The Energy Efficiency Design Index (EEDI) expresses the grams of CO2 emitted per tonne of cargo transported per nautical mile. Its level is directly proportional to the fuel consumed (and hence the propulsive power required) at the design speed, divided by the cargo capacity and speed. Anything that cuts the power needed to move a given deadweight at the design speed, or that raises propulsive efficiency, lowers the EEDI. The three methods below act on exactly those levers.

A fine hull with a low block coefficient (Cb) presents less displaced volume for a given length and thus a smaller wetted surface area, reducing the frictional resistance component at a given displacement. A fine (low Cb) forebody and afterbody also weaken the bow wave and the pressure (residual or wave-making) resistance at the design speed, because the water is pushed aside more gradually. Lower residual resistance means the installed propulsive power at the design speed is less, so less fuel is burnt per tonne-mile, lowering the EEDI. Being longer and narrower for the same displacement also raises waterline length, which reduces the length-related frictional and Froude-number-dependent resistance. The penalty is reduced cubic cargo capacity and somewhat less form stability, so the hull form is optimised rather than simply fined down. Lower block coefficient is therefore one of the strongest design levers for meeting EEDI limits.

A long-stroke (high stroke-to-bore ratio) slow-speed diesel extracts more work from each unit of fuel in the expansion stroke and achieves higher thermal (brake) efficiency, typically up to about 50 per cent, with a correspondingly lower specific fuel consumption per kWh. Because the stroke is long, the engine can turn slowly at the same piston speed, enabling direct coupling to a large slow-turning propeller with no reduction gearbox and no associated transmission losses. A more efficient engine burns less fuel for each kW it delivers, hence produces less CO2 per tonne-mile, which reduces the attained EEDI directly. Long-stroke engines also operate at low revolutions, which marries perfectly with the large-diameter, low-rev propeller of part (c).

Propeller open-water efficiency rises as the disc-area loading (thrust per unit swept area) falls. A large-diameter propeller turning slowly accelerates a large mass of water by a small amount, giving low disc loading and high efficiency for the same thrust and therefore less shaft power and less fuel per tonne of cargo. Large slow propellers also stay further away from cavitation, reducing blade erosion, vibration and noise. Because EEDI is fixed by the fuel (shaft power) needed at the design speed, optimising hull, engine and propeller together — a fine low-block hull at a low Froude number, a long-stroke low-speed engine, and a large-diameter low-rev propeller — minimises energy consumption per tonne of cargo and is the classic route to satisfying EEDI limitations.