For displacement boats, speed is usually the largest controllable factor. Power demand rises steeply as the hull approaches its practical displacement speed. In simplified propeller analysis, absorbed power is often treated as roughly proportional to the cube of shaft rpm within a comparable operating region. Real boats depart from the simple rule, but it explains why a modest speed reduction can save substantial fuel.
High-value efficiency work
- Keep hull and propeller clean.
- Ensure the engine can reach its specified loaded rpm.
- Select propeller and reduction ratio as a system.
- Maintain injectors, air path, cooling and exhaust.
- Use accurate rpm, speed and fuel measurements.
- Reduce unnecessary weight and windage.
- Plan with current, tide and sea state.
- Avoid charging or hotel loads that force inefficient engine operation when a better energy source is available.
Do not chase efficiency by running so slowly that the engine remains cold or lightly loaded for its whole life. Compare litres per nautical mile, not only litres per hour.
Sea-trial data should include reciprocal runs to reduce wind and current error. Log rpm, GPS speed, fuel flow, coolant temperature, exhaust appearance, trim and displacement. Repeat after antifouling, propeller changes or engine work.