Tier III is being implemented to regulate the nitrogen oxides (NOx) emitted from ships, and selective catalytic reduction

(SCR) systems are installed in ships to comply with this regulation. However, it is diffi cult to apply an SCR system to

small or remodeled ships, because the engine room has limited space. Moreover, an SCR system should be installed in the

consideration of the back-pressure of the engine exhaust gas, and the pressure should be less than the allowable back-pressure

(less than 150 mmH 2 O) downstream of the turbocharger. The purpose of this study was to identify the design parameters to

install a retrofi tted SCR system in the limited space inside the engine room. A performance test was performed to develop

the retrofi tted SCR system applicable to small and size ships. A CFD analysis was performed by varying the exhaust gas

velocity (6–12 m/s) and the porosity of honeycomb (60–90%) after a verifi cation of CFD analysis and experiment results,

obtained from an actual SCR system in a small-size ship. It was confi rmed that when the exhaust gas velocity decreased,

the back-pressure downstream of the turbocharger was reduced. In addition, the pressure variation was minimal when the

catalyst porosity was 70% or more at the constant 8 m/s gas velocity. The results of the current study can guide the design

of the retrofi tted SCR system in small-size ship that has limited engine room space.