Slow Steaming

What slow steaming is and why it works

Slow steaming is not a single number. It’s a band of operating practice defined against each ship’s design speed, the speed the yard contracted at the design draft and a stated sea margin. A 9,000 TEU container ship designed for 25 knots that runs liner schedules at 18 knots is slow steaming. A Capesize bulker built for 14.5 knots that loads iron ore at Tubarao and crosses to Qingdao at 11 knots is doing the same thing. The word covers any sustained reduction below the design point that an operator chooses for fuel, cost, or compliance reasons, rather than because of weather, draft, or a fault.

The physics is the reason any of this pays. For a displacement hull below the hump in its resistance curve, roughly Froude numbers under 0.30, the power the propeller must absorb climbs with about the cube of speed. Cut speed, and power drops far faster than the speed itself. That single fact, the cubic speed-power law, is what makes 3 knots off a 24-knot ship worth tens of thousands of dollars a day in bunkers.

The cubic speed-power law

The Admiralty relation states that shaft power $P$ scales as displacement to the two-thirds times the cube of speed, divided by a roughly constant Admiralty coefficient $C$ for a given hull and loading:

Two resistance components drive the cube. Frictional resistance, the larger share on a slow full-form ship, grows with about $V^2$ because skin friction follows the square of the relative flow. Wave-making resistance grows faster, as $V^4$ to $V^6$ near the design Froude number, but flattens toward $V^2$ once the ship drops well below the hump. Shaft power is resistance times speed, so $V^2 \times V$ gives the cube. The approximation holds to within 5 to 15% across the normal operating envelope of bulkers, tankers, & container ships; it breaks down near hull speed & at very light loads, covered in Limitations.

For the cleaner speed-power curve fit operators actually use, with a fitted exponent $n$ that’s often 2.7 to 3.3 rather than exactly 3, the form is:

A measured exponent below 3 usually means the ship spends part of its envelope where wave-making is small & friction dominates; an exponent above 3 flags a ship operating closer to its hump, where shaving speed buys even more. The voyage slow steaming calculator takes a noon-report power-speed pair, fits $n$, & projects the burn at any target speed. For the resistance background see ship resistance and powering, hull form design, block coefficient, & the Admiralty coefficient article.

Instantaneous power versus voyage fuel

The cube governs the rate at which fuel leaves the tank, not the total burned over a fixed distance. Those differ because a slower ship takes longer to cover the same miles, & a longer voyage means more hours of burning. The distinction is where most back-of-envelope slow-steaming estimates go wrong.

Take a ship dropping from 20 to 16 knots, a 20% cut. The speed ratio is 16/20 = 0.80. Instantaneous power & fuel rate fall by $1 - 0.80^3 = 1 - 0.512$, about 48.8%. But voyage time stretches by 20/16 = 1.25, so the fuel burned over the leg is the rate times the time: the $V^3$ rate against a $1/V$ time leaves $V^2$. Voyage fuel falls by $1 - 0.80^2 = 1 - 0.64$, about 36%.

So the headline saving on a fixed route at a 20% speed cut is near 36%, not 49%. Because the cargo per voyage is unchanged, that 36% is also the cut in CO2 per voyage & in per-tonne-mile well-to-wake intensity. Push the cut to 30% (20 to 14 knots) & voyage fuel falls about 51%; a 40% cut gives near 64%; a 50% cut near 75%. The marginal saving per knot shrinks as speed falls, the reason there’s a practical floor rather than an open-ended race to zero.

Auxiliary load does not follow the cube

The cubic law applies to propulsion only. The auxiliary plant, generators feeding the accommodation, pumps, reefer plugs, cargo systems, bow thrusters at port, draws a roughly steady load per hour that doesn’t fall when the main engine throttles back. A longer voyage therefore burns proportionally more auxiliary fuel.

On a typical slow-speed two-stroke container ship, auxiliary load runs 5 to 10% of propulsive load at design speed. Slow steam to half speed & that share climbs to 12 to 20% of the now-smaller total, because the propulsion term shrank while the auxiliary term grew with voyage time. The real voyage saving sits a few points below the pure cubic figure. On an LNG carrier, where reliquefaction or sub-cooling can match the propulsion load, the auxiliary term is large enough that aggressive slow steaming barely moves total burn, which is why LNG carriers slowed least.

A worked voyage: 8,500 nautical miles

Put numbers on it. Take a Panamax bulker on the 8,500 nm Newcastle to Qingdao coal run, design speed 14.5 knots, main-engine burn near 38 t/day of VLSFO at 14.5 knots, auxiliary load a steady 3 t/day, bunkers at USD 600/t.

At 14.5 knots the leg takes 8,500 / (14.5 x 24) = 24.4 days. Main engine burns 38 x 24.4 = 928 t; auxiliary burns 3 x 24.4 = 73 t; total 1,001 t, costing about USD 601,000.

Slow to 11.5 knots, a 20.7% cut. The main-engine rate falls by the cube: $38 \times (11.5/14.5)^3 = 38 \times 0.499 = 19.0$ t/day. The leg now takes 8,500 / (11.5 x 24) = 30.8 days. Main engine burns 19.0 x 30.8 = 585 t; auxiliary burns 3 x 30.8 = 92 t; total 677 t, costing about USD 406,000.

Propulsion fuel fell 37%, close to the $1-V^2$ figure for a 20.7% speed cut. But total fuel fell only 32%, because the auxiliary term grew from 73 to 92 t as the voyage stretched by six days. The fuel saving is about USD 195,000; against it sits 6.4 extra days the ship can’t earn elsewhere, which is why the slow-steaming decision is never about fuel alone. The voyage slow steaming calculator & the fuel savings per voyage calculator run this for any ship & route; the lost-voyage side comes from the voyage TCE calculator.

The profit-maximising speed

There’s an economic optimum, & it’s not the slowest the ship can run. Slowing saves fuel but loses voyages, & a lost voyage is lost revenue. The balance point depends on the bunker price, the daily earning rate, & the steepness of the fuel curve.

The standard result drops out of differentiating voyage profit with respect to speed. With profit equal to time-charter-equivalent earnings minus fuel cost, & fuel scaling as $V^3$, the profit-maximising speed is

where $TCE_{daily}$ is the daily time-charter equivalent in USD, $B$ is the bunker price in USD per tonne, & $k$ is the ship’s fuel coefficient (tonnes per day at unit speed cubed). The factor 3 is the cube exponent. The result is intuitive in its sensitivities: higher bunker price pushes the optimum slower, higher earnings push it faster, & both enter through a cube root, so the optimum moves slowly against large input swings.

Run a 14,000 TEU ship through it. At a TCE of USD 60,000/day & VLSFO at USD 600/t, the optimum lands near 17 to 18 knots. Halve the market to USD 20,000/day & it falls to 13 to 14 knots. At a peak USD 100,000/day it climbs back to 21 to 22 knots, near the design speed, the reason ships speed up when freight spikes (see the rebound effect). The voyage TCE calculator computes the earning side of this for any voyage, & the demurrage deposit calculator handles the late-arrival penalty that constrains how slow a chartered ship can actually go.

The 2008 financial crisis and the Maersk programme

Slow steaming existed as a tactic long before 2008. Tanker operators throttled back in soft markets through the 1970s & 1980s. What changed in late 2008 was scale: the practice went from an occasional choice to a fleet-wide standing policy across container shipping inside a few months.

Three things hit at once. Container demand on the headhaul Asia-Europe trade fell by roughly a quarter through the winter of 2008 to 2009. The pre-crisis orderbook kept delivering newbuildings into that hole, so the fleet grew while the cargo shrank. And bunkers, after spiking near USD 800/t in mid-2008, stayed expensive relative to the freight a ship could earn. Slowing down absorbed the surplus capacity (a slower loop needs more ships to keep weekly frequency, soaking up idle tonnage) & cut the fuel bill at the same time. It solved both problems with one lever.

A.P. Moller-Maersk drove the shift. Maersk cut its large container ships from a design speed near 25 knots toward 17 to 18 knots over 2008 & 2009, & published the result: order of USD 80 million a year in fuel across the operated fleet, with a stated reduction in CO2 per container moved. MSC, CMA CGM, & Hapag-Lloyd followed between 2009 & 2012; combined fuel savings across the largest lines were estimated in the high hundreds of millions of dollars a year. Bulk & tanker operators slowed too, by smaller margins, because their fuel-cost-to-charter-rate ratios were less extreme. By the end of 2009 the aggregate world-fleet fuel reduction from speed alone was put at roughly 15 to 25%. The IMO Fourth GHG Study 2020 attributes a large part of the 2008 to 2018 carbon-intensity improvement of the international fleet to this operational speed reduction rather than to technical efficiency gains.

The crisis-era cut was meant to be temporary. The conditions that justified it, overcapacity, costly fuel, thin freight rates, did not lift, so the temporary measure set into structural slow steaming through the early 2010s.

Super slow steaming and the engine learning curve

As operators pushed lower, the industry coined the gradations. Super slow steaming meant below roughly 16 knots for container ships & below 11 for bulkers; ultra slow steaming meant below about 14 & 10 respectively. The barrier wasn’t the hull, which only got more efficient at lower speed, but the main engine, designed to run near 80% of its maximum continuous rating, now asked to hold 25 to 40% load for weeks.

Three engine fears surfaced: cylinder lubrication failure at low combustion temperature, turbocharger surge at low exhaust energy, & injector & exhaust fouling from cold, incomplete combustion. MAN B&W & Wartsila answered with part-load tuning packages, slide-valve fuel injectors that atomize cleanly at low flow, cylinder-oil feed-rate cuts matched to the lower sulfur uptake, & turbocharger cut-out (taking one turbocharger of a multi-turbo engine offline so the rest see proper flow). Operators added hot-stack runs, brief high-load periods every several days to burn off deposits. By 2014 super slow steaming was routine on container ships & common on dry bulk & tankers. The engine side now has its own treatment in engine derating for slow steaming.

Structural normalisation and lower design speeds

By 2015 the market had repriced around the slow pattern. Newbuild contracts specified design speeds 2 to 4 knots below the pre-2008 norm, partly chasing the EEDI reference line that rewards a lower installed power. Owners ordered bulbous bow retrofits to re-fair the hull for the new operating point, since a bulb tuned for 24 knots adds resistance at 18. Some permanently capped engine output through derating. Charter terms & liner schedules were rewritten around slower transit times.

Operating speeds settled into a band that has held with minor drift:

LNG carriers slowed least, both because their charter rates are high (the optimum speed sits high) & because their large auxiliary load blunts the propulsion saving.

Regulation turns a choice into a default after 2023

What began as an economic choice became a regulatory one. EEXI & the CII rating both took effect in 2023 under MARPOL Annex VI as revised by MEPC.328(76). EEXI checks a one-time technical index against a required value; the cheapest route to compliance for most existing ships is engine power limitation, which caps maximum power & so caps top speed, formalizing slow steaming in the ship’s papers. The CII, governed by MEPC.336(76) & MEPC.339(76), grades annual operational intensity A to E & tightens its reduction factor each year.

The CII is what locks slow steaming in. Speeding up raises annual fuel per transport-work & typically pushes a ship from a C to a D band; three straight D years or one E year forces a corrective action plan. The cheapest corrective action is almost always to slow back down. Layer on the EU ETS from 2024 (EUA at EUR 70 to EUR 100/t-CO2 puts a direct price on every tonne burned on EU-touching legs), FuelEU Maritime from 2025, & the IMO Net-Zero Framework GHG fuel intensity standard from 2027, & every regulatory line points the same way. The link between speed & rating is set out in detail in slow steaming and CII.

Engine considerations: derating, EPL, and part-load tuning

A two-stroke main engine is sized for a contracted maximum continuous rating (MCR) & tuned to be efficient near 75 to 85% of it. Run it at 25 to 40% for months & the optimum tuning is wrong, the turbocharger is undersized for the new operating point, & combustion runs cold. The industry’s three responses, listed roughly in order of permanence, are part-load tuning, turbocharger cut-out, & engine derating.

Part-load tuning re-times fuel injection & exhaust-valve events so the efficiency peak moves toward the new working load. Slide-valve injectors, retrofitted across much of the two-stroke fleet after 2009, keep the spray pattern clean at low fuel flow where the old plunger valves dribbled & coked. Turbocharger cut-out takes one of two or three turbochargers offline below a set load so the remaining units see proper exhaust mass flow & don’t surge; the cut unit is valved back in for maneuvering or speeding up.

Engine derating goes further: it permanently lowers the rated power & re-optimizes the whole engine around the lower figure, gaining 2 to 4 g/kWh of specific fuel consumption at the new rating. Engine power limitation (EPL) is the regulatory cousin, a software or mechanical cap on maximum power that satisfies EEXI without re-tuning. EPL is cheaper & reversible (the limit can be overridden in an emergency, with the override logged); derating is the deeper, irreversible optimization. The mechanics, the SFOC trade, & the worked MCR math are in engine derating for slow steaming, with the numbers in the engine MCR derating calculator & the EPL limit in the EPL required MCR calculator.

The market shifted from add-on kits to factory part-load tuning over the 2010s. Early slow steaming bolted modifications onto engines designed for high load; by the mid-2010s MAN B&W’s G-type & Wartsila’s low-speed designs were tuned from the drawing board around a lower contracted output, with the efficiency peak placed where the ship would actually run. Variable turbine geometry & hydraulically actuated exhaust valves let one engine hold good combustion across a wider load range without manual turbocharger cut-out. The result is that a newbuild ordered today is, in effect, a slow-steaming engine from delivery, which is part of why the speed band has settled rather than fallen further.

The durability story turned out better than the 2008 fears. After the 2009 to 2012 learning period, properly tuned slow-steaming engines run cooler & with less thermal cycling than the same engine at full load. Time between overhaul for piston rings has stretched from about 8,000 running hours at high load toward 12,000 to 16,000 hours under steady part-load operation. The trade is responsiveness: a heavily derated or EPL-capped engine has less reserve power for heavy weather or a strong head current, so the master has a narrower margin to push through, & the EPL override exists for exactly that case. The deeper engineering treatment lives in the marine diesel engine article.

Cold corrosion, fouling, and the cleanliness penalty

The win is not free. Sustained low load creates two slow-acting penalties that erode the saving if ignored: cold corrosion inside the cylinder & fouling on the hull & in the gas path.

Cold corrosion is the sharper risk. At low load the cylinder liner wall runs colder, & if its temperature drops below the sulfuric-acid dew point of the combustion gases, condensed acid attacks the liner. Operators with high-sulfur fuel saw liner wear rates jump on early slow-steaming engines until cylinder-oil base number (BN) & feed rate were matched to the lower load. After the 2020 sulfur cap cut fuel sulfur to 0.50%, the acid load fell, but very low load can still drive cold-end attack, & low-BN oils on low-sulfur fuel can leave deposits if overdosed. Hot-stack runs, brief high-load periods every 5 to 10 days, raise liner temperature, burn off deposits, & clear the gas path; they cost a few hours of higher fuel rate against weeks of protected wear. The full mechanism & the lube-oil management are covered in slow steaming and engine cleanliness.

Hull & propeller fouling is the second penalty. A slower hull has weaker flow over the coating & calls at port more often per year, both of which let weed & barnacle settle. A fouled hull can add 8 to 30% to required power, swamping the slow-steaming gain. ISO 19030-1:2016 sets the method for measuring this performance loss from speed-power data over time, separating fouling drift from weather noise. Slow-steaming operators answer with fouling-release silicone coatings rather than basic self-polishing copper paint, in-water hull cleaning every 3 to 6 months, & propeller polishing at each drydock interval of 30 to 60 months. The hull-cleaning economics sit alongside the speed lever in the CII corrective action plan.

Measuring the saving and proving it

A claimed slow-steaming saving is only as good as the baseline it’s measured against, & the baseline drifts. A new hull at 14.5 knots needs one power; the same hull two years later, fouled & with a roughened propeller, needs 10 to 25% more for the same speed. Compare this year’s slow-steaming burn against the design-trial figure & the saving looks larger than it is, because part of the gap is fouling penalty, not speed benefit. ISO 19030-1:2016 sets the method to separate the two: it defines how to build a speed-power reference from sea-trial or model data, how to filter noon-report or high-frequency sensor data for weather, draft, & current, & how to express the residual as a percentage performance change over time. Parts 2 & 3 give default & alternative methods.

The data feeding all of this is the noon report or, increasingly, continuous shaft-power & flow-meter telemetry. A noon report logs speed over ground, speed through water, observed weather, fuel consumed, & shaft RPM once a day; the speed-power fit in the voyage slow steaming calculator is built from exactly these pairs. The weakness is that noon data is coarse & self-reported, & the IMO Fourth GHG Study 2020 flagged the gap between bottom-up activity-based emission estimates & top-down fuel-sales figures partly to data quality. High-frequency monitoring closes the gap by sampling power & flow every few seconds, which is what makes a defensible ISO 19030 trend possible & what feeds the verified CII submission under MEPC.336(76).

The verification chain matters because the saving is now a regulatory asset. A ship’s annual fuel & distance feed its CII rating, its EU MRV report, & its FuelEU balance, all of which are audited. A slow-steaming program that can’t show a clean, weather-corrected speed-power trend can’t defend its rating if challenged, & can’t price its compliance position for a charter negotiation. The measurement discipline of ISO 19030 is what turns a fuel-curve estimate into a number an auditor & a charterer will accept.

Schedule, charter party, and just-in-time arrival

Slow steaming runs into contracts long before it runs into physics. A container ship on a liner loop owes schedule reliability, typically arrival inside a 12 to 24 hour window; head seas & current can push a slow ship outside it. Lines answer by building 5 to 15% slack into the rotation & adding a ship to the string so the slower loop still sails weekly, then leaning on weather routing to protect the arrival probability.

The charter party is where slow steaming gets negotiated. Under a time charter party the charterer pays for bunkers & wants the ship slow, but the standard speed-&-consumption warranty obliges the owner to maintain a stated speed; ordering the ship below it can trip an off-hire or performance claim. BIMCO’s CII Operations Clause for Time Charter Parties (2022) & its Slow Steaming Clause exist to reconcile this, letting the charterer instruct economical speed while protecting the owner’s CII rating; the BIMCO CII clauses article walks through the allocation. Under a voyage charter the owner pays for fuel & gives a speed warranty to the charterer, so the slow-steaming decision is the owner’s, bounded by laytime & demurrage; arrive late & demurrage on the cargo can wipe out the fuel saving.

Just-in-time arrival closes the loop. The old pattern was sail fast, then drift or anchor off the port waiting for a berth, burning the saving twice over. Just-in-time arrival replaces it: the port shares a confirmed berth time, the ship slows en route to hit it, & the waiting fuel becomes saved fuel. BIMCO’s Virtual Arrival & Just in Time Arrival clauses give the contractual frame; the result is slow steaming targeted at the one window that matters rather than across the whole leg. The required-speed math for a given arrival time runs through the market & time charter calculator.

Who pays, and how it shows up in the rating

The economics turn on who holds the fuel bill. Under a time charter party the charterer buys the bunkers, so the charterer is the fuel-cost-sensitive party & the one pressing for slow speed; the owner carries the CII rating risk & the engine. Under a voyage charter or bareboat the owner buys the fuel, so the owner decides, weighing the saving against laytime, demurrage, & any speed warranty. Either way slow steaming costs voyages: a ship that completes fewer round trips a year earns fewer freights, & in a hot market that lost revenue can dwarf the fuel saved, which is exactly what the optimum-speed formula prices.

Liner & tramp trades play it differently. A liner needs schedule integrity, so it slows the whole string at once & adds a ship to keep weekly sailings; the saving survives net of that extra ship’s cost. A tramp operator on dry bulk or tankers (bulk carrier, chemical tanker, general cargo) sets speed voyage by voyage against the prevailing TCE & bunker price, so its speed swings more with the market.

The CII makes slow steaming a rating lever, not just a cost lever. The Annual Efficiency Ratio (AER) measures grams of CO2 per deadweight-tonne-mile across the calendar year. Slowing cuts fuel per voyage but also cuts the deadweight-miles carried per year, since the ship finishes fewer voyages, so the two effects partly cancel. The net move on AER is biggest for a modest 2 to 4 knot cut, which often lifts the rating one or two bands; a 4 to 6 knot cut helps less per knot; & past 6 knots the saving can stall or reverse as the steady auxiliary load comes to dominate a thin annual transport-work figure. For most ship types the rating sweet spot sits 3 to 5 knots below design speed. The sensitivity of the band to speed is modeled in the slow steaming CII sensitivity calculator, with the underlying AER math in the CII Attained calculator & CII Required calculator. The full speed-to-rating chain is in slow steaming and CII.

Some fleets baked the speed lever into the design. Vale specified design speeds of 15.0 to 15.4 knots for its 400,000 DWT Valemax ore carriers ordered 2009 to 2014, letting the yards draw a fuller, lower-resistance hull around the slow operating point; the class rates well on CII as a result. NYK’s PEACE program from 2009 wrapped slow steaming into a wider operational-efficiency push & reported fleet fuel cuts of 8 to 12%. The EU ETS extension to shipping in 2024, with EUA at EUR 70 to EUR 100/t-CO2, added a further 1 to 2 knot incentive on EU-touching legs on top of the existing baseline.

The rebound effect and the CII-gaming critique

Slow steaming is not a fixed gain. It’s a function of the freight market, & it reverses when the market turns. The container boom of 2021 to 2022 is the clearest case: with charter rates spiking & ships scarce, the profit-maximising speed jumped & operators sped up, eating into the CO2 they’d saved in the soft years. This is the rebound effect, & it’s why speed-based emission savings can’t be banked as permanent. The optimum tracks the cube root of the earnings-to-bunker ratio, so any sustained jump in freight rates pulls speed back up.

The harder critique is aimed at the CII metric itself. Because the AER divides annual fuel by deadweight-tonne-miles, a ship can improve its rating by sailing more miles at the same cargo, not by burning less in absolute terms. Idling time in port or at anchor, when the engine is off, lowers the average intensity over the year even though it does no transport work. Analysts have flagged that a ship can game a better CII band through ballast-leg routing or by padding sailed distance, decoupling the rating from real emissions per tonne delivered. The IMO’s review of the CII mechanism, ongoing under the MEPC.346(78) SEEMP guidance & the broader Net-Zero Framework, is meant to close some of this, but the structural point stands: an intensity metric rewards distance, & slow steaming sits awkwardly against it because the slower ship covers fewer miles. None of this makes slow steaming a poor fuel choice; it makes the regulatory accounting of it imperfect.

Limitations

The cubic speed-power law that makes slow steaming pay is an approximation, & it fails at both ends of the speed range. Near hull speed, the Froude number where wave-making resistance climbs as $V^4$ to $V^6$, the exponent is well above 3, so the saving from the first knot off design speed is larger than the cube predicts. At very low load the opposite holds: friction dominates, the curve flattens toward $V^2$, & extra slowing buys less than expected. The 5 to 15% accuracy band quoted earlier is the middle of the envelope, not the edges. ISO 19030-1:2016 exists precisely because the real curve drifts with hull condition & has to be re-measured, not assumed.

The engine sets a hard floor that the economics ignore. Sustained operation below roughly 10 to 20% load risks cold corrosion of the liner & cold-end fouling of the gas path, covered in slow steaming and engine cleanliness; the hot-stack & cylinder-oil management that hold this off cost fuel & attention, so the net saving at extreme low speed is smaller than the bare $V^2$ math shows. Older engines, typically over 15 years & never tuned for part load, can wear fast enough that a premature overhaul erases years of fuel savings. The deeper the slow steaming, the more the auxiliary load, steady per hour & unmoved by the cube, takes over the fuel total, which is why high-auxiliary ships (LNG carriers, cruise ships, ro-pax) gain least.

The commercial constraints bite before the physics does. Schedule integrity, charter-party speed warranties, laytime & demurrage windows, & time-sensitive cargo (reefer, project, pharmaceutical) all cap how slow a ship can actually run regardless of the fuel curve. Charter-rate volatility means the optimum speed moves between contracts, so a ship can’t be tuned once & left. And the rebound effect plus the CII-gaming critique above mean speed-based savings are neither permanent nor cleanly captured by the current intensity metric. Slow steaming is the cheapest operational decarbonization lever shipping has, but it’s a lever with a floor, a moving optimum, & an accounting problem, not a free win.

Where slow steaming sits among the other levers

Slow steaming rarely runs alone now. The SEEMP under MEPC.346(78) treats it as one entry in a stacked efficiency plan. A ship re-faired with a bulbous bow retrofit for the new operating point, capped by engine power limitation, fitted with energy-saving devices ahead of the propeller, wind-assisted propulsion to add speed-independent thrust, air lubrication to cut friction, & trim optimisation for the slow draft, stacks several single-digit gains on top of the speed cut. The savings don’t add linearly, since each measure changes the operating point the next one optimizes against, so the combined figure has to be modeled rather than summed; the SEEMP Measures Combined calculator does that, & the marine propeller article covers the device side.

The forward view is for the speed band to hold rather than fall much further. The cumulative pull of CII tightening, FuelEU Maritime, EU ETS, & the IMO Net-Zero Framework GHG fuel intensity standard from 2027 keeps the pressure on, but the marginal saving per knot is already small at today’s speeds & the engine floor is close. Most fleet forecasts put the residual room at 1 to 3 knots of further average reduction through 2030, then a plateau.

Alternative fuels change the arithmetic without ending the practice. LNG, methanol, ammonia, & biofuels run 2 to 4 times the cost per gigajoule of heavy fuel oil, so each unit of energy saved by slowing is worth more, which argues for more slow steaming, not less. Pulling the other way, their lower well-to-wake intensity eases the regulatory pressure that the CII & FuelEU put on speed. The two effects offset, & the working expectation is that slow steaming stays the default operating pattern through 2030 & likely 2040, with the exact band tracking the fuel mix & the freight market rather than any single rule.

See also

Additional calculators:

• Time-Charter Equivalent (TCE) - Voyage
• Slow Steaming - Fuel Savings per Voyage
• Slow Steaming Fuel Savings Calculator
• Slow-Steaming GHG - CII Rating Sensitivity

Additional formula references:

• Charter Tce Voyage
• Voyage Slow Steaming
• Charter Freight Demurrage Deposit
• Charter Mpa Market Time

Additional related wiki articles:

• Admiralty Coefficient
• Time charter party

Operational and technical efficiency

• Wind-assisted propulsion
• Air lubrication systems
• Just-in-time arrival
• Weather routing
• Trim optimisation
• Bulbous bow retrofits
• Energy-saving devices
• Battery-hybrid propulsion
• Onboard carbon capture
• Cold ironing / shore power

Hull form and resistance

• Hull form design
• Block coefficient
• Hydrostatics and Bonjean curves
• Trim and list
• Metacentric height
• Ship resistance and powering
• Marine propeller

Marine fuels

• LNG as marine fuel
• Methanol as marine fuel
• Ammonia as marine fuel
• Biofuels in shipping
• Heavy fuel oil
• Marine gas oil
• Well-to-wake intensity
• RFNBO under EU rules
• Methane slip from LNG dual-fuel
• N2O emissions from marine engines
• Black carbon and Arctic shipping

Engines, exhaust and machinery

• Marine diesel engine
• Marine gas turbine
• Marine propeller
• Exhaust gas cleaning system
• Bow thruster and stern thruster

Regulatory and reporting frameworks

• MARPOL Annex VI
• IMO Net-Zero Framework
• IMO GHG Strategy
• EEXI, EPL and ShaPoLi
• SEEMP I, II, III
• CII corrective action plan
• EU MRV Regulation
• EU ETS for shipping
• FuelEU Maritime
• FuelEU penalties, pooling and multipliers
• UK ETS for shipping
• China DCS
• IMO DCS vs EU MRV
• CARB at-berth rule
• Emission control areas
• NOx Tier I, II, III
• IMO 2020 sulphur cap

Voluntary frameworks

• Poseidon Principles
• Sea Cargo Charter
• RightShip GHG Rating
• Green Shipping Corridors
• BIMCO CII clauses
• EUA market mechanics for shipping
• Voluntary carbon credits in shipping

Conventions, codes and class

• SOLAS Convention
• MARPOL Convention
• Ballast Water Management Convention
• Hong Kong Convention
• COLREGs Convention
• ISM Code
• ISPS Code
• Classification society

Ship types

• Bulk carrier
• Container ship
• Chemical tanker
• LNG carrier
• General cargo ship

Calculators

• Voyage slow steaming calculator
• Slow steaming fuel savings per voyage
• Engine MCR derating calculator
• Voyage TCE calculator
• EPL required MCR calculator
• SEEMP Measures Combined calculator
• CII Attained calculator
• CII Required calculator
• Calculator catalogue

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.

Further reading

• ITTC. Recommended Procedures and Guidelines: Predicted Power of Ships in Service. International Towing Tank Conference, 2017.
• IRENA. A pathway to decarbonise the shipping sector by 2050. International Renewable Energy Agency, 2021.
• ICS. Catalysing the Fourth Propulsion Revolution. International Chamber of Shipping, 2022.
• Lloyd’s Register. Slow Steaming and the Bulbous Bow. LR Technical Briefing, 2014.

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