The shape of the ship
An LNG carrier is unusual-looking for a reason. The cargo is light, roughly 45 per cent the density of water, and it must be surrounded by insulation and a double hull. So the ship is voluminous rather than heavy: high-sided, with a large enclosed volume and a modest deadweight for its size.
Inside, four or five cargo tanks run the length of the cargo area. Between them are cofferdams, empty spaces that separate one tank from the next. Around them is the double hull. Above them, on a membrane ship, is a flat trunk deck with domes where the pumps and pipework come through; on a spherical ship, the tank tops break the deck line.
Aft sits the machinery space and accommodation. Forward is a conventional bow, though newer ships often have a reshaped one for fuel efficiency.
Cargo tanks
The tanks are the ship. Their type dictates the hull’s shape, the yard that can build it, and much of the cost. Membrane systems line the hull with a thin barrier; free-standing systems drop self-supporting tanks into the holds. The membrane versus Moss explainer covers the trade-offs in full.
What matters for the ship as a whole is that the tank is not refrigerated in service on a conventional carrier. There is no machine keeping the cargo cold. It stays cold because the insulation is good and because a small fraction is continuously boiling, taking its latent heat with it.
Pumps and cargo machinery
Getting the cargo out is not trivial. You cannot use a normal pump with a shaft passing through the tank wall, because that seal would be a leak path at minus 162 degrees.
The answer is the submerged motor pump: pump and electric motor built as one unit and lowered inside the tank, running immersed in the liquid. Nothing rotates through the tank boundary. Nikkiso and a small number of others build these, and the same design principle is used at terminals.
Around the pumps sits the rest of the cargo system: boil-off gas compressors that route vapour to the engines, heaters, vaporisers, the cargo control system, and on many ships a reliquefaction plant. Cryostar, Wärtsilä and Air Liquide are among the firms supplying that machinery, and the carriers hub lists them with sources.
Propulsion
Propulsion has changed more than anything else on these ships in the past two decades.
Steam turbines dominated for fifty years. They are inefficient by modern standards but have one decisive advantage: a boiler will burn anything, including cargo boil-off, without complaint. Many older vessels still afloat are steamships for this reason.
Dual-fuel diesel electric (DFDE) and tri-fuel (TFDE) plants use medium-speed engines driving generators, with electric propulsion motors. They are considerably more efficient than steam and can burn gas or oil.
Two-stroke dual-fuel engines are today’s standard. MAN’s ME-GI injects gas at high pressure on the diesel cycle; its ME-GA is a low-pressure alternative. WinGD’s X-DF is a low-pressure design that mixes gas with the scavenge air. All three burn boil-off directly and are markedly more efficient than steam, which is why a modern ship’s fuel bill and emissions look nothing like a 1990s vessel’s.
The choice interacts with the cargo. High-pressure designs need a compressor to raise gas pressure, which uses power; low-pressure designs avoid that but handle methane slip differently. Whether a ship reliquefies surplus boil-off or burns it is partly a consequence of which engine it has.
Sizes and classes
Cargo capacity is quoted in cubic metres of liquid, not tonnes.
Around 125,000 to 145,000 cubic metres describes older conventional tonnage. 174,000 is the modern standard, sized to fit the expanded Panama Canal locks and the majority of terminals. 210,000 to 217,000 is Q-Flex and 263,000 to 266,000 is Q-Max, both built for Qatari trades, with Q-Max sized to the biggest ship its loading terminal could take. At the other end, small-scale carriers and bunkering vessels range from a few thousand cubic metres up, often with Type C pressure tanks rather than membranes.
Our fleet directory lists every tracked vessel with its capacity, containment, propulsion, yard and delivery year, and groups sister ships built at the same yard in the same year to the same size. What it does not carry is position data: this site describes ships, not voyages.