Slow Steaming

Slow steaming is the deliberate operation of a merchant vessel at a service speed materially below its design speed, typically 3 to 6 knots below design speed, to reduce fuel consumption and (under modern regulatory frameworks) to improve CII rating and to reduce EU ETS, FuelEU Maritime and IMO Net-Zero Framework exposure. The practice exploits the cubic-power relationship between speed and resistance for displacement hulls (the Admiralty coefficient approximation), under which a 20% speed reduction yields approximately a 49% reduction in instantaneous propulsive power, partially offset by the longer voyage time and the resulting larger total fuel burn for the voyage; the net fuel saving for a typical voyage at 20% reduced speed is approximately 35 to 40%, equivalent to approximately the same percentage reduction in CO2 emissions and per-tonne-mile WtW intensity. Slow steaming was widely adopted from late 2008 onwards in response to the financial crisis (collapsing trade demand and the resulting overcapacity), the high bunker fuel prices (USD 600 to USD 800 per tonne in 2008) and the introduction of EEDI (which incentivised lower design speed for newbuilds). It became structural through the 2010s and 2020s as the dominant operational decarbonisation lever and as a major driver of the bulbous bow retrofit, EEXI engine power limitation and SEEMP III voyage optimisation programmes. Average operating speeds in 2024 are approximately: container ships 16 to 19 knots (down from 23 to 25 knots in 2007), bulk carriers 11 to 13 knots (down from 14 to 15 knots), crude oil tankers 12 to 14 knots (down from 14 to 15 knots). ShipCalculators.com hosts the principal computational tools: the slow steaming fuel savings calculator, the optimal speed calculator, the Admiralty coefficient calculator, the TCE economics of slow steaming calculator, the EEXI EPL calculator and the CII Attained calculator. A full listing is available in the calculator catalogue.

Contents

Background and physics

The cubic-power relationship

For a displacement ship operating at speeds below the design speed (i.e. at Froude numbers below approximately 0.30), the instantaneous propulsive power required ($P$) is approximately proportional to the cube of the speed ($V$):

$$ P \approx K V^3 $$

where $K$ is a constant for a given hull form and displacement (the Admiralty coefficient relationship). The cubic dependency arises from the combination of:

Frictional resistance ($R_F \propto V^2$): proportional to the square of the speed.
Wave-making resistance ($R_W$): rises faster than $V^2$ at high Froude numbers but is approximately proportional to $V^2$ at low to moderate Froude numbers.
Shaft power = resistance × speed = $V^2 \times V = V^3$ (approximately).

For an actual hull, the relationship is more complex (the wave-making resistance can rise as $V^4$ to $V^6$ at high Froude numbers, and the frictional resistance has a Reynolds number dependency), but the cubic approximation is generally accurate to within 5 to 15% across the operating envelope of typical displacement merchant ships.

The fuel consumption rate is proportional to the propulsive power (modulo the engine’s specific fuel consumption):

$$ \dot{m}{fuel} = P / LHV / \eta{engine} \approx K’ V^3 $$

A 20% speed reduction (e.g. from 20 knots to 16 knots) yields:

$V$ ratio: 16/20 = 0.80.
$V^3$ ratio: 0.80^3 = 0.512.
Power reduction: 1 - 0.512 = 48.8% reduction.
Instantaneous fuel rate reduction: 48.8%.

But the voyage time lengthens proportionally to 1/V:

Voyage time ratio: 20/16 = 1.25.

So the total voyage fuel is:

$V^2$ ratio: 0.80^2 = 0.64.
Voyage fuel reduction: 1 - 0.64 = 36% reduction.

Therefore a 20% speed reduction reduces the per-voyage fuel consumption by approximately 36%, which is also the per-tonne-mile reduction in WtW intensity (because the cargo carried per voyage is unchanged).

A 30% speed reduction (e.g. from 20 to 14 knots) yields a per-voyage fuel reduction of approximately 51%; a 40% reduction yields approximately 64%; a 50% reduction yields approximately 75%.

For a fuller treatment of the underlying physics see ship resistance and powering, hull form design and block coefficient.

Auxiliary load consideration

The cubic relationship applies to the propulsive power but not to the auxiliary load (electrical loads, accommodation services, cargo handling, bow thrusters at port). Auxiliary load is approximately constant per unit time, so a longer voyage from slow steaming proportionally increases the total auxiliary fuel consumption.

For a typical slow-speed container ship, the auxiliary load is approximately 5 to 10% of the propulsive load at design speed. The auxiliary contribution to total voyage fuel grows from 5 to 10% at design speed to 12 to 20% at 50% slow steaming. The net voyage fuel saving must account for this, reducing slightly from the pure cubic-power calculation.

For LNG carriers the auxiliary load is much higher (cargo refrigeration and reliquefaction can equal the propulsion load), so the slow-steaming benefit is proportionally smaller.

Optimal speed economics

The optimal operating speed for a vessel depends on:

Bunker fuel cost: higher fuel cost shifts the optimum toward slower speeds.
Time charter rate (TCE - Time Charter Equivalent): higher freight or charter rates shift the optimum toward higher speeds (the value of completing voyages quickly outweighs the fuel saving).
Cargo demurrage: late delivery of cargo can incur demurrage payments that offset the fuel saving.
Schedule constraints: container line schedule reliability requirements (typical promised port-call arrival within ± 12 hours), tanker contractual delivery windows, and dry-bulk cargo loading/discharge windows all constrain the practical operating speed.
Vessel age and engine condition: older vessels with worn engines have a steeper fuel-vs-speed curve than new vessels.

The Time Charter Equivalent (TCE) profit-maximising speed is given by:

$$ V_{optimal} = \left( \frac{TCE_{daily}}{3 \cdot K \cdot Bunker_{price}} \right)^{1/3} $$

where the constant 3 reflects the cubic power relationship. For a typical 14,000 TEU container ship with a TCE of USD 60,000/day and a VLSFO price of USD 600/t, the optimal speed is approximately 17 to 18 knots; at TCE USD 20,000/day, the optimal speed is approximately 13 to 14 knots; at TCE USD 100,000/day (peak market), the optimal speed is approximately 21 to 22 knots.

The TCE economics of slow steaming calculator implements the full economic optimisation.

Historical adoption

2008 to 2010: emergency response

The 2008 financial crisis triggered an immediate and widespread adoption of slow steaming as an emergency response to:

Collapse in trade demand: container freight demand fell approximately 25% in late 2008 and early 2009.
Vessel overcapacity: the orderbook from the pre-crisis boom continued to deliver capacity, exacerbating the imbalance.
High bunker prices: VLSFO at USD 600 to USD 800 per tonne in mid-2008.

Major container lines (Maersk, MSC, CMA CGM, Hapag-Lloyd) reduced operating speeds from approximately 23 to 25 knots to approximately 17 to 19 knots within months. Bulk and tanker operators followed with smaller reductions. The aggregate fuel saving across the world fleet was estimated at approximately 15 to 25% by end-2009.

Slow steaming was initially intended as a temporary response, but the conditions that justified it (overcapacity, high fuel prices, modest freight rates) persisted through the early 2010s, leading to structural slow steaming.

Super slow steaming and ultra slow steaming (2010 to 2014)

As the practice became established, several operators adopted super slow steaming (typically below 16 knots for container ships, below 11 knots for bulkers) and ultra slow steaming (below 14 knots for container ships, below 10 knots for bulkers). The operational and engine implications of very low speeds prompted concerns about engine wear, fuel injector fouling, and turbocharger surge, which were progressively addressed through:

Engine modifications: new injection profiles, modified turbocharger settings, additional auxiliary heating to maintain combustion temperature.
Slide valve injectors and common-rail systems: better tolerance to part-load operation.
Operating procedures: scheduled higher-load periods to clean the engine (“hot stack” procedures) every 5 to 10 days.

By 2014 super slow steaming was the norm for container ships and a common practice for bulkers and tankers.

2015 to 2022: structural normalisation

By 2015, slow steaming had transitioned from emergency measure to structural reality:

Newbuild orders specified lower design speeds (often by 2 to 4 knots vs the pre-2008 standard).
EEDI requirements progressively pushed design speeds lower for new orders.
Bulbous bow retrofits were implemented to optimise hulls for the lower service speed.
Engine derating was undertaken to permanently cap maximum power at the slow-steaming level.
Schedule and contract structures were renegotiated to accommodate slower delivery times.

Average operating speeds stabilised at approximately:

Container ships: 17 to 19 knots (down from 23 to 25 knots).
Bulk carriers: 12 to 13 knots (down from 14 to 15 knots).
Crude oil tankers: 12 to 14 knots (down from 14 to 15 knots).
LNG carriers: 18 to 19 knots (down from 19 to 20 knots, modest reduction due to high TCE values).

2023 onwards: regulatory consolidation

The introduction of EEXI in 2023 and CII rating in 2023, plus the EU ETS for shipping from 2024 and FuelEU Maritime from 2025, created strong regulatory drivers for continued slow steaming. The IMO Net-Zero Framework GFI standard from 2027 will further reinforce the slow-steaming pattern.

The CII rating in particular has the effect of locking-in slow steaming: a vessel that increases speed (and therefore fuel consumption) typically downgrades from C rating to D rating, triggering the CII corrective action plan requirement. This regulatory mechanism makes slow steaming the default operating pattern for the foreseeable future.

Operational and engineering considerations

Engine wear and durability

Slow steaming was initially feared to cause:

Cylinder lubrication problems: at low load, cylinder temperature drops and lube oil can fail to atomise correctly, causing increased ring wear.
Turbocharger surge: low-load operation can cause turbocharger compressor surge if not managed carefully.
Fuel injection fouling: low fuel demand can cause injector nozzles to coke up.
Combustion instability: low cylinder temperature can cause incomplete combustion and reduced engine durability.

In practice, after a 2008-to-2012 learning period, engine manufacturers developed slow-steaming kits (modified injection profiles, cylinder oil dosing changes, turbocharger waste-gating, additional auxiliary heating) that effectively addressed these concerns. Modern slow-steaming engines have better durability than the same engine running at high load, principally because of reduced thermal stress.

The Time Between Overhaul (TBO) for major engine components has actually lengthened in the slow-steaming era, with typical TBO for piston rings increasing from 8,000 hours (high-load operation) to 12,000 to 16,000 hours (slow-steaming operation).

Hot-stack procedures

To prevent injector fouling and exhaust system carbon buildup, operators typically schedule hot-stack procedures every 5 to 10 days: a brief (typically 4 to 8 hour) period of high-load operation to clean the combustion chamber and exhaust system. The hot-stack procedure consumes additional fuel but is required for engine durability.

Hull and propeller fouling

A vessel operating at slow speed accumulates hull fouling more rapidly than a vessel at design speed (because the slower flow reduces the self-cleaning effect, and because port-call frequency typically increases per voyage). The fouling penalty offsets some of the slow-steaming saving.

To mitigate, slow-steaming vessels typically:

Adopt premium anti-fouling coatings (silicone-based or hard fouling-release coatings) instead of basic copper-based coatings.
Implement periodic in-water hull cleaning (typically every 3 to 6 months).
Implement propeller polishing at every drydocking (every 30 to 60 months).

The hull cleaning savings discussion in the CII corrective action plan article covers these measures in more detail.

Schedule reliability

For container ships operating on liner services, schedule reliability is a contractual obligation. The reliable arrival window is typically ± 12 to 24 hours of the scheduled time. Slow steaming in head winds and rough weather can push arrivals outside the window, triggering penalty payments.

The standard solution is to build buffer time into the schedule (typically 5 to 15% slack) and to use weather routing to maximise the on-time arrival probability.

Crew time and fatigue

Slow steaming means longer voyages and longer time at sea between port calls. For some routes, this exceeds the crew comfortable working interval. The MLC 2006 (Maritime Labour Convention) sets minimum rest hours; longer voyages do not directly violate the MLC but can stretch the practical crew rotation timing.

Economic and financial implications

Owner perspective

For the shipowner, slow steaming delivers direct fuel saving (the owner pays for the fuel under bareboat charter, shares cost under voyage charter, and does not pay under time charter). The saving is partially offset by:

Reduced voyage volume: fewer voyages per year per vessel reduces total revenue per vessel.
Time charter rate sensitivity: if the time charter rate is high, the lost voyage value can exceed the fuel saving.

Charterer perspective

For the time charterer, slow steaming reduces the fuel cost (which the time charterer pays under standard time charter). The charterer’s perspective is the most fuel-cost-sensitive and the most willing to push for slow steaming.

For the voyage charterer (typical for bulk carriers and tankers on tramp trade), the freight is typically a lump sum or per-tonne rate; the owner pays for the fuel. Voyage charter rates can include speed warranties (the owner agrees to maintain a minimum average speed) that constrain slow steaming.

Liner trade vs tramp trade

The economic logic of slow steaming differs between liner and tramp trades:

Liner trade (container ships, some ro-ro): schedule reliability is paramount. Slow steaming requires a fleet-wide approach with additional vessels to maintain frequency. The slow-steaming savings net of the additional capital cost of the extra vessels are still typically positive.
Tramp trade (bulk carriers, tankers, some general cargo): schedule flexibility is greater. Slow steaming is a per-voyage decision based on prevailing TCE, bunker price and voyage commitments.

CII rating economics

Under the CII rating framework, the AER (Annual Efficiency Ratio) drives the rating. AER is approximately proportional to fuel consumption per dwt-mile carried. Slow steaming reduces fuel per voyage, but a longer voyage time reduces the dwt-miles carried per year (because the vessel completes fewer voyages). The net effect on AER is typically:

Modest slow steaming (2 to 4 knot reduction): AER improves significantly, CII rating improves typically 1 to 2 bands.
Aggressive slow steaming (4 to 6 knot reduction): AER improves, but the marginal benefit declines, and the lost revenue can outweigh the regulatory benefit.
Very aggressive slow steaming (6+ knot reduction): AER may not improve further or may even degrade if the voyage frequency falls so much that auxiliary load dominates.

The optimal slow steaming for CII rating is typically around 3 to 5 knots below design speed for most vessel types.

Notable slow-steaming policies

Maersk 2008 super slow steaming

A.P. Moller-Maersk was an early adopter of super slow steaming, reducing operating speeds on its E-class and S-class container vessels to approximately 17 knots (from a design speed of 25 knots) by mid-2009. Maersk reported that the slow-steaming initiative saved approximately USD 80 million per year in fuel costs across its operated fleet.

MSC, CMA CGM and Hapag-Lloyd

MSC (Mediterranean Shipping Company), CMA CGM and Hapag-Lloyd all adopted similar slow-steaming policies between 2009 and 2012. The combined fuel saving across these four lines (the world’s largest container operators) is estimated at approximately USD 800 million per year.

Vale slow-steaming and the Valemax fleet

Brazilian iron-ore exporter Vale specified relatively low design speeds (15.0 to 15.4 knots) for its 70-strong Valemax fleet (400,000 DWT class, the largest dry bulk carriers ever built) at the time of ordering (2009 to 2014). The low design speed allowed for an exceptionally efficient hull form and contributed to the Valemax fleet’s leading CII rating performance.

NYK, MOL and K Line

The Japanese “three big” shipping lines (NYK, MOL, K Line) progressively adopted slow steaming through 2009 to 2012 across their container, bulk and LNG fleets. NYK’s PEACE project (Plan for Energy Efficient Activities and Conservation in Environment, launched 2009) was an integrated slow-steaming and operational efficiency programme that delivered approximately 8 to 12% fleet-wide fuel savings.

EU ETS-driven slow steaming (from 2024)

The EU ETS extension to maritime in 2024, with EUA prices of approximately EUR 70 to EUR 100 per t-CO2, has provided a measurable additional incentive for slow steaming on EU-calling vessels. ICCT analysis (2024) suggests an additional 1 to 2% speed reduction across the EU-calling fleet attributable to ETS economics on top of the prevailing slow-steaming baseline.

Limitations and risks

Schedule degradation

Aggressive slow steaming degrades schedule reliability. For container ship liner services, reliability degradation can erode the line’s market position; for LNG carrier charters, schedule slippage can cause demurrage on the cargo and downstream LNG buyer obligations.

Engine condition risk

Older engines (typically over 15 years) may not have been designed for slow-steaming operation and may experience accelerated wear. The cost of premature engine overhaul can offset the slow-steaming saving.

Auxiliary fuel cost

The longer voyage time increases auxiliary fuel consumption proportionally. For vessels with high auxiliary loads (LNG carriers, cruise ships, ro-pax ferries) this can substantially erode the slow-steaming benefit.

Charter rate volatility

The optimal speed depends on the prevailing TCE rate. In a volatile freight market (typical of bulk carriers and tankers), the optimal speed can shift dramatically between contracts, making operational consistency difficult.

Impact on cargo

For some perishable or time-sensitive cargo (refrigerated containers, time-critical project cargo, some chemical or pharmaceutical shipments), slow steaming may not be acceptable. Premium charter rates for fast service offset the lost fuel savings.

Speed warranty complications

Time charter contracts often include speed warranties specifying a minimum average speed. Slow steaming below the warranty speed is a contractual breach unless renegotiated. The BIMCO CII clauses provide a framework for renegotiating speed warranties to accommodate CII compliance requirements.

Future outlook

Continued reduction or floor?

DNV’s Maritime Forecast to 2050 (2023) projects continued modest slow-steaming through 2030 (additional 1 to 3 knots reduction across the fleet on average), driven by the cumulative effect of FuelEU Maritime, IMO Net-Zero Framework GFI, EU ETS and CII rating tightening. Beyond 2030, the slow-steaming pattern is expected to stabilise as the fleet transitions to alternative fuels (where the speed-fuel-cost relationship is fundamentally different).

Combination with other measures

Slow steaming is increasingly combined with:

Bulbous bow retrofits optimising the hull for the new operating speed.
Engine power limitation (EPL) permanently capping maximum power.
Energy-saving devices reducing propulsive demand at all speeds.
Wind-assisted propulsion providing additional speed-independent thrust contribution.
Air lubrication systems reducing frictional resistance.
Trim optimisation optimising for the new operating point.
Just-in-time arrival and weather routing further reducing the time-vs-fuel trade-off.

The SEEMP Measures Combined calculator implements the cumulative-savings calculation across these measures.

Alternative fuel implications

As the fleet transitions to alternative fuels (LNG, methanol, ammonia, biofuels), the cost structure changes. Alternative fuels are typically 2 to 4 times more expensive per GJ than HFO, which actually increases the value of slow steaming (each GJ saved is worth more). Conversely, the WtW intensity of alternative fuels is typically lower, which reduces the regulatory pressure for slow steaming.

The net effect is uncertain. The expectation is that slow steaming will remain the dominant operational approach through 2030 and likely 2040, gradually evolving as fuel mixes change.

References

IMO Resolution MEPC.328(76): 2021 Revised MARPOL Annex VI. International Maritime Organization, 2021.
IMO Resolution MEPC.336(76): 2021 Guidelines on Operational Carbon Intensity Indicators (CII Guidelines, G1). International Maritime Organization, 2021.
IMO Resolution adopted MEPC 83 (April 2025): IMO Net-Zero Framework. International Maritime Organization, 2025.
Regulation (EU) 2023/1805 of the European Parliament and of the Council of 13 September 2023 on the use of renewable and low-carbon fuels in maritime transport (FuelEU Maritime). Official Journal of the EU, 2023.
Regulation (EU) 2023/959 of the European Parliament and of the Council of 10 May 2023 amending Directive 2003/87/EC (EU ETS Maritime). Official Journal of the EU, 2023.
DNV. Maritime Forecast to 2050. DNV Energy Transition Outlook, 2023.
ICCT. Slow steaming and CO2 emissions from container shipping. International Council on Clean Transportation, 2017.
ICCT. Speed reduction in shipping: An updated CO2 mitigation analysis. International Council on Clean Transportation, 2024.
A.P. Moller-Maersk. Sustainability Report 2017. Maersk Group, 2018.
NYK Group. PEACE Project: Plan for Energy Efficient Activities and Conservation in Environment. NYK, 2014.
BIMCO. BIMCO CII Operations Clause for Time Charter Parties. BIMCO, 2022.