Built for anyone who has to read a ship's spec sheet and act on it: charterers matching a ship to a berth, cargo owners sizing a parcel, operators planning a port call, brokers circulating tonnage. A ship's particulars — a dozen tonnages and dimensions that each mean something different — decide whether a ship fits, how much it carries and what it costs. The definitions are scattered across class glossaries and half-understood on the desk. We assembled them into one reference, with a worked "will it fit" example.
The rynda voyage-economics calculator classifies and models a ship from exactly these numbers; the tonnage board lets you filter by them.
📖 See also: Ship types & sizes · Canal transit limits · Vessel identity — IMO vs MMSI
Contents
- At a glance
- The tonnages — DWT, GT, NT, displacement
- The dimensions — LOA, LBP, beam, depth
- Draft, freeboard and the load line
- Air draft
- TPC — tonnes per centimetre
- The full particulars glossary
- A worked "will it fit" example
- For developers and modellers
- FAQ
- References
At a glance
The short version, if you read nothing else:
- A ship's "particulars" split into tonnages (weights and volumes) and dimensions (lengths, widths, depths). Confusing the two is the classic beginner error.
- DWT (deadweight) is the carrying capacity — the total weight of cargo + fuel + stores + water a ship can carry. It's the number the size classes are built on.
- GT (gross tonnage) is a volume measure of the whole enclosed ship — used for regulations, manning and port dues, not weight. NT (net tonnage) is the earning-space volume. Displacement is the actual weight of water the ship pushes aside.
- Dimensions decide fit: LOA (overall length) and beam (width) against berths and canal locks; draft (depth below the waterline) against water depth; air draft (height above the water) against bridges.
- TPC (tonnes per centimetre) tells you how much weight sinks the ship one centimetre — the number that turns a draft restriction into a cargo limit.
- The load line ("Plimsoll mark") is the legal limit: a ship may not load past the mark for the zone and season it's in.
This is a reference, not stability or loading advice. Actual loading is governed by the ship's stability book, the load line certificate and the master's calculation; the figures here explain the terms.
The tonnages — DWT, GT, NT, displacement
Four different "tonnages", four different meanings:
- Deadweight tonnage (DWT) — the carrying capacity in tonnes: everything the ship can load (cargo + bunkers + stores + fresh water + crew) when down to her marks. A "180,000 DWT" bulker can carry ~180,000 t all-in. This is the cargo number.
- Displacement — the actual weight of the ship at a given draft (= the weight of water displaced). Light displacement is the empty ship (steel + machinery); loaded displacement is the ship at her marks. DWT = loaded displacement − light displacement.
- Gross tonnage (GT) — a volume measure of the whole enclosed ship (a dimensionless index from total volume), used for regulations, safety rules, manning and port dues. ECA, EU ETS and CII thresholds are all in GT. GT is not a weight — never convert it to DWT.
- Net tonnage (NT) — the volume of the earning spaces (cargo holds), derived from GT. Used for some canal and port dues (the idea: charge for the space that makes money).
The one-line memory aid: DWT = what it carries (weight), GT/NT = how big it is (volume), displacement = what it weighs.
The dimensions — LOA, LBP, beam, depth
- LOA (Length Overall) — the extreme length, bow to stern. The number berths and locks care about.
- LBP (Length Between Perpendiculars) — length along the waterline between the rudder post and the bow at the summer waterline. Used in naval-architecture and some tonnage/hydrostatic calcs, and shorter than LOA.
- Beam (breadth) — the width at the widest point. The binding dimension for canal locks (Panamax 32.31 m, Neopanamax 51.25 m) and many berths.
- Depth (moulded depth) — the vertical distance from the keel to the main deck. Not the same as draft — depth is the whole hull height; draft is only the submerged part.
LOA and beam answer "does it physically fit the berth/canal"; depth relates the hull to how deep it can load.
Draft, freeboard and the load line
- Draft (draught) — the depth of the hull below the waterline: how much water the ship needs. The binding number against a port's or canal's water depth. It changes with loading — more cargo, deeper draft.
- Freeboard — the height of the hull above the waterline (deck edge to water). Draft + freeboard relate to the hull depth.
- The load line ("Plimsoll mark") — a legally-required mark on the hull showing the maximum draft a ship may load to, with separate lines for different zones and seasons: S (summer), W (winter), T (tropical), F (fresh water), TF (tropical fresh), WNA (winter North Atlantic). Fresh water is less dense, so a ship floats deeper — the FWA (fresh water allowance) is how much extra draft is permitted in fresh water (the reason Panama quotes a TFW — Tropical Fresh Water — draft).
Loading past the applicable load line is illegal and a serious PSC detention item.
Air draft
Air draft is the height of the highest point of the ship above the waterline — mast, funnel or container stack. It's the dimension that decides whether a ship passes under a bridge: the Panama locks' Bridge of the Americas (57.91 m), the Suez Canal Bridge (68 m), the Bosphorus bridges (58 m), the fixed bridges of the Kiel Canal (40 m). A ship can satisfy every length/beam/draft limit and still be too tall — and air draft falls as the ship loads deeper, so a lightly-laden boxship with a high stack is the worst case.
TPC — tonnes per centimetre
TPC (Tonnes Per Centimetre immersion) is the weight that sinks the ship exactly one centimetre deeper. It's the bridge between a draft limit and a cargo limit:
TPC ≈ (waterplane area (m²) × water density (t/m³)) / 100
The waterplane area is the area of the ship's horizontal cross-section right at the waterline — roughly how much "footprint" the hull presents to the water surface. In seawater (density 1.025), a ship with a 3 000 m² waterplane area has TPC ≈ 3 000 × 1.025 / 100 ≈ 30.75 t/cm — so every extra 30.75 t of cargo pushes her 1 cm deeper. Turn it around: if a port restricts you to 0.5 m less draft, you must load **50 cm × TPC** less — about 1 540 t less cargo in this example. TPC is how a draft restriction becomes a tonnes-of-cargo decision.
The full particulars glossary
The reference the desk assumes you know:
| Term | Meaning | Measures |
|---|---|---|
| DWT | Deadweight — carrying capacity | Weight (t) |
| Displacement | Weight of the ship (= water displaced) | Weight (t) |
| Light displacement (LDT) | The empty ship (used in scrap sales) | Weight (t) |
| GT | Gross tonnage — whole-ship volume index | Volume (dimensionless) |
| NT | Net tonnage — earning-space volume | Volume (dimensionless) |
| LOA | Length overall | Length (m) |
| LBP | Length between perpendiculars | Length (m) |
| Beam | Breadth at the widest point | Length (m) |
| Depth (moulded) | Keel to main deck | Length (m) |
| Draft | Hull below the waterline | Length (m) |
| Air draft | Highest point above the waterline | Length (m) |
| Freeboard | Hull above the waterline | Length (m) |
| TPC | Tonnes per cm of immersion | Weight/length (t/cm) |
| Load line | Max legal loading draft, by zone/season | Mark on hull |
| FWA | Extra draft allowed in fresh water | Length (mm) |
A worked "will it fit" example
Question: can a 82 000 DWT Kamsarmax (LOA 229 m, beam 32.26 m, summer draft 14.4 m, TPC ≈ 60 t/cm) load a full cargo and sail to a river berth with a 12.5 m draft limit?
Beam 32.26 m ≤ 32.31 m → fits the old Panama locks and standard berths ✓
LOA 229 m → within Kamsarmax port limits ✓
Summer draft 14.4 m > 12.5 m → CANNOT load to marks; draft-restricted
Draft to shed = 14.4 − 12.5 = 1.90 m = 190 cm
Cargo to shed = 190 cm × 60 t/cm ≈ 11 400 t
So the ship physically fits the berth, but the 12.5 m draft limit forces it to load ~11,400 t light — a "part cargo." That single TPC calculation is the difference between a full and a part fixture, and it's why draft limits, not just DWT, decide what a ship earns on a trade. Model the whole thing — cargo intake, draft, TCE — in the voyage-economics calculator.
For developers and modellers
- Classify and model from the particulars: the voyage-economics calculator maps DWT + type into a size class and applies the right speed/consumption and canal fit.
- Never interchange GT and DWT — they're different scales (volume vs weight). Store both; use GT for regulatory thresholds (ECA/ETS/CII), DWT for cargo capacity.
- A draft limit is a cargo limit via TPC —
cargo_to_shed = (marks_draft − port_limit) × TPC. Expose it so a user sees the part-cargo, not just a "doesn't fit" flag.
FAQ and common confusions
Q: What's the difference between DWT and GT?
A: DWT (deadweight) is a weight — the ship's carrying capacity in tonnes, the basis of the size classes. GT (gross tonnage) is a volume index of the whole enclosed ship, used for regulations, manning and dues. Different scales — never convert one to the other.
Q: Is draft the same as depth?
A: No. Draft is only the part of the hull below the waterline (what decides if a ship clears the seabed). Depth (moulded) is the whole hull height, keel to main deck. Draft changes with loading; depth doesn't.
Q: What is air draft?
A: The height of the ship's highest point above the water — the number that decides whether it clears a bridge. It falls as the ship loads deeper, so a lightly-laden ship with a tall stack is the tightest case.
Q: What is TPC and why does it matter?
A: Tonnes Per Centimetre — the weight that sinks the ship 1 cm. It converts a draft restriction into a cargo limit: cargo to shed = (draft over the limit in cm) × TPC. It's how you know how much a shallow berth costs you in cargo.
Q: What is the Plimsoll / load line?
A: The load line is the legal maximum loading draft, marked on the hull with lines for different zones and seasons (summer, winter, tropical, fresh…). Loading past it is illegal and a serious PSC detention item.
Q: Why can a ship load deeper in fresh water?
A: Fresh water is less dense than seawater, so a ship sits deeper for the same weight. The fresh water allowance (FWA) permits extra draft in fresh water — the reason Panama and river ports use a fresh-water draft reference.
Q: What's the difference between LOA and LBP?
A: LOA is the extreme overall length (what berths and canals limit). LBP is the length between perpendiculars at the waterline — shorter, used in naval-architecture and hydrostatic calculations.
Q: How do I know if a ship fits a berth or canal?
A: Check the binding dimension: beam and LOA against the berth/lock, draft against the water depth, air draft against any bridge. The tightest one decides — and a draft limit further caps the cargo via TPC.
References
- International Convention on Tonnage Measurement of Ships (1969) — GT and NT.
- International Convention on Load Lines (1966) — the load line, zones and freeboard.
- SOLAS / class-society tonnage and hydrostatic definitions (Lloyd's Register, DNV, ClassNK).
- rynda voyage-economics calculator, tonnage board and the Ship types & sizes reference.
Document version
- 2026-07 — first public version. The tonnages, dimensions, draft/load line, air draft and TPC in one glossary, with a worked "will it fit" example. Actual loading is governed by the ship's stability book and load line certificate.