"How much can she lift?" is the first question in every chartering conversation, and the answer is the deadweight. But deadweight is not one number — it changes with the season, the zone, and the water density. Understanding it properly is core seamanship, not just commercial knowledge.

The three weights of a ship

  • Lightweight — the ship herself: hull, machinery, outfit, with no cargo, fuel, water, or stores. Fixed for the ship's life (apart from modifications).
  • Deadweight (DWT) — everything the ship can carry: cargo, fuel oil, diesel, lube oil, fresh water, stores, crew and effects. This is the earning capacity.
  • Displacement — the total: lightweight + deadweight. The actual weight of the ship at any moment.

So: Deadweight = Displacement (loaded) − Lightweight. When someone quotes a ship's DWT, they mean the summer deadweight unless stated otherwise.

The Plimsoll mark: the law on the hull

Amidships on both sides you will find the load line mark — a circle with a horizontal line through it, crossed by a deck line, with a set of horizontal marks beside it. These are the maximum drafts the ship may legally load to:

  • TF — Tropical Fresh (deepest allowed: warm fresh water gives least buoyancy)
  • F — Fresh water
  • T — Tropical seawater
  • S — Summer (the standard reference mark, level with the circle's line)
  • W — Winter (shallower: rougher seas need more freeboard)
  • WNA — Winter North Atlantic (shallowest, for the harshest winter zone)

Which mark applies depends on where the ship is and where she is going — the load line zones and seasonal areas are published in charts and tables, and the stricter of the departure and arrival requirements governs. Loading beyond the applicable mark is illegal and unsafe: it eats into freeboard, reserve buoyancy, and the stability margins the designers built in.

Density matters

The marks assume standard seawater at 1.025 tonnes per cubic metre. In fresh or brackish water the ship floats deeper for the same weight — that is exactly what the Fresh Water Allowance (FWA) is for: the extra draft permitted in fresh water so the ship arrives at the summer mark once she reaches seawater. Always measure the dock water density; "about 1.010" is not a measurement.

Why overloading is never an option

Pressure to "just take a few hundred tonnes more" is as old as shipping. The answer is always no: overloading voids the load line certificate, invalidates insurance, risks detention by port state control, and — most importantly — reduces the freeboard and stability that keep the ship and crew alive in bad weather. No cargo is worth that.

Practical tip

Before every loading, work out the limiting draft yourself: the applicable load line mark, corrected for the actual water density and for fuel and water you will burn before reaching the next zone. Tell the terminal the maximum lift in tonnes — in writing — before the first lift goes aboard.

Check deadweight and load limits with SMART SEAMEN's free deadweight calculator — get started.

← All guides