INEC 24’s core theme is ‘lean, mean and green’ navies, encompassing reduced carbon emissions through new fuels. Literature abounds with shallow claims that navies need comply with IMO edicts or otherwise demonstrate ‘net zero’ performance. This is misleading, as legitimate as claiming that navies need do nothing. Navies adopting new fuels need navigate a sensible course, and avoid replicating the naïve, largely unconditional adoption of MARPOL regulations in the 1990s, a legacy still saddling many navies with unnecessary regulatory burdens offering minimal environment protection while emasculating capability. Selection of future fuels will necessarily be based upon engineering evaluations, but reliance upon technical assessments in isolation would be inadequate. Effective, focused and technically literate naval engineering policy must always form the bedrock underpinning naval engineering deliberations, as it must for new fuels. Warships, and the navies that operate them, exist for one fundamental reason – to fight and win at sea. There is no virtue in being ‘cleaner and greener’ if the fight is lost. No government will thank their navy for losing a ship and crew while doing so with lower carbon intensity than the adversary. This is an inviolable, and for some perhaps inconvenient, truth. Amid societal expectations and the pursuit of ‘net zero’, in concert with the anticipated decline in diesel availability, navies must, in time, seek alternatives, preferably where there is parallel operational advantage. Fuels suitable for commercial use can be inappropriate for military applications. Whether naval transition is undertaken in an objective, informed manner, or in a less orderly and dysfunctional way in response to external forces may determine the operational effectiveness of navies and their logistical overheads for decades to come. Navies are now at or approaching that threshold of decision, as warships currently in design are those that will be built into the 2040s and 2050s. Powertrains and energy sources are clearly a critical consideration for warship design. In the absence of any evident replacement for diesel, most navies are compelled to reply upon current fuels for new designs, or else significantly modify or prematurely decommission extant and nascent ship classes. Technical assessments need to be bounded within objective engineering policy frameworks, in-turn linked to capability intents. Any ‘opportunities’ for alternative fuels adoption that may present as technically feasible must be rigorously evaluated in terms of the unique and more exacting naval requirements. ‘Net zero’ does not mean ‘zero’, and the span of the ‘net’ need not be limited to fleet units. This gives scope, opportunity and licence to develop innovative means of reducing carbon emissions while maintaining a superior naval capability. This paper explores how effective navy engineering policy can improve carbon efficiencies while safeguarding naval capability. Any quest for ‘net zero’ must be about what a navy needs to do, not what it could. Engineering policy must be in the vanguard of navies’ pursuits of alternative fuels, so that any aspired ‘green’ future does not render a navy as an ‘inconsequential and vanilla’ outfit rather than a ‘formidable and green’ maritime force.