Gross Tonnage (GT — IMO 1969)

Anyone delivering a new hull, clearing a ship in a Brazilian port or pricing pilotage works from the same figure: the gross tonnage printed on the tonnage certificate. Port authorities bill on it, and it decides whether a vessel falls inside or outside requirements such as AIS from 300 GT, the MARPOL IOPP certificate from 400 GT and the SOLAS and ISM package from 500 GT. A design that lands just past one of those thresholds carries a permanently different operating cost, and the arithmetic separating the two sides is logarithmic, so nobody does it in their head.

The 1969 International Convention on Tonnage Measurement of Ships defines GT = K1 × V, with K1 = 0.2 + 0.02 × log10(V). V is the moulded volume of every enclosed space, in cubic metres, measured to the inner face of the shell plating. The coefficient climbs slowly: 0.220 at 10 m³, 0.260 at 1,000 m³ and 0.300 at 100,000 m³. In practice GT settles near a quarter of the volume. With the 12,500 m³ already loaded on screen, K1 works out at 0.2819 and the answer reads 3524.23 — a bare number, which is why no unit follows it.

The convention covers ships of 24 metres and over on international voyages. Smaller craft get measured under simplified national rules; in Brazil those come from the maritime authority and use a block expression built from length, beam and depth, so running the logarithmic formula there yields a figure with no registry value. The other trap is V itself. Only enclosed spaces count, and Regulation 2 lists exclusions — recesses open to the weather through a permanent opening, among others — that a design model knows nothing about. Gross tonnage is also distinct from net tonnage, which has its own expression built on K2, K3 and passenger count.