Since the new “Regulations for the Prevention of Air Pollution from Ships” of the International Maritime Organization (IMO; MARPOL Annex VI Tier III) became effective, new technologies in marine applications are needed to fulfill the exhaust-gas limits. The reduction rate of the permissible emissions in the emission control areas (ECA) is about 75 % from Tier II to Tier III. To meet these limits, it is necessary to take additional measures, such as installing a Selective Catalytic Reduction (SCR) system.Because harbors are specifically in focus regarding the air quality, a hybrid propulsion system (Diesel-electric) and Exhaust Aftertreatment (EAT) to reduce the emissions and the lifecycle costs by reducing the fuel consumption were planned back in 2012. With the goal in mind of decreasing all relevant emissions, the described compact EAT consists of a Diesel Oxidation Catalyst (DOC), a Particulate Matter (PM) removal and a SCR-catalyst. It was especially designed to fit into the available narrow space in that tugboat and to meet the maximum allowed backpressure for this engine family. According to these requirements, the approach of a one box design was chosen with U-turns in coordination with the catalyst shape and placement, in order for the exhaust gas flow has to be optimized, e.g. integrated hydrolysis catalyst incl. effective simple mixing device. Special safety features like a self-regulated bypass system and a spark arrestor are also integrated.Since the team had a very short lead time to develop the product, several simulation tools were used. Type and dimensions of the catalysts (e.g. metal substrates) as well as the necessary high uniformity of the exhaust flow and the urea/ammonia were calculated before producing the first prototype which was going directly in use. With these tools the time from the first idea & concept to a EAT system was less than one year. During that short realization phase in 2012 to 2013, the necessary IMO III Emissions Certification ran on an engine test bed with the intent to conform to the requirements of the Lloyds Register (service organization for improving safety, quality and performance) and SOLAS (International Convention for the Safety of Life at Sea). Also included was the application on board into the Tugboat and at last but not least the first sea test trials. Currently two similar types of boats are successfully running in harbour areas in the Netherlands.This article shows the product development process from prototype directly into series. The good NOx- and PM-reduction of this very compact, special and fully closed loop EAT system are also shown.