This manual covers the Yanmar 2YM15, 2YM15C, 3YM20, 3YM20C, 3YM30, and 3YM30C marine diesel engines. These engines are engineered for reliability and performance, featuring direct injection and raw water cooling systems. The 2YM15 offers 14.0 PS from its 0.763 L displacement, while the 3YM20 provides 20.0 PS from 1.115 L, and the 3YM30 delivers 27.0 PS from 1.267 L. All models utilize a marine gearbox with forward, neutral, and reverse gears, designed for efficient propulsion in various marine applications. This comprehensive guide provides essential information for the operation, maintenance, and servicing of your Yanmar engines. It includes detailed procedures for routine maintenance, such as oil and filter changes, along with troubleshooting guidance and technical specifications. Understanding the information within this manual will assist owners and technicians in ensuring the longevity, optimal performance, and safe operation of these robust marine diesel engines.