Engine Derating for Slow Steaming

What de-rating means on the engine layout diagram

Every slow-speed two-stroke is sold as a layout field, not a single number. MAN Energy Solutions and WinGD both publish their engines as a quadrilateral on a power-versus-speed plot bounded by four corner points: L1 (maximum power, maximum speed), L2 (maximum power, lower speed), L3 (lower power, lower speed) and L4 (lower power, maximum speed). L1 is the nominal maximum continuous rating. Everything inside the L1-L2-L3-L4 box is a valid place to put the specified maximum continuous rating, the SMCR, for a given ship.

De-rating is the act of choosing an SMCR low and usually left of L1, then committing to it as the engine’s contract rating. A 12,000 kW L1 engine specified at 9,000 kW and 85 rpm hasn’t been crippled. It’s been ordered as a 9,000 kW engine, with the fuel-injection, turbocharger and timing all matched to that point. The physical castings, the bore and the stroke are identical to the L1 build; what’s different is the matching. That’s the whole idea: pick the SMCR that sits on the propeller curve for the speed you’ll actually run, then tune the engine so its SFOC minimum lands on or near that point.

The move down the diagram is usually a move down and to the left. A ship that will steam at 16 knots instead of 22 doesn’t just need less power; it needs less power at lower shaft rpm, because the efficient propeller for 16 knots turns slower. So the specified point drops from, say, L1 at 100 percent power and 100 percent speed to something like 70 percent power and 90 percent speed. The exact landing spot is a negotiation between the naval architect’s resistance prediction, the propeller designer’s open-water curve, and the engine builder’s permissible-operating envelope. The engine load diagram and operating envelope sets the hard fences: the torque/speed limit line, the overload region, and the light-running margin that keeps the engine off its own scavenge-air and thermal limits.

There’s a real efficiency prize here. An engine matched at a de-rated point can be given a longer stroke-to-bore tune, a turbocharger sized for the lower air demand, and an injection profile built for the lower mean effective pressure. The published SFOC at the de-rated SMCR is typically 2 to 6 g/kWh below what the same physical engine would burn if you simply throttled an L1-rated build back to the same load. That gap is the difference between buying a 9,000 kW engine and running a 12,000 kW engine at 75 percent. Over a 25-year hull life burning 30 to 50 tonnes a day, a few grams per kWh is real money.

A worked feel for the numbers helps. Take a hypothetical six-cylinder engine with an L1 of 12,000 kW at 105 rpm and an L3 of about 8,400 kW at 84 rpm. A liner ship that re-strings its service speed from 22 to 16 knots sees its calm-water power demand fall by roughly the cube of the speed ratio, $(16/22)^3 \approx 0.385$, so a hull that needed close to L1 at 22 knots now needs on the order of 4,600 kW for propulsion at 16. Add sea margin and engine margin and the naval architect lands an SMCR near 5,500 to 6,000 kW at perhaps 78 to 82 rpm. That point sits low and left in the field, well inside the L3-L4 edge. An L1-matched 12,000 kW build asked to deliver 5,800 kW is running at 48 percent of its matched load, off the efficient point and probably leaning on its blowers; an engine ordered and matched at 5,800 kW puts that same daily output near the bottom of its own SFOC bowl. Same castings, same bore and stroke, different identity on paper and several grams per kWh apart in the bunker tank.

The layout-field width is what makes this possible. MAN Energy Solutions sets the speed ratio between L1 and L2 around 1.0 to the lower-speed corner near 0.8 of L1 speed depending on the type, and the power ratio L1 to L3/L4 often near 0.7 to 0.8, so the box is wide enough to hold a wide spread of ship designs without a custom casting. WinGD publishes its RT-flex and X-DF range with comparable fields. The naval architect’s job is to find the one point in that box where the propeller curve, the resistance prediction and the SFOC minimum all agree, then hold the engine builder to it as the contract SMCR.

How slow steaming created the de-rating market

De-rating as a routine engineering option is younger than the engines it’s applied to. Through most of the twentieth century, commercial ships were specified at or near their service speed continuously, with the engine matched near L1 and a fuel price low enough that part-load SFOC barely entered the design. The 2008 financial crisis broke that. Container freight rates collapsed against sudden overcapacity, and because hull resistance power rises with roughly the cube of speed, operators found that dropping from a design 24 or 25 knots to 16 or 18 cut fuel burn by something close to half. The Fourth IMO GHG Study 2020 traced how this practice, which the industry called slow steaming, reshaped fleet fuel demand across the following decade.

The problem was that engines matched for 75 to 85 percent continuous load behaved badly at 30 to 40 percent. Scavenge air ran short, combustion temperatures fell, acid condensed on liner surfaces and cold corrosion accelerated, and SFOC climbed away from its minimum. Engine builders responded with two distinct kinds of fix. The first was operational: part-load tuning maps, turbocharger cut-out on multi-turbocharger engines, cylinder cut-out for super-slow operation, and revised cylinder-oil feed-rate strategies, all aimed at making an unchanged engine tolerate sustained low load. The second was the design fix this article is mostly about: re-matching the engine to a lower SMCR so the efficient point moved to where the ship now lived. Slow steaming became the steady state for much of the box fleet, and de-rating moved from a downturn improvisation to a line item on newbuild specifications.

True de-rating versus an EPL or ShaPoLi power cap

The single most confused point in this whole subject is the difference between mechanical de-rating and a power limitation. They look similar on a dyno sheet and they’re driven by overlapping motives, but they’re not the same thing, and class treats them differently.

True de-rating re-matches the engine. The injection equipment may be changed: a smaller fuel pump plunger, fewer or smaller injector nozzle holes, a different cam or, on electronically controlled engines, a different injection map. The turbocharger is re-matched, sometimes to a smaller frame or a different nozzle ring, because the air demand at the new SMCR is lower. The result is a new layout point with its own shop-test SFOC curve, its own NOx Technical File parent/member engine data, and its own EIAPP certificate. It is the engine’s permanent identity. Reversing it means re-engaging the OEM, re-testing, and re-certifying. People don’t do this casually.

An Engine Power Limitation (EPL) or a Shaft Power Limitation (ShaPoLi) is the opposite philosophy. The engine keeps its original rating and its original matching. A limiter caps the deliverable power: EPL works in the engine control system on fuel index or rail pressure; ShaPoLi works on a shaft torque/speed measurement and trips the governor. The cap is reversible, it’s sealed and logged, and it can be overridden in an emergency to recover full power for safe navigation. IMO built EPL and ShaPoLi specifically as the cheap retrofit route to EEXI compliance under MEPC.328(76), with the implementation rules in MEPC.335(76) covering the sealing, the tamper-evidence, the overridable-for-safety provision, and the on-board records.

The EEXI math is why a cap works at all. Attained EEXI is essentially CO2 per transport work; the main-engine term uses 75 percent of the limited MCR, and the reference speed used in the index falls as power falls. Because attained EEXI scales roughly with $P^{2/3}$, a modest power cut buys a useful index reduction. The required fraction of nameplate MCR to retain is $k_\text{MCR} = (\text{EEXI}_\text{required} / \text{EEXI}_\text{attained}^\text{before})^{3/2}$, which is exactly what the EPL calculator solves. Many existing ships found that capping at 70 to 85 percent of nameplate MCR was enough to pass, with no engine surgery at all.

ShaPoLi solves the same equation on shaft power rather than engine power, so the ShaPoLi calculator returns the same fractional form. Owners pick ShaPoLi when they have a shaft generator (PTO) drawing off the line, or twin-engine plants, or a propeller arrangement where measuring at the shaft is cleaner than limiting at the fuel index. Both methods satisfy the same MEPC.335(76) framework and both are reversible.

So which do you do? A cap is reversible and cheap, but it leaves a fully matched high-power engine running at part load, which means the part-load SFOC penalty stays. True de-rating costs more upfront and is hard to undo, but it recovers the SFOC. The practical answer most owners reached: cap an existing ship for EEXI day-one compliance, and reserve true de-rating for a newbuild design, or for an older ship being fundamentally re-engined, where the SFOC gain pays back over the remaining life. The honest framing for a CII strategy matters too, because EEXI is a one-time design index while CII is an annual operational rating; a power cap helps both, but only true matching delivers the operational fuel saving year after year.

There’s a third category worth naming, because it gets lumped in with the other two and shouldn’t be: a pure operational power cap with no regulatory standing. An operator can simply instruct the bridge and engine room to hold a maximum daily load, with nothing sealed and nothing verified. That’s not de-rating and it’s not an EEXI EPL; it’s a voluntary speed policy. It saves fuel exactly as long as the crew honors it, contributes nothing to the attained-EEXI calculation because there’s no verified limited MCR, and leaves the SFOC bowl untouched. It’s a legitimate tactic for a tramp ship reacting to a soft freight market, but conflating it with a sealed EPL, which counts toward EEXI and is auditable at survey, is a common mistake in fleet planning. The distinction is whether the limit is verified and documented, not whether the ship happens to be steaming slowly.

The SFOC benefit of matching the engine to the service load

Specific fuel oil consumption isn’t flat across the load range. On a modern slow-speed two-stroke the SFOC curve is roughly a shallow bowl: high at very low load, dropping to a minimum somewhere between 65 and 85 percent of the engine’s matched MCR, then creeping up again toward full load. The minimum is where the turbocharger is on-design, the scavenge pressure is right, and combustion is complete. Push the engine well below the bowl and you fall off the efficient point; the turbocharger can’t make enough air, peak pressures drop, and SFOC climbs.

Here’s the mechanism that makes de-rating worth the trouble. Run an L1-matched engine at 30 percent of its L1 load and you’re operating far left of its SFOC bowl, in the region where the single available turbocharger is undersupplied and the engine wants an auxiliary blower running continuously. Re-match the same engine to a de-rated SMCR where 30 percent of L1 becomes, say, 50 to 55 percent of the new matched MCR, and you’ve moved the operating point back toward the bowl. The same daily power output now costs several grams per kWh less.

It’s easier to feel the size of the effect in efficiency terms. Brake thermal efficiency is $\eta_{BT} = 3600 / (\text{SFOC} \cdot \text{NCV})$, with NCV around 40.5 to 42 MJ/kg for heavy fuel oil. A modern de-rated two-stroke at its matched point can sit near 50 percent BTE, with SFOC in the 165 to 170 g/kWh band on its tuned point. Drag that same engine to a badly off-design part load and SFOC can rise into the 180s or higher, which is two to three points of efficiency thrown away as heat. The SFOC curve article walks through why the bowl has the shape it does and how part-load tuning maps reshape it.

The shape of the bowl is set by how the turbocharger and the injection work together across load. At the matched point, the turbocharger delivers the scavenge pressure the cylinder needs for a complete, well-mixed burn, and the injection timing puts peak pressure where the thermal load and the indicated efficiency are both acceptable. Fall well below the match and the single-stage turbocharger can’t keep scavenge pressure up; the air-fuel ratio tightens, combustion gets later and dirtier, and the exhaust temperature can drop toward the acid dew point in the manifold and turbine. Electronically controlled engines fight this with part-load injection maps, advancing the injection and trimming rail pressure to hold combustion quality, which flattens the left side of the bowl somewhat. But a map can only do so much against an air supply that’s fundamentally undersized for the operating point. Re-matching the air system is what actually moves the bowl.

This is also why a power cap alone leaves money on the table. EPL doesn’t move the SFOC bowl; it just forbids you from operating past a ceiling. The matched minimum is still up at the original high-power point. True de-rating moves the bowl to where the ship actually lives. On a Tier II engine the de-rated tune can lean harder on early injection for efficiency, since the NOx budget is looser than Tier III; on a Tier III engine with SCR or EGR fitted, the de-rating match and the NOx-control match have to be solved together, because the after-treatment or recirculation changes the exhaust conditions the turbocharger sees. The bowl, the NOx settings and the air system are one coupled problem, which is exactly why a re-rate is an OEM job rather than a field adjustment.

Re-matching the propeller for the lower service speed

De-rating the engine without touching the propeller is half a job. The propeller fitted for the original design speed is wrong for the new one, and leaving it on forces a compromise that erases part of the engine gain.

The efficient propeller for a slow ship is generally a larger-diameter, lower-rpm propeller. Propulsive efficiency rises as you spread the same thrust over a bigger disc turning slower, because you accelerate a larger mass of water by a smaller velocity increment, which is where the ideal efficiency of a screw comes from. So a vessel re-engined for 16 knots will often want a propeller of larger diameter and lower design rpm than the one it carried at 22 knots, subject to hull aperture clearance and the draft-dependent tip immersion. Where a full re-blade isn’t economic, re-pitching the existing propeller to a lower pitch is the cheaper partial fix: less thrust and less torque per revolution, letting the engine sit at a lower speed without overloading.

The hull and propeller interact, and the de-rated match has to respect both. The propeller demand line on the engine layout diagram is set by the law that absorbed power varies roughly with the cube of shaft speed for a fixed-pitch propeller on a given hull. The naval architect places that demand line with a deliberate light-running margin, typically 4 to 7 percent, so a fouled hull or heavy weather, which both stiffen the propeller and pull the demand line up toward the torque limit, don’t push the engine into its overload region. Get the margin wrong and a slightly fouled ship can’t reach its contract speed without tripping the load-diagram torque limit. The interaction with slow steaming and engine cleanliness is direct here: a fouled hull at low load drives the engine toward the very region de-rating was meant to avoid.

A new propeller also changes the SMCR landing spot. Once the propeller demand line is redrawn for the lower-rpm screw, the engine’s specified point is set at the intersection of that line with the chosen power level. So propeller and engine are designed together, not in sequence; the cylinder bore and stroke selection criteria for a newbuild and the propeller pitch/diameter choice are one coupled optimization.

Re-pitching, the cheaper option, has a specific catch that catches people out. Lowering the pitch shifts the propeller demand line to the right on the layout diagram, toward higher rpm for the same power, because a flatter blade absorbs less torque per revolution and the engine has to spin faster to deliver the same thrust. Done carelessly that can push the demand line toward the engine’s speed limit rather than away from it. The propeller change that actually lands the engine low and left, the one that matches a de-rated SMCR, is usually a larger diameter at lower design rpm, not just a pitch trim, which is why a genuine re-match often means a new propeller rather than a re-pitch. For a fixed-pitch propeller the diameter is bounded by the stern aperture and the need to keep the tips immersed at ballast draft; a controllable-pitch propeller buys flexibility at the cost of hub efficiency and a more complex installation. Each of these trades sits inside the same engine load diagram the engine has to respect, so the propeller designer and the engine builder are looking at the same plot.

The Speed-Power relationship that underpins all of this is the cubic-ish fit between shaft power and ship speed for a given hull. The speed-power cubic fit calculator lets you fit that curve from trial or noon-report data and read off the power demand at a candidate service speed, which is the first number the de-rating exercise needs. From there the slow-steaming fuel-savings calculator turns the speed reduction into tonnes per voyage, the figure that has to justify the yard cost of a re-rate.

Auxiliary blower operation at low load

Below a load threshold the engine’s own turbocharger can’t supply enough scavenge air, and electrically driven auxiliary blowers cut in to keep the cylinders charged. On a two-stroke this isn’t optional: with no intake stroke, the engine depends entirely on scavenge pressure to push exhaust out and fresh air in through the uniflow scavenging ports. Lose scavenge pressure and combustion collapses.

The blowers normally start automatically when scavenge air pressure falls below a set value, around 0.3 to 0.5 bar gauge on many designs, which corresponds to somewhere in the 20 to 35 percent load region depending on the engine and its matching. They stop when the turbocharger has spooled up enough to carry the load alone. The point of de-rating is to move that crossover. An engine matched to a lower SMCR reaches self-sustaining turbocharger operation at a lower absolute power, so at your actual service load the blowers are off and you’re not paying their electrical demand or wearing them out.

That electrical demand isn’t trivial on a ship that steams slowly for weeks. Continuous auxiliary-blower running adds auxiliary engine load, and the auxiliary plant has its own fuel cost; the tech aux engine load side of the energy balance matters when you’re tallying the true saving from a speed cut. A ship that’s mechanically de-rated to live above its blower threshold avoids that parasitic load entirely; a ship merely capped by EPL, still matched high, may run its blowers continuously at the same service speed. That’s another reason the SFOC argument and the blower argument point the same way.

Turbocharger matching is the other half of the constraint. A turbocharger sized for L1 air demand is oversized for a de-rated point; it surges or runs inefficiently at the lower flow. True de-rating re-matches it, sometimes with a smaller frame, a different nozzle ring, or, on multi-turbocharger engines, by permanently sizing the installation for the lower flow. On large engines with two or three turbochargers, cutting one out at sustained low load lets the survivors run nearer their efficient point, with better surge margin and higher scavenge pressure than two units each at half flow.

Turbocharger cut-out is itself a low-cost cousin of de-rating, worth distinguishing. On a two- or three-turbocharger engine the operator can isolate one unit at sustained low load by closing its air and gas-side flaps and letting its rotor coast or lock. The remaining units carry the full exhaust flow and run nearer their efficient point. It’s reversible in minutes when load rises, it needs no class re-rate, and it’s a standard feature on the larger MAN Energy Solutions and WinGD engines. It doesn’t re-match the engine the way a true de-rate does, but it pushes the blower-crossover load down and props up scavenge pressure without any permanent commitment, which makes it the natural companion to an EPL cap on an existing ship that can’t justify a full re-match.

Cold corrosion is the failure mode all of this is fighting. When scavenge pressure and combustion temperature fall together at low load, sulphur in the fuel oxidizes incompletely, sulphuric acid condenses on the cooler parts of the liner, and corrosive wear climbs. The mitigations are well established: hold jacket cooling-water temperature up so the liner wall stays above the acid dew point, match the cylinder-oil base number to the fuel sulphur, tune the feed rate for the load, and run the engine up to a cleaning load periodically. De-rating helps by keeping the engine nearer its efficient point where temperatures stay up, but it doesn’t remove the discipline; a de-rated engine pushed below its own matched range hits the same problem, just at a lower absolute power.

The loss of sea margin and reserve power

Every gain here is bought with a sacrifice, and the sacrifice is reserve power. A ship’s sea margin is the extra installed power, commonly 15 percent over the calm-water trial requirement, that lets it hold schedule in wind, waves and a fouled hull. De-rate the engine and you’ve spent part of that margin permanently. The engine that comfortably overcame a Beaufort 7 head sea at the original rating may now sit close to its torque limit doing the same thing.

This is the difference that makes a power cap attractive for safety. An EPL or ShaPoLi limit is overridable: MEPC.335(76) requires the system to allow the master to recover full power for safe navigation in adverse conditions, with the override logged. A capped ship caught in worsening weather can break the seal and call on the reserve. A de-rated ship has no such reserve to call on, because the power was never built into the matched engine. The injection equipment, the turbocharger, the whole tune assume the lower ceiling.

The operational answer is weather routing. A de-rated ship has to plan voyages with less power in hand, which means earlier departure decisions, more conservative routing around heavy systems, and tighter margins on arrival windows. Just-in-time arrival planning, the kind the JIT economic-speed calculator supports, becomes more than a fuel optimization; it’s part of staying inside the de-rated envelope. The trade is deliberate: you accept reduced heavy-weather capability in exchange for the SFOC and capital saving, and you manage the residual risk operationally. For a liner ship on a fixed string with predictable conditions that’s a good trade; for a tramp going anywhere in any season it needs harder thought.

Class and OEM approval, and the certificate trail

You can’t quietly re-rate a main engine. True de-rating and EPL/ShaPoLi both pull in the classification society, the engine OEM, and the flag administration, and both leave a documentary trail that a port-state control officer can ask to see.

For true de-rating, the OEM owns the new layout point. The builder issues revised project documentation for the de-rated SMCR, and if the change touches anything affecting NOx, the injection equipment, the timing, the turbocharger match, the engine has to be re-certified against the NOx Technical Code. That means a revised NOx Technical File describing the new engine’s adjustable components and their permitted settings, and potentially a new EIAPP certificate, because the EIAPP is issued against the engine as matched. Class reviews and surveys the change. None of this is optional; an engine running outside its approved NOx Technical File settings is non-compliant regardless of how clean it actually runs.

For a power cap, the trail is about the limiter, not the engine. MEPC.335(76) sets out what the verifier checks: the power-limitation system itself, the sealing and tamper-evidence, the alarm and recording functions, the override arrangement, and the on-board management manual. The ship’s EEXI Technical File documents the limited MCR used in the attained-EEXI calculation, and the EEXI verification confirms the limited figure is what the index relied on. If the seal is broken for a weather override, the event is logged and the records are available at survey. The point of the sealing-and-logging regime is exactly that the cap is reversible and auditable: it can be overridden, but it can’t be quietly defeated.

The control-system side matters on electronically controlled engines. On an ME-C type engine, the de-rated matching, the part-load timing maps, and an EPL ceiling all live in software, which makes the change less of a workshop job and more of a re-parameterization plus re-verification. That’s convenient, but it raises the verification bar: the surveyor has to confirm the software settings match the approved Technical File, since nothing physical has visibly changed. The NOx Technical Code under MARPOL Annex VI anticipates this; it defines the adjustable components and their permitted ranges precisely so that an on-board parameter check, not just a teardown, can establish compliance.

There’s a survey-cycle dimension as well. A power cap is verified once at the EEXI survey, then confirmed at the periodical Annex VI surveys; the seal and the records are the evidence. A true de-rate, because it alters the certified engine, folds into the engine’s own survey trail: the revised EIAPP and NOx Technical File travel with the engine for the rest of its life, and any later adjustment within or outside the documented range is judged against them. The asymmetry is deliberate. A cap is designed to be examined and reversed; a re-rate is designed to be permanent, so the certification weight sits where the permanence is.

The economics: newbuild design de-rating versus retrofit

The money case splits cleanly along the newbuild/retrofit line, and that split drives almost every real decision.

On a newbuild, de-rating is nearly free and almost always right when the design speed is modest. You’re specifying the engine from scratch, so ordering a smaller matched engine, or a larger engine matched low with the right propeller, costs little or nothing extra and may even cost less in first price. You get the SFOC bowl placed where the ship will live, the propeller designed for the service rpm, and the turbocharger sized correctly, all from day one. The Fourth IMO GHG Study 2020 documented how widespread slow steaming reshaped fleet fuel demand after 2008; newbuild designs since have largely internalized it, with engines specified at lower SMCR and ships drawn around lower design speeds. The design-speed choice is the lever, and de-rating is just the engineering that follows it.

On an existing ship the calculus inverts. True de-rating means re-engineering injection equipment, possibly the turbocharger, the propeller, plus class and OEM fees and certification: a yard-period cost that only pays back if the ship will steam slowly for years. A power cap, by contrast, is a control-system change and a sealing exercise, cheap enough that it became the default EEXI compliance route for the existing fleet under MEPC.328(76). So the typical retrofit answer is: cap now for EEXI, and only commit to true de-rating, propeller re-blade included, when a major overhaul or re-engining opens the window anyway.

The regulatory driver is what forced the question for everyone. EEXI under MEPC.328(76) put a one-time design-index floor under every existing ship above the size threshold; the operational CII rating under the same Annex VI revision then made annual carbon intensity a graded, reportable number. Together they turned slow steaming from a fuel-price tactic into a compliance obligation. A ship can pass EEXI with a sealed cap, but holding a good CII band year after year needs the operational fuel saving that only a properly matched, de-rated plant, or genuinely disciplined slow steaming, delivers. The fuel-price case and the regulatory case now push in the same direction, which is why de-rating moved from a downturn improvisation to a standing design choice.

Limitations

De-rating engineering carries practitioner caveats that the headline fuel saving hides.

Permanent de-rating sacrifices reserve power and sea margin. The capability to overcome a heavy head sea or a fouled hull at the original rating is gone, and unlike a power cap it can’t be recovered by breaking a seal. Heavy-weather routing and conservative scheduling become part of operating a de-rated ship, not optional refinements.

Re-rating in either direction needs class and OEM involvement. You can’t move the SMCR, change the injection equipment, or alter timing without the OEM’s revised project data, classification-society review, and, where NOx-relevant components change, a revised NOx Technical File and potentially a new EIAPP certificate. Running outside the approved Technical File settings is non-compliant even if the engine runs clean.

The de-rating-versus-cap distinction is easy to blur and shouldn’t be. True de-rating is a permanent re-match that recovers SFOC; an EPL or ShaPoLi cap is a reversible, sealed, overridable power ceiling that leaves the matched SFOC bowl where it was. They satisfy different needs. Treating a cap as if it delivered the SFOC of true de-rating overstates the operational saving; treating de-rating as reversible understates the cost of undoing it.

Auxiliary-blower and turbocharger matching set hard low-load limits. Below the blower threshold the engine depends on continuous electrically driven scavenge air, with its own fuel and maintenance cost; a turbocharger matched for a higher point surges or runs inefficiently at low flow. De-rating moves these thresholds but doesn’t abolish them, and super-slow operation below roughly 25 to 30 percent of even the de-rated MCR reintroduces the same part-load problems, rising SFOC, cold corrosion risk, and the need to lift cylinder oil feed rate, that de-rating was meant to escape.

The SFOC figures, light-running margins, and blower thresholds quoted here are representative of modern slow-speed two-strokes, not values for any specific engine. The matched SFOC curve, the permissible operating envelope, and the certified NOx settings for a given installation come from that engine’s own shop-test data and project guide. Use the engine MCR de-rating calculator, the EPL calculator, and the ShaPoLi calculator for first-pass estimates, then confirm against the OEM documentation and the classification society before committing a rating change.

See also

• Slow Steaming
• Slow Steaming and CII
• Slow Steaming and Engine Cleanliness
• What Is EEXI
• MARPOL Annex VI
• Engine Power and BMEP Relationships
• Specific Fuel Oil Consumption Curves on Marine Engines
• Engine Load Diagram and Operating Envelope
• MAN B&W ME-C Electronic Control Overview

Related calculators

• Engine MCR De-rating Calculator
• EPL (Engine Power Limitation) Calculator
• ShaPoLi (Shaft Power Limitation) Calculator
• Engine Thermal Efficiency from SFOC
• Slow Steaming Fuel Savings per Voyage
• Speed-Power Cubic Fit
• Just-In-Time Arrival Economic Speed