What Is a Boat’s Tonnage (Gross and Net Ton)?
Most “tons” in paperwork are not weight but volume — knowing the difference between gross ton, net ton and displacement is how you make sense of the documents.
In boat paperwork, most “tons” are about volume, not weight. Not knowing the difference is a bit like answering “will the crane lift it?” with the documents. Gross ton and net ton in the paperwork are administrative measures based on volume; the real equivalent of weight is displacement.
The table below sets the three apart.
Tonnage measurements
Measure | Meaning |
|---|---|
Gross ton (GT) | Volume-based, calculated from enclosed spaces |
Net ton (NT) | Volume-based, administrative size |
Displacement | Weight: the weight of the water the boat displaces |
This distinction comes up all the time in paperwork: berth registry, licences and some fee bands are defined by tonnage. Displacement, on the other hand, is the number that matters in practice: it is what you use for crane capacity, road transport and trim calculations.
At the haul-out, the number you give the crane operator is always the fully loaded displacement; “it says 9 tons on the papers” is the classic story of boats left hanging. Write the three figures separately in your boat file: GT (paperwork), light and loaded displacement (crane/transport/trim).