This operation manual covers the Yanmar YM Series marine diesel engines, specifically models 2YM15, 3YM20, and 3YM30. These vertical, water-cooled, 4-cycle diesel engines are engineered for reliability and performance, featuring a freshwater cooling system with a heat exchanger. The 2YM15 offers 14.2 PS, the 3YM20 provides 20.0 PS, and the 3YM30 delivers 30.0 PS, all at 3600 rpm. With a bore and stroke of 70 mm x 75 mm and displacements ranging from 1.155 L to 1.267 L, these engines are designed for demanding marine applications. This comprehensive English manual is designed to assist owners, mechanics, and technicians in understanding and maintaining their Yanmar YM Series engines. It details essential information including operation, maintenance schedules, and technical specifications. Key maintenance intervals are outlined, with initial service due at 50 hours or 1 month, periodic oil service every 50 hours or monthly, and major periodic service at 1000 hours or 4 years. The manual also includes a section on EPA requirements, ensuring compliance and proper operation.