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Engine Derating for Slow Steaming

Slow steaming is the operational practice of running a ship at substantially less than its design speed to reduce fuel consumption. Engine derating is the related engineering practice of reducing an engine’s SMCR below its layout point to optimise it for sustained low-load operation. The two practices arose together during the 2008-2014 freight market downturn and have remained common since. Derating involves propeller re-matching, turbocharger cut-out for two-turbocharger engines, and tuning of injection timing and exhaust valve timing to recover SFOC at the lower load point. This article covers derating procedures, operational implications, and the engineering trade-offs. Visit the home page or browse the calculator catalogue for related propulsion engineering tools.

Contents

Background

Through most of the twentieth century, commercial ships were designed to operate at or near their service speed continuously, with engines rated to deliver that speed at moderate margin. Fuel was inexpensive enough that the engineering optimum favoured high specific power output and acceptable SFOC at the design point, with relatively poor efficiency at lower loads.

The 2008 financial crisis disrupted this norm. Container shipping faced sudden overcapacity; freight rates collapsed; ship operators searched for any cost reduction. The fact that fuel consumption scales approximately with the cube of speed (for hull resistance) meant that reducing service speed by 20 percent could cut fuel consumption by 50 percent. Operators began running ships at 16-18 knots rather than the design 22-25 knots, accepting longer voyage times in exchange for fuel cost reduction.

The resulting slow steaming practice produced unforeseen engine problems. Engines designed for 80 percent MCR continuous operation were now running at 30-40 percent MCR. At these loads, scavenge air supply was inadequate, combustion temperatures were too low, and acid condensation produced cold corrosion. Engine manufacturers responded with derating packages that re-tuned engines for sustained low-load operation, including:

  • Reduced SMCR (the formal rated power)
  • Propeller re-matching for the new operating profile
  • Turbocharger cut-out (running one of two turbochargers, leaving the other inactive at low load)
  • Injection and exhaust valve timing maps optimised for the new operating range
  • Cylinder cut-out for super-slow-steaming (running fewer cylinders to maintain higher per-cylinder load)

Slow steaming became permanent for much of the world container fleet, and derating became a routine engineering option for both new builds and retrofits.

This article covers the procedures, the engine modifications, the operational benefits, and the risks associated with derating for sustained low-load operation.

Derating principles

What changes

Derating shifts the engine’s nominal operating point to a lower power and (typically) lower speed. The original layout diagram MCR point may be displaced by:

  • 20 percent reduction in SMCR: a typical derating margin
  • 5 to 15 percent reduction in rated rpm: matching the lower-speed propeller
  • 15 to 25 percent reduction in BMEP at the new SMCR: lower combustion intensity

The engine remains capable of higher power; derating is a re-rating, not a hard limit.

What stays the same

The physical engine (bore, stroke, cylinder count, crankshaft, turbocharger, bedplate) is unchanged. Derating is a tuning operation, not a hardware change. Engineers can return the engine to its original rating if conditions warrant.

Propeller re-matching

The most consequential change in derating is propeller re-matching. The original propeller was sized for the design power and speed; the new operating profile demands a different propeller match.

Re-pitching

The most common change is to reduce propeller pitch. Lower pitch produces less thrust per revolution but also requires less torque, allowing the engine to operate at lower speed and lower power without overloading.

Re-blading

A more substantial change is to install a new propeller with different blade geometry. New propellers can be optimised for the slow-steaming operating profile, achieving better efficiency in the low-power range than the original propeller.

Effect on engine layout

The re-matched propeller demand line on the engine layout diagram shifts to lower power and lower rpm. The engine’s SMCR is set at the intersection of this new demand line with a chosen power level.

Turbocharger cut-out

Two-turbocharger engines (common on larger slow-speed two-strokes) can shut down one turbocharger at sustained low load:

Why turbocharger cut-out helps

A single turbocharger operating at higher load is more efficient than two turbochargers each at half load. Turbocharger surge margins improve, scavenge air pressure is maintained, and exhaust gas temperatures stay within target range.

Implementation

The cut-out turbocharger is bypassed mechanically (close the inlet and exhaust valves) and the rotor is allowed to coast or is locked. The active turbocharger handles the full exhaust flow at sustained load.

Re-engagement

When higher load is needed, the cut-out turbocharger is re-engaged. The procedure is automated on modern engines, with re-engagement taking 5 to 15 minutes including spool-up to operating speed.

Cylinder cut-out

For super-slow-steaming (sustained operation below 30 percent MCR), some engines support cylinder cut-out: running fewer than the full complement of cylinders.

Why cylinder cut-out helps

The remaining active cylinders run at higher per-cylinder load, with better combustion temperatures, better scavenging, and lower SFOC. The cut-out cylinders are mechanically driven by the crankshaft but receive no fuel and may have their exhaust valves held open to minimise pumping losses.

Implementation

Cylinder cut-out is implemented in the engine control system. The operator selects a cut-out pattern (e.g. cylinders 3 and 6 of an 8-cylinder engine), and the system manages fuel and valve scheduling automatically.

Limitations

Cylinder cut-out increases torsional vibration and produces an irregular firing pattern. It is restricted to specific engine designs that have been certified for cylinder cut-out operation, and to specific cut-out patterns within those engines.

Timing map adjustment

Modern electronically controlled engines include timing maps optimised for various operating regimes. Derating activates the slow-steaming timing map:

  • Injection timing advanced at lower load to compensate for slower combustion
  • Exhaust valve closing earlier to retain heat and reduce residual fraction
  • Common rail pressure maintained at moderate levels rather than dropping with load
  • Cylinder lubricator timing optimised for lower load conditions

The timing map can be switched in operation when the operator initiates a derating mode.

Operational benefits

Fuel consumption

The primary benefit is fuel consumption reduction. For a typical container ship:

  • Original design: 22 knots, 80 percent MCR, ~150 tonnes/day fuel
  • Derated slow-steaming: 16 knots, 30 percent MCR, ~50 tonnes/day fuel

The reduction is approximately 67 percent of the original fuel rate, achieving the cubic-with-speed scaling.

SFOC at the operating point

With derating, the engine operates near its new SMCR point, which is the local minimum of the SFOC curve. SFOC may be 5 to 10 g/kWh higher than the original optimum (because the engine was not designed from scratch for the lower point), but much better than running an undated engine at 30 percent of its design MCR.

CO2 reduction

Lower fuel consumption means lower CO2 emissions per nautical mile, supporting EEDI/EEXI and CII (Carbon Intensity Indicator) compliance.

Reduced wear at high BMEP

Slower steaming removes the engine from the high-BMEP region of the layout diagram, reducing peak cylinder pressures, thermal load, and wear rates on heavily loaded components. Engine life is typically extended, although the gains are partly offset by cold corrosion risks.

Operational risks

Cold corrosion

The principal operational risk of slow steaming is cold corrosion of cylinder liners. Lower combustion temperatures produce less complete sulphur oxidation, more SO3, more sulphuric acid, and more acid condensation on liner surfaces.

Mitigation:

  • Maintain higher cooling water temperature (above 75 degrees Celsius)
  • Use appropriate cylinder oil BN for fuel sulphur
  • Optimise feed rate for low-load conditions
  • Run engine at higher load periodically to clean liner surfaces

Combustion deposits

Low-load operation produces more incomplete combustion, more deposits in piston ring grooves, more carbon on injector tips, and more soot in scavenge box. Periodic high-load runs are recommended to burn off deposits.

Turbocharger fouling

At low scavenge air mass flow, turbocharger compressor and turbine surfaces can accumulate fouling. Periodic water washing of the turbocharger is recommended for slow-steaming operation.

Cylinder cut-out vibration

Cylinder cut-out, while permitted on certified engines, increases torsional vibration. Sustained operation in cut-out mode requires monitoring of torsional vibration levels and may require avoidance of certain rpm ranges.

Exhaust temperature limits

At very low load, exhaust gas temperatures may fall below the dew point of acid, producing acid condensation in the exhaust manifold and turbocharger. Some engines have minimum exhaust temperature alarms.

Strategic considerations

When to derate

Operators consider derating when:

  • Sustained low-load operation is anticipated for the engine’s remaining life
  • The cost of derating modifications is justified by fuel savings
  • Regulatory considerations (CII, EEXI) favour reduced power
  • The propeller can be re-pitched or re-bladed

Short-term slow steaming without derating is also possible but produces higher SFOC and more wear at the low load.

When to undo derating

Derating can be reversed if conditions change:

  • Freight rates rise, encouraging higher service speed
  • Fuel prices fall
  • Vessel repurposed for different trade
  • Major overhaul provides opportunity for rerating

Reversing derating involves restoring the original timing maps, possibly re-pitching the propeller, and recommissioning the engine at higher load.

Super-slow-steaming limits

Below a certain load (typically 25 to 30 percent of derated MCR), even derated engines have difficulty operating safely. Auxiliary scavenge air blowers running continuously become necessary. SFOC rises rapidly. Cylinder oil feed rates may need to be raised.

Industry experience

The shipping industry has now had over 15 years of slow-steaming experience. Key lessons:

  • Engine designs adapted: new engines are designed with broader operating envelopes, supporting both fast and slow operation
  • Cylinder oil formulations evolved: oils specifically designed for slow-steaming operation are widely available
  • Operating practices matured: routine high-load runs, water washing, monitoring practices have become standardised
  • Regulatory pressure: IMO CII and similar requirements create direct incentives for slow steaming
  • Container line consolidation: large operators (Maersk, MSC, CMA CGM) standardised slow-steaming practices across their fleets

The industry has effectively shifted to a slower, more fuel-efficient steady state since 2008.

See also

References

  • MAN Energy Solutions. (2023). Slow Steaming Service Letter. MAN Energy Solutions.
  • WinGD. (2023). Engine Derating and Slow Steaming Guidelines. Winterthur Gas & Diesel.
  • IMO. (2021). MEPC.327(75): Carbon Intensity Indicator (CII).
  • DNV. (2020). Slow Steaming and Engine Health: Industry Guidance.
  • Lloyd’s Register. (2020). Engine Operating in Low Load: Best Practices.