Neptune Atlas
Reading a ship's particulars
A vessel description (the ship’s particulars) is a short list of numbers that a broker must read in seconds and make an offer from. Most of the numbers do not measure the same thing, and the two most often quoted are not weights at all. Every figure on the sheet answers one of three questions: can the ship carry this weight, will the cargo fit inside it, and can the ship reach the berth and handle the cargo?
Three tonnages, and only one of them is cargo
Displacement is the weight of water the ship pushes aside, which equals the weight of the ship and everything on it. It is the only one of the three that is a real weight.
Lightweight, usually written LDT, is the empty ship: steel, machinery and permanent fittings. It has almost no use in chartering; it is the weight used when a ship is sold for demolition.
Deadweight is the loaded displacement minus the lightweight. So it is everything the ship can carry when loaded down to its load line marks: cargo and bunkers (fuel), fresh water, stores, spares, crew and any ballast still on board. This means deadweight is not cargo capacity, and on a long voyage with full fuel tanks the difference can be thousands of tonnes. What the ship actually carries is its deadweight cargo capacity, which changes with each voyage.
The deadweight in a circular is normally the summer deadweight, measured at the summer load line in salt water. Load lines change 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 its best case.
GRT and NRT are volumes, not weights
People misread this field more than any other. Gross tonnage measures the enclosed volume of the ship, worked out by a formula under the 1969 Tonnage Convention. Since that convention it is a pure index number with no unit, although the older label GRT is still used every day. Net tonnage is the same idea, but it counts only the spaces that earn money, the holds and tanks, with machinery and crew spaces taken out.
Neither figure tells you what the ship carries. What they decide is how much the ship pays. Port dues, pilots, tugs, light dues, canal tolls and tonnage tax are often charged on gross or net tonnage. A canal may also apply its own special measurement. Crewing and survey rules use gross tonnage limits too. Two ships with the same deadweight can have quite different gross tonnages, and so different port costs, and that difference shows up in the voyage calculation, not in the cargo quantity.
A ship shown at 20,000 gross does not carry 20,000 tonnes. Brokers who read it that way have offered the wrong hull to a charterer.
The dimension that excludes a ship is draught
LOA, length overall, is checked against the length of berths and locks. Many terminals publish a maximum length and enforce it strictly. Beam (width) is checked against locks and channel width and, at some berths, against how far a shore crane can reach across to the far side of the hatch.
Draught, how deep the ship sits in the water, is the figure that rules ships out, long before deadweight becomes the limit. The ship is kept out if it sits deeper than the water over the bar, in the channel, at the berth, or in the tidal window it is given. A load port with limited depth lets a smaller ship load fully but forces a larger one to load part of the cargo and finish loading somewhere else, and that changes the cost of the voyage. Deadweight tells a broker the upper limit; draught tells them whether the ship can get close to it at this port.
Air draught matters wherever something crosses above the water: bridges on a river, a seaway, an overhead conveyor. It is measured from the waterline to the ship’s highest fixed point, and it is greatest when the ship is empty, which is exactly how it arrives to load. A ship that passes under a bridge when leaving loaded may not pass under it when arriving empty.
Grain and bale: whether the cargo fits
Grain capacity is the total space inside the cargo holds that a loose, free-flowing cargo would fill, reaching out to the ship’s side between the frames and up inside the hatch openings. Bale capacity is the space for 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 difference is the space a sack cannot reach but loose barley can.
Compare this with the cargo’s stowage factor, the space one tonne of it takes up. Multiply the quantity by the stowage factor and compare the result with the ship’s cubic capacity. If the cargo needs more space than the ship has, the ship cubes out: it is full, but not down to its marks. If the cargo needs less space, the ship deadweights out: it is down to its marks with the holds partly 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. The ship is down to its 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 the ship has 67,000.
It cubes out near 47,800 t and never reaches its marks.
Two more things apply on top of that arithmetic. Broken stowage is the space lost because the cargo’s shape does not match the hold’s shape. It is small for grain and large for steel. A dense cargo, on the other hand, raises a question of strength, not space. Tank top strength is given in tonnes per square metre, and ore or concentrate loaded in a few tight piles can go over that limit even when the ship is far below its deadweight.
Gear decides the middle of the range
Gear means the ship’s own cranes, and for ships between Handysize and Supramax whether the ship has gear is the most important fact on the sheet. A geared ship can work at a berth with no shore cranes, at anchor into barges, and at a terminal whose crane is broken. A gearless ship of twice the deadweight cannot go to any of them, whatever its other particulars say. That is why gear is given in the circular line itself: in “4 x 30 t cranes”, the gear matters more than the deadweight next to it.
Read the rest of the gear line just as carefully:
- Crane count and SWL. The safe working load limits the weight of one piece the ship can lift, so one heavy piece can be too much, even when the total tonnage is small. Ask whether cranes can work in pairs for a heavier lift, and whether grabs are fitted.
- Holds and hatches. The number sets how many work teams can work at once. That sets the loading and discharge rate, and the laytime clause is based on that rate.
- Hatch size and shape. Box-shaped holds and wide hatches decide whether steel and large units can be loaded at all, or must be moved into the sides of the hold by hand.
- Fittings and history. Log fittings, grain fittings, CO2, hold coatings, last three cargoes. Cleanliness decides whether the ship is acceptable for a food grade cargo, before anyone discusses a rate.
What a fleet register actually holds
No database holds all of this, so here is what one register does hold. Across the 101,900 hulls in the Neptune Atlas register, every one with an IMO number:
| Field | Hulls carrying it |
|---|---|
| Flag | 101,608 |
| Year built | 96,174 |
| Gross tonnage | 88,634 |
| Deadweight | 75,596 |
To see what is on file for one ship, the free IMO number lookup shows its name, type, flag, tonnage and year built.
Hulls with an IMO number and no deadweight at all. More of the register has a gross tonnage than a deadweight, which is the opposite of what a broker expects. Tonnage is certified and stays with the registry entry, while deadweight comes from commercial sources that never covered small ships.
Below that, coverage drops quickly: cubic capacities, crane count and SWL, hold and hatch numbers and last three cargoes are hardly present in bulk vessel data from anyone. They come from the owner’s particulars sheet or a dry cargo questionnaire, and that is still how the trade works: a register narrows the choice, and the owner’s sheet gives the final answer.
Every field is a constraint
| Figure | The question it answers |
|---|---|
| Summer deadweight | How much the ship carries at best, before fuel and stores are taken off |
| Grain and bale capacity | Whether this cargo fits inside the ship at its stowage factor |
| Draught | Whether the ship can reach this berth at all |
| LOA and beam | Whether the berth, lock and channel will take the ship |
| Air draught | Whether the ship passes under what crosses the channel |
| GRT and NRT | What the ship costs in dues, pilots and tolls |
| Cranes, holds, hatches | Whether the ship can handle the cargo, and how fast |
Read a particulars sheet as a list of the reasons the ship may not be able to carry this cargo at this berth. The quickest way through it is to check the limits in the order they apply: draught first, then gear, then cubic capacity, 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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Neptune Atlas
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