🔍 Key Concepts

• Typical material properties needed for main propulsion shafts (strength, toughness, fatigue resistance) • Difference between forged and cast metals in terms of grain structure and strength • Why lightweight metals like aluminum are or are not suitable for high‑load rotating machinery

💭 Think About

• Think about the types of loads and stresses a main reduction gear shaft experiences during operation. Which material handles those best? • Consider which materials are commonly used for highly stressed rotating machinery in marine engines, such as crankshafts and propulsion shafts. • Which choice would provide both high strength and reliability under shock loads, rather than just light weight or corrosion resistance?

✅ Before You Answer

• Verify which materials are typically used for high‑strength rotating shafts in marine propulsion, not just for propellers or small fittings. • Check the difference in mechanical properties between forged and cast forms of the same metal, especially regarding fatigue and impact strength. • Eliminate any materials that would be too weak, too brittle, or primarily chosen only for weight savings rather than strength in a main reduction gear shaft.