Nowadays the interest for environmental issues is steadily gaining ground. Inescapably, the shipping industry, is also called to contribute its fair share towards the reduction of air pollution. Harbour tugboats need to comply both with international but also with local exhaust air emission regulations, which are anticipated to become more stringent in the future. Kotug, is interested to proactively explore suitable powering solutions, which can be applied at its Rotortugs’ newbuilding scheme, to comply with upcoming regulations of the intended port of operation. The goal of this thesis is to develop a decision-support tool to facilitate the selection process between alternative prime mover combinations, based on what is needed in terms of operation while considering technical challenges, environmental performance and economic returns. The effect of a variety of promising alternative fuels matched with suitable prime movers was investigated for the period 2018-2033, which was set as the study’s time horizon. LNG, Methanol, Biodiesel and DME were assessed as feasible solutions. The first two were associated with gas-burning engines, whereas the rest were associated with conventional 4-stroke compression ignition engines, similar to the ones already installed in the existing Rotortugs running on MGO. The environmental performance differs substantially to a conventional prime mover running on MGO. However, to adhere to future stricter regulations in certain sea areas, additional after-treatment systems are necessary. Research indicated three post-treatment solutions are the most effective in limiting the majority of the combustion-related emissions; Selective catalytic reduction, diesel particulate filter and oxidation catalysts. The total environmental fingerprint of a vessel can be however assessed only under the context of its propulsion configuration layout. The operational profile of a tugboat is highly variable, allowing room for the exploitation of different drivetrains than the traditional diesel-direct layout, which could lead in benefits mainly in terms of fuel efficiency and maintenance savings. Three drivetrains were selected; a diesel-electric and two hybrid, based on AC and DC topology. Before deciding to invest on a future boat a ship-owner is expected to be eager to comprehend the cost-related issues between alternatives. Moreover, the already proven design of the diesel-direct Rotortug, complemented by the necessary after-treatment technologies, makes sense to be the first option to consider, provided it complies with the anticipated regulations. This option is considered the baseline case. It is reasonable to compare all other options against the baseline case. To this end, a decision framework based on a techno-economic evaluation is proposed for supporting ship-owners to take informative decisions on investment considerations; Three objectives were set; economic performance, environmental performance and cost-effectiveness. To quantify economic performance a comparative TCO analysis was implemented, while for environmental performance the emission output of alternatives is compared. Last, cost-effectiveness adds valuable insight into how good money are allocated towards emission reduction targets and provides a methodology to compete for funding from a regulatory authority. Two case studies are presented that demonstrate the utility of these methods and enhance the understanding of the impact of decisive factors in decision-making. It is concluded that the proposed decision-support tool enables decision-support at an early-stage; nonetheless can still be improved, by incorporating additional factors and expanding certain modules with more details.