Slow steaming is operating a ship's propulsion engine well below its designed maximum continuous rating (MCR), typically at 40 to 60 percent of MCR (some operators at even lower), to reduce fuel consumption, fuel cost and emissions (SOx, CO2, NOx). Because fuel consumption varies approximately as the cube of speed (P ∝ V^3 for resistance and hence fuel roughly ∝ V^3), a modest reduction in speed produces a disproportionately large reduction in fuel. It is achieved without engine modification simply by limiting the fuel injection per cycle (reducing the fuel pump index / governor speed setting / electronic load limit), i.e. de-tuning or derating the engine by running at reduced speed and load, and by selecting the appropriate propeller pitch (for fixed pitch propeller, simply the engine speed is set low; for CPP, the pitch is adjusted). The engine is operated on a lower percentage of MCR by controlling the governor and the load, without altering the engine physically.

Operational issues with slow steaming:

1. Cold corrosion: at low load the cylinder liner wall and combustion chamber temperatures fall below the dew point of the sulphuric acid formed from fuel sulphur/combustion, so acid condenses on the liner causing corrosion wear of the liner, rings, and could promote bore polishing.
2. Poor combustion: low load, low charge air pressure from the turbocharger (which runs in the low-efficiency region), giving a rich air/fuel ratio, poor atomization, smoke, carbon and soot formation, fouling of the turbocharger air side and exhaust turbine.
3. Over-lubrication: the cylinder oil feed rate based on MCR may over-lubricate at low load, causing excess oil in the scavenge space, carbon deposits on ring grooves and piston crown, stuck rings, and increased risk of a scavenge fire.
4. Turbocharger surging: the single turbocharger may come close to its surge line at low load; inadequate scavenge pressure can lead to pulsation and surging, reducing charge air and worsening combustion.
5. Exhaust gas temperature too low: the low exhaust temperature makes the waste heat boiler/economiser inefficient and can cause acid/soot deposition and corrosion in the boiler, and on dual layer it can lead to boiler upkeep problems.
6. Deposits/carbon in exhaust valves, fuel injectors and turbocharger blades, requiring more frequent cleaning.
7. Watchkeeping/fuel management: more careful control, and coking up of injectors.

How these issues are dealt with:

1. Cylinder lubrication: use two-level/electronic lubrication with a reduced low-load feed rate matched to the load and fuel sulphur; keep the BN of the oil appropriate; avoid over-lubrication.
2. Keep liner temperature up by raising the jacket cooling water temperature and insulating the scavenge space; maintain adequate cooling water temperature control.
3. Prevent cold corrosion by maintaining the wall temperature above dew point, and possibly by the use of appropriate additive/cylinder oil and by periodic higher-load running to burn off deposits and reheat the liner.
4. Manage turbocharger: keep it in its efficient/safe speed region; use two turbochargers/turbocharger cut-out on multi-TC engines, or clean the air side; avoid running for excessive time at very low load; adjust scavenge pressure; some engines use a Variable Turbine Area or waste gate.
5. Operate boilers correctly with soot blowing, monitor economiser temperatures/pressure, and run the boiler as per plan.
6. Periodic operation at higher load (e.g. weekly) to burn off carbon deposits and recondition the liners.
7. Careful fuel quality/temperature management to give good atomization at low load, i.e. correct viscosity at injector.

These measures keep the engine reliable at slow-steaming load while capturing the fuel savings.