Answer
ShaPoLi (Shaft Power Limitation) and EPL (Engine Power Limitation) are overridable technical measures approved under IMO Resolution MEPC.335(76) to limit a vessel's power output and ensure compliance with the Energy Efficiency Existing Ship Index (EEXI).
The key distinction lies in where and how the power is measured and controlled.
Core Structural & Operational Differences| Feature | Engine Power Limitation (EPL) | Shaft Power Limitation (ShaPoLi) |
| Control Parameter | Direct fuel delivery to the main engine. | Actual mechanical power transmitted through the intermediate shaft. |
| Working Mechanism | Mechanical: Sealed mechanical stopper on the fuel rack/governor. Electronic: Password-protected software fuel index limiter in the ECS. | Optical/strain-gauge torsion meters measure shaft torque + RPM in real time to compute Shaft Power ($P_{Shaft} = 2\pi \cdot N \cdot Q$). |
| System Complexity | Low. Primarily mechanical or software-based. | High. Requires optical/strain sensors, processing units, bridge display, and data logging devices. |
| Shaft Generator Impact | Disadvantageous: If a Shaft Generator/PTO is engaged, the fuel index limit applies to the total engine load, leaving less power available for propulsion. | Advantageous: Measures power delivered only to the propeller. Engine can produce extra power to drive a Shaft Generator without tripping the limit. |
| Override Mechanism | Breaking a physical wire seal on the mechanical stopper or overriding the soft limiter via password/switch. | Software/bridge override switch that bypasses automated engine load control. |
| CapEx & Installation | Very low cost; quick installation (often achievable by ship's crew under manufacturer guidance). | Higher cost due to precision shaft instrumentation and data logger installation. |
For vessels with a shaft generator, ShaPoLi is often preferred. Because EPL limits total fuel input at the fuel pump, engaging a shaft generator reduces the power left for the propeller. ShaPoLi measures torque past the PTO coupling, restricting only the actual power going into the propeller.