LPG: The Quiet Option
LPG never makes the headlines methanol and ammonia do, yet it holds a steady third place in the alternative-fuel orderbook, 139 ships in the January 2026 count from lesson 1, and the ships that choose it choose it for the most robust reason in shipping: they are carrying the fuel anyway.
What the fuel is
Liquefied petroleum gas is propane and butane, alone or blended, by-products of natural-gas processing and oil refining. The storage story is where LPG quietly beats the glamour fuels. Propane liquefies at about −42 °C at atmospheric pressure, or under roughly eight to nine bar at ambient temperature; butane is milder still. So an owner chooses between moderate pressure, moderate refrigeration, or a semi-refrigerated blend of the two, all routine engineering with ordinary low-temperature steels. Compare the axes from lesson 2: where LNG demands −162 °C cryogenics and hydrogen −253 °C, LPG asks for a pressure vessel or a fridge.
The energy content is generous, around 46 MJ/kg, comfortably above fuel oil, and combustion is clean of sulphur and low in particulates. The CO2 saving at the funnel is real but modest, in the mid-teens of percent depending on the propane/butane split and the engine. One safety particular deserves respect: LPG vapour is heavier than air. A leak does not rise and disperse like methane; it pools in bilges and low spaces, which is why gas detection and ventilation layouts for LPG ships are designed from the bottom up. The Indian Register's guidelines on LPG-fuelled vessels (open) show how class translates that hazard picture into arrangement and system requirements.
Why the gas carriers moved first
Almost the entire LPG-fuelled fleet is LPG carriers, VLGCs above all, and the logic is worth understanding because it generalises. A gas carrier burning her own cargo has no availability problem: the fuel is on board by definition, in every port her trade touches. Her crew already hold gas competencies. Her systems already handle the product. The conversion from "carries LPG" to "burns LPG" is the shortest technical and organisational distance in the whole fuel transition, and two-stroke LPG engines built for exactly this trade have accumulated solid service experience. Lloyd's Register's Fuel for thought LPG report (free after registration) tracks the adoption drivers and the supply picture in detail.
Outside the gas trades, the case thins quickly, not on chemistry but on the framework's availability axis. A bunkering network for ships that do not carry LPG as cargo barely exists, and building one competes with LNG's head start and methanol's simplicity. That is the honest reading of 139 ships against LNG's 1,259: a durable niche, not a general answer.
The renewable path
LPG's long-term climate case cannot rest on a mid-teens CO2 saving; like LNG it depends on a renewable version of itself. Here LPG holds an underrated card: renewable LPG already exists as a by-product of renewable diesel (HVO) production, hydrotreating bio-feedstocks yields a propane-rich stream, with renewable dimethyl ether (rDME) as a blendable companion. The supply chain therefore grows with the renewable-diesel industry rather than waiting to be invented, though today's volumes are small and the aviation and heating sectors compete for them.
One forward-looking connection makes LPG more interesting than its niche suggests: its storage conditions sit close to ammonia's. Propane liquefies at −42 °C or under moderate pressure; ammonia at −33 °C or similar pressures. Tanks, materials and handling philosophy overlap, which is why LPG-fuelled ships and LPG-trained crews are widely seen as a head start on the ammonia future of lesson 6: a quiet option that keeps a louder one open.
On the framework, then: energy density strong by mass and reasonable by volume; storage the mildest of any gas; tank-to-wake CO2 modestly better than fuel oil, with no methane slip to erode it; engines proven in the niche; availability excellent inside the gas trades and thin outside; renewable pathway real but small. LPG is the transition's quiet professional: unlikely to conquer the deep-sea fleet, very likely to keep its own house in order while louder fuels argue.