How to Compare Fuels Without Fooling Yourself
Every fuel in this course will be sold to you at some point, by an engine maker, a consultant, a headline. The defence is a fixed set of questions asked in the same order every time. This lesson builds that framework; the next nine lessons apply it.
Energy: per tonne, and per cubic metre of tank
Start with the physics, because it is non-negotiable. Fuel oil carries roughly 40 MJ/kg. LNG does better, around 50. Methanol holds about 20, half of fuel oil, and ammonia about 18.6. Hydrogen is the outlier in both directions: roughly 120 MJ/kg, the best of any chemical fuel by mass, and among the worst by volume.
Volume is where ship design feels the pain. A fuel with half the energy per tonne needs twice the tonnage bunkered; but the tank space penalty is always worse than the mass ratio, because low-flashpoint and cryogenic fuels cannot sit in the hull's odd corners the way fuel oil does. They need cylindrical or prismatic tanks, insulation, cofferdams and ventilated surrounds. As a rule of thumb per unit of energy stored, LNG installations consume roughly twice the volume of conventional arrangements, methanol around two and a half times, ammonia around three, and liquid hydrogen four or more. That is cargo space, and cargo space is revenue. The ABS advisory on gas and other low flashpoint fuels (open) tabulates these properties side by side and is a solid desk reference.
Emissions: well-to-wake or nothing
Tank-to-wake counts what leaves the funnel. Well-to-wake adds everything upstream: production, processing, transport. The distinction decides arguments. Ammonia and hydrogen emit no CO2 on board, but made from natural gas without carbon capture, their well-to-wake footprint can exceed the fuel oil they replaced. Conversely, a biofuel burns to CO2 in the cylinder yet scores well because the carbon was recently in the air. Regulation is converging on lifecycle accounting: FuelEU Maritime is explicitly well-to-wake, and the IMO framework follows the same logic (both are covered properly in Maritime Regulations, Explained). When someone quotes an emissions saving, your first question is always: measured from the well, or from the tank?
The other four axes
Cost, of the fuel per unit energy, of the installation, and of compliance avoided. A dearer fuel can be the cheaper choice once carbon pricing is counted, which is why cost comparisons dated before lifecycle regulation are obsolete.
Availability: the axis that most often kills a fuel for a specific trade. A global bunkering map is irrelevant if none of it sits on your rotation. Availability is also the chicken-and-egg of the transition: suppliers wait for demand, owners wait for supply.
Safety character, not "safe or unsafe" but which hazard: cryogenic temperature (LNG, hydrogen), toxicity (ammonia, methanol), low flashpoint (methanol, hydrogen), pressure (LPG). Every fuel here can be handled safely; the question is the cost and training burden of doing so.
Engine and regulatory readiness, whether type-approved engines, class rules and crew competence standards exist today, or must be pioneered at your expense. Bureau Veritas's Alternative Fuels Outlook (free after registration) scores the candidate fuels across exactly these maturity dimensions, and DNV maintains an open comparative overview of the fuel options that makes a useful second opinion.
Using the framework
Two disciplines make it work. First, ask all six questions every time: a fuel pitch is usually built on the two axes where the candidate shines and silence on the rest. Second, weight the axes for your ship: deep-sea tramping weights availability and volumetric density hard; a fixed-route ferry barely feels either. Try it on a claim you have already heard "ammonia is the zero-carbon fuel". True on tank-to-wake chemistry; now ask about well-to-wake production, tank volume, toxicity, bunkering ports and engine maturity, and the sentence acquires the qualifiers it always needed. The lessons that follow run each fuel through all six axes honestly: strengths, weaknesses, and the trades where each one is already the right answer.