🔍 Key Concepts
• Propeller law for displacement vessels (how power changes with RPM)
• Relationship between shaft RPM, delivered power, and fuel consumption for a diesel at constant slip
• Why small increases in RPM cause large changes in required power at the propeller
💭 Think About
• If shaft RPM increases by 10%, does the thrust and power demanded from the propeller go up a little, or a lot? Think about whether this change feels linear or much steeper in practice.
• For a diesel engine, fuel consumption is roughly proportional to the power output. So once you know how power varies with RPM, you also know how fuel varies with RPM.
• Compare what would happen to fuel use if you doubled the RPM: for each choice (directly, square, cube, inverse), imagine the numerical effect on fuel. Which best matches the known propeller law for ships?
✅ Before You Answer
• Be clear that we are talking about a displacement vessel at constant slip, not planning or highly variable slip conditions.
• Remember that fuel ≈ proportional to power output, not directly to RPM itself.
• Verify which exponent (1, 2, or 3) is used in the standard propeller power–RPM relationship taught as the ‘propeller law’.