms the largest single line item on most port bills. Pilotage, towage and mooring fees frequently use the same GT figure as their starting point, with adjustments for vessel type or time of day layered on top.
The Curaçao Ports Authority’s own harbor tariff schedule confirms this structure, calculating harbor fees, wharfage and pilotage from a vessel’s registered tonnage rather than its cargo weight. The definitions behind that calculation are not arbitrary either.
Regulatory bodies such as the U.S. Coast Guard codify gross and net tonnage as separate, precisely defined measurements, which is why an agent working from a tonnage certificate can predict a port bill with real accuracy before a vessel even arrives.
Knowing which figure drives which fee saves time before a vessel ever reaches the roadstead. A ship’s tonnage certificate, checked against the current tariff schedule, gives an accurate estimate of dues, pilotage and towage before the agent submits a final disbursement account. For operators calling Curaçao, working with an experienced port agency in Curaçao means these calculations are handled correctly the first time, with tonnage figures verified against certificates rather than assumed from a charter party or class record.