In a recent bulletin released by EBI – European Boating Industry, the European association representing the recreational boating and nautical tourism industry at EU institutional level – one of the news items concerns ammonia and its possible use as a marine fuel. In recent years, anhydrous ammonia (NH₃, a molecule composed of one nitrogen atom and three hydrogen atoms) has entered the debate on alternative fuels for marine propulsion. We therefore sought to examine the topic more closely.
Interest stems from a distinctive characteristic of this molecule: it contains no carbon and therefore, when used as a fuel, does not generate carbon dioxide. This potentially aligns it with the ambitious decarbonisation targets set for international shipping.
However, to properly understand the issue, certain technical concepts need clarification.
When assessing the environmental impact of a fuel, two key metrics are used. The first is “tank-to-wake”, measuring emissions generated during onboard fuel use. Since ammonia contains no carbon, its CO₂ emissions at this stage are zero. On paper, this appears ideal.
The second metric is “well-to-wake”, which considers the entire supply chain, from production to final use. This is where complexity arises. If ammonia is produced through conventional natural gas-based processes, the overall CO₂ balance remains significant. If instead it is produced using hydrogen from renewable sources – so-called green ammonia – the overall emissions profile is drastically reduced.
This distinction is not merely technical: it separates a genuinely sustainable solution from a mere displacement of emissions upstream.
Technology for ammonia use already exists, and suitable engines and fuel cells are under development. Major marine engine manufacturers are pursuing three main pathways: direct ammonia-fuelled internal combustion engines; dual-fuel engines capable of operating on diesel and ammonia, using a pilot fuel for ignition; and electric conversion systems with fuel cells powered directly or indirectly by ammonia.
Full-load tests on high-power engines have already been announced. Initial commercial installations are targeting cargo ships and offshore units. The technology is therefore at an advanced stage and moving beyond laboratory development.
From a logistical standpoint, ammonia offers a key advantage over hydrogen: it is already produced and transported on an industrial scale and can be stored in liquid form at less extreme pressures and temperatures. This relatively simplifies onboard tank management.
However, limitations are substantial. Ammonia is highly toxic to humans, poses environmental risks in case of accidental release, is corrosive to certain materials and burns more slowly than conventional fuels, often requiring pilot fuel assistance. Combustion also produces nitrogen oxides (NOx), potentially nitrous oxide (N₂O), and the phenomenon known as “ammonia slip” – traces of NH₃ escaping in the exhaust.
As a result, ventilation, leak detection, containment and exhaust after-treatment systems must be significantly more complex than those used for traditional fuels.
The use of ammonia in yachting is under study, but must be approached realistically, distinguishing by size class and operational profile. Large yachts may represent the only truly compatible segment. If realistic first applications emerge, they will likely involve yachts above 70–80 metres, with substantial technical volumes, advanced diesel-electric or hybrid architecture, and ocean-going or expedition profiles.
The reasons are primarily technical. Dedicated safety systems require space: ammonia demands segregated compartments, double containment barriers, forced ventilation, sensors and protected technical zones. Only large-displacement yachts can accommodate this without compromising layout and comfort.
Secondly, highly trained technical crews and complex bunkering procedures are required, closer to merchant shipping standards than traditional leisure boating. Diesel-electric platforms also facilitate integration of alternative fuels through dedicated generators or fuel cell modules.
Operational coherence is another factor. Explorer and expedition yachts, designed for long-range navigation and high autonomy, are more compatible with alternative fuels featuring lower volumetric energy density than diesel.
Within the yacht sector, additional constraints apply compared with commercial shipping. Guest safety is paramount: the presence of non-professional passengers raises safety thresholds beyond those typical for cargo vessels. Insurance criteria are yet to be defined.
Infrastructure represents another major limitation. Ammonia bunkering facilities are currently non-existent in marinas. Its characteristic odour and the management of even minor releases are problematic in tourist environments. Moreover, tanks and safety systems significantly reduce usable onboard space.
For these reasons, several industry studies indicate ammonia is more suitable for deep-sea commercial vessels than for passenger or leisure units.
For yachts below 50–60 metres, ammonia currently appears largely impractical. Volumes are insufficient, system complexity excessive, bunkering networks absent, and the risk-benefit ratio unfavourable.
In these size ranges, more feasible medium-term solutions include HVO and drop-in biofuels, methanol, diesel-electric hybrid systems, limited-scale hydrogen applications or batteries for short-range navigation.