Tonnage

Reading a ship's particulars

Published 20 August 2026 · updated 22 September 2026

A vessel description is a short list of numbers a broker is expected to read in seconds and offer from. Most of them are not measuring the same thing, and the two quoted most often are not weights at all. Every figure on the sheet answers one of three questions: can she lift it, will it fit inside her, and can she get alongside and work it.

Three tonnages, and only one of them is cargo

Displacement is the weight of water the ship pushes aside, which is the weight of the ship and everything in her. It is the only one of the three that is a real weight.

Lightweight, usually quoted as LDT, is the ship herself with nothing aboard: steel, machinery, permanent fittings. It has almost no chartering use: it is the weight a ship is sold against for demolition.

Deadweight is loaded displacement minus lightweight, so it is everything she can carry when she is down to her marks: cargo and bunkers, fresh water, stores, spares, crew and any ballast still aboard. Deadweight is therefore not cargo capacity, and on a long haul with full bunkers the gap runs to thousands of tonnes. What she actually lifts is deadweight cargo capacity, and it is voyage-specific.

The deadweight in a circular is normally summer deadweight, taken at the summer load line in salt water. Load lines vary by zone and season, so the same hull loads less in a winter North Atlantic zone and more in the tropics, and fresh water at a river berth changes it again. A ship offered at 52,000 dwt is offered at her best case.

GRT and NRT are volumes, not weights

This is the field misread most often. Gross tonnage measures the enclosed volume of the ship, worked out by formula under the 1969 Tonnage Convention. Since that convention it is a pure index number with no unit attached, though the older GRT label survives in daily use. Net tonnage is the same idea restricted to the earning spaces, holds and tanks, with machinery and crew spaces deducted.

Neither says anything about what she carries. What they set is what she costs to move: port dues, pilotage, towage, light dues, canal tolls and tonnage tax are commonly assessed on gross or net tonnage, and a canal may apply its own special measurement on top. Manning and survey regimes key off gross tonnage thresholds too. Two ships of identical deadweight can carry noticeably different gross tonnages and therefore different port costs, and that difference lands in the voyage calculation rather than the cargo quantity.

A ship shown at 20,000 gross does not carry 20,000 tonnes, and reading her that way has put the wrong hull in front of a charterer.

The dimension that excludes her is draught

LOA, length overall, is checked against berth and lock length, and plenty of terminals publish a maximum and enforce it flatly. Beam is checked against locks and channel width, and at some berths against how far a shore crane reaches across to the offside coaming.

Draught is the figure that does the excluding, and it does so long before deadweight becomes the limit. She is kept out by the water she draws against the depth over the bar, in the channel, at the berth, and inside whatever tidal window she is given. A load port with limited depth takes a smaller ship down to her marks and forces a larger one to load part cargo and top up elsewhere, which is a different piece of economics. Deadweight tells a broker the ceiling; draught tells them whether she can get near it here.

Air draught matters wherever something crosses the water above her: bridges on a river approach, a seaway, an overhead conveyor. Measured from the waterline to her highest fixed point, it is worst when she is light, which is exactly how she arrives to load. A ship that clears a bridge outbound and loaded may not clear it inbound in ballast.

Grain and bale: whether the cargo fits

Grain capacity is the total volume inside the cargo spaces as a free-flowing cargo would fill them, out to the shell between the frames and up inside the hatch coamings. Bale capacity is the volume open to cargo in packages, bags or bales, measured to the inside of the frames and under the beams. Bale is always the smaller number, and the gap is the space a sack cannot reach but barley can.

Against that sits the stowage factor of the cargo, the volume one tonne of it occupies. Multiply the quantity by the stowage factor and compare it with her cubic. If the cargo wants more volume than she has, she cubes out: full, but not down to her marks. If it wants less, she deadweights out: down to her marks with the holds part empty.

Ship      52,000 dwt summer, grain capacity 67,000 m3
          less 2,000 t of bunkers, water and stores,
          so 50,000 t of cargo by weight

Iron ore  stowage factor around 0.4 m3 per tonne
          50,000 t wants 20,000 m3. She is down to her marks
          with the holds less than a third full.

Grain     stowage factor around 1.4 m3 per tonne
          50,000 t wants 70,000 m3 and she has 67,000.
          She cubes out near 47,800 t and never sees her marks.

Two things sit on top of that arithmetic. Broken stowage is the volume lost to the shape of the cargo against the shape of the hold: small for grain, large for steels. And a dense cargo raises a strength question rather than a volume one. Tank top strength is quoted in tonnes per square metre, and ore or concentrate loaded in a few tight piles can exceed it while the ship is nowhere near her deadweight.

Gear decides the middle of the range

Whether she carries her own cranes is the single most consequential fact on the sheet for anything between Handysize and Supramax. A geared ship can work at a berth with no shore equipment, at anchorage into barges, and at a terminal whose crane is out of service. A gearless ship of twice the deadweight cannot go to any of them, whatever the rest of her particulars say. That is why gear is quoted in the circular line itself: in “4 x 30 t cranes” the gear does more work than the deadweight beside it.

The rest of the gear line rewards the same attention:

• Crane count and SWL. Safe working load caps the unit weight she can lift, so a single heavy piece can be beyond her while the total tonnage is trivial. Whether cranes pair for a heavier lift, and whether grabs are fitted, are both worth asking.
• Holds and hatches. The count sets how many gangs can work at once, which sets the load and discharge rate, which is the number the laytime clause is written against.
• Hatch size and shape. Box-shaped holds and wide hatches decide whether steels and units can be landed at all, or worked into the wings by hand.
• Fittings and history. Log fittings, grain fittings, CO2, hold coatings, last three cargoes. Cleanliness decides whether she is acceptable for a food grade parcel before any rate is discussed.

What a fleet register actually holds

Not all of this exists in any database, and it is worth saying which parts do. Across the 101,850 hulls in the Neptune Atlas register, every one carrying an IMO number:

Below that line coverage thins fast. Cubic capacities, crane count and SWL, hold and hatch numbers and last three cargoes are barely present in bulk vessel data from anyone. They come off the owner’s particulars sheet or a dry cargo questionnaire, and that is still how the trade works: a register narrows the field, and the owner’s sheet settles it.

Every field is a constraint

A particulars sheet is not a description of a ship. It is the list of ways she can fail to do this cargo at this berth, and the quickest route through it is to check the constraints in the order they bite: draught first, then gear, then cubic, and deadweight last.

References

• IMO, International Convention on Tonnage Measurement of Ships, 1969. imo.org
• IMO, International Convention on Load Lines, 1966. imo.org
• IMO, International Code for the Safe Carriage of Grain in Bulk, resolution MSC.23(59), 1991. imo.org
• IMO, International Maritime Solid Bulk Cargoes (IMSBC) Code. imo.org
• Britannia P&I, Draft survey calculation on a ship: practical guidance, 2023. britanniapandi.com

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