đ 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
đ 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?
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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 â proportional to power output, not directly to RPM itself.
⢠Verify which exponent (1, 2, or 3) is used in the standard propeller powerâRPM relationship taught as the âpropeller lawâ.