A conceptual design of main components sizing for series-parallel Plug-in Hybrid Electric Recreational Boats (PHERB) powertrain is introduced in this paper. The PHERB powertrain has only one electric machine to function as either an electric generator or motor in different time intervals specified by a special developed energy storage strategy that control the power flow according to desired operating mode. The PHERB uses a hybrid energy storage system combining batteries and ultra-capacitors, which can work together effectively to improve drive performance and energy efficiency. Subsequently the size of the internal combustion engine can be reduced since it is operated during certain operation mode only. The PHERB powertrain component sizing begins with calculation of boat energy and power requirements for typical driving conditions according to the boat power parameters, specifications and performance requirements. The size and capacity of the main components are determined through a power flow analysis so as to fulfill the PHERB powertrain design specifications and requirements. After that, the parameters and specifications for each component that make up the overall structure of the PHERB powertrain are defined based on the developed Kuala Terengganu river drive cycle. The results obtained from this analysis are within reasonable range and satisfactory.