As shipping faces growing pressure to cut greenhouse gas emissions, new doctoral research from Tallinn University of Technology shows that the cleanest solution for small vessels is not one-size-fits-all. Instead, the best decarbonization pathway depends on route length, electricity supply, port infrastructure, vessel duty cycle, and investment costs. The thesis focuses on vessels below 5,000 gross tonnage (GT), a segment that includes ferries, pilot boats, and regional service vessels that have received far less attention than large ocean-going vessels.
Riina Otsason’s doctoral thesis, Environmental and Techno-Economic Assessment of Decarbonization Pathways for Ships Below 5,000 GT , combines life-cycle assessment, techno-economic modelling, and scenario analysis across four case studies in Estonia and beyond. The studies compare electric and diesel ferries, alternative fuels for pilot vessels, fleet-level decarbonization scenarios for regional ferry lines, and a ground-effect vehicle concept for inter-island transport. Together, they show how environmental performance, economic feasibility, and operational constraints interact in small-vessel decarbonization.
The findings suggest that meaningful emission reductions are technically possible, but the best option depends strongly on context. In one case study, a battery-electric ferry produced about 75% lower greenhouse gas emissions than its diesel sister vessel under the prevailing electricity mix. In another, biomethane delivered the highest emissions reduction potential for a pilot fleet, at roughly 59% compared with marine diesel oil. Across the full thesis, electrification performed best on short, predictable routes with low-carbon electricity and adequate charging infrastructure, while bio-based fuels and hybrid systems offered more practical transitional solutions in more variable operating environments.
The research also shows that technical feasibility does not automatically translate into financial viability. Battery-electric propulsion can reduce operating costs over time, but it typically requires higher upfront capital expenditure than diesel alternatives. In the ferry case study, the estimated payback period was about 17 years without subsidies, improving to around 10 years when financial support mechanisms were included. For small fleets operating with narrow margins, that makes investment risk a decisive factor.
According to the thesis, infrastructure readiness is just as important as vessel technology. Small vessels are especially sensitive to trade-offs between energy density, storage capacity, weight, range, safety requirements, and port infrastructure. Route length, charging availability, fuel supply chains, and grid capacity all shape what can realistically be deployed. The conclusion is clear: decarbonizing smaller vessels is not simply a matter of replacing one engine with another, but of matching the right technology to the right route and the right supporting infrastructure.
The work is particularly relevant for regions with islands, ferries, and coastal services, where maritime transport is part of everyday mobility and local air quality. By focusing on a segment that has been comparatively underexamined in maritime climate research, the thesis offers evidence that could help ship operators, port planners, and policymakers identify practical transition pathways for short-sea and regional transport.
Otsason’s thesis was defended at the Estonian Maritime Academy, Tallinn University of Technology, on 11 June 2026. The work was supervised by Prof. Ulla Pirita Tapaninen, with opponents Magnus Gustafsson of Åbo Akademi University and Prof. Tae Eun Kim of UiT The Arctic University of Norway.
About the thesis
The dissertation integrates four peer-reviewed publications and evaluates alternative propulsion systems and fuel pathways in small-vessel contexts. It argues that small-vessel decarbonization is achievable, but only when environmental goals are aligned with vessel characteristics, route conditions, and infrastructure readiness.
Computational simulation/modeling
Not applicable