Answer
The load diagram is plotted in percentage (rather than absolute kW or BHP) to provide a normalized, universal standard across engines of completely different sizes and power outputs.
Proportional Stress: 100% load always represents the Maximum Continuous Rating (MCR). Whether you are on a handysize bulker with a 6,000 kW engine or a VLCC with a 30,000 kW engine, running at 85% MCR represents the exact same proportional thermal and mechanical stress on the engine components.
Defining Operational Limits: The boundaries of safe operation—such as the torque limit, the thermal limit, and the continuous service rating (CSR)—are defined relative to the MCR.
Propeller Matching: It simplifies plotting the propeller curve (the cubic law). It allows engineers to easily visualize slip, hull fouling, and sea-margin without having to calculate absolute torque figures, making it easier to instantly identify if the engine is experiencing a heavy running condition.
Load diagram and load diagram for variable pitch propeller
Fixed Pitch Propeller (FPP) Load Diagram
For an FPP, the engine speed and propeller pitch are physically fixed. The engine load diagram defines the safe operating envelope bordered by:
Torque Limit (Left Boundary): Prevents heavy thermal loading and mechanical stress at low RPMs (surge limit of turbocharger).
Speed Limit (Right Boundary): Prevents mechanical overspeed.
Overload Limit (Top Boundary): The absolute maximum continuous rating (110% MCR).
Nominal Propeller Curve: The ideal operating line, governed by the propeller law: Power=Constant×RPM3. The engine must stay on or slightly to the left of this curve (light running margin) to account for hull fouling over time.
Controllable Pitch Propeller (CPP) Load Diagram
A CPP can alter its pitch angle, breaking the rigid Power=c×RPM3 relationship.
Instead of a single propeller curve, a CPP has a family of curves, one for each pitch angle.
Combinator Curve: The control system uses a combinator curve to automatically match the optimum engine RPM with the most efficient propeller pitch to minimize fuel consumption and thermal stress.
Constant RPM Operation: A CPP diagram often shows a vertical operating line where the engine runs at a constant RPM (typically to drive a Shaft Generator for heavy electrical loads), and ship speed is dictated purely by altering the propeller pitch.
Key Takeaways from the Diagram:
Fixed Pitch Propeller (FPP): Operates on a single, set propeller curve (following the propeller law, where Power is proportional to RPM cubed). The available power is restricted by how it intersects with the engine's torque limit line, potentially underutilizing the engine in heavy weather or loaded conditions.
Variable Pitch Propeller (CPP): The pitch is variable (shown as a family of curves, 0% to 100%). This flexibility allows the engine to deliver full power across a wide speed range by adjusting the pitch rather than changing RPM. It highlights two critical operational modes:
Combinator Curve (Green): Automatically and simultaneously optimizes both RPM and pitch for maximum efficiency across all speed ranges.
Constant RPM Operation (Vertical Bar): Keeps the engine at a fixed RPM (ideal for shaft generators) and varies ship speed purely through pitch control.