CII Corrective Trajectory Projector

What this projector returns

A CII corrective trajectory is the year-by-year forward path of a ship’s attained CII across its corrective action plan period, read against the annually tightening required CII line to find the year the ship returns to a C rating.

After a ship is rated D for three consecutive years, or E in a single year, Regulation 28 of MARPOL Annex VI obliges the owner to file a SEEMP Part III corrective action plan that sets out how the ship returns to at least C. This page takes the improvement that plan assumes and projects the result. You give it the attained and required CII at the trigger year, the combined savings the measures are expected to deliver, the year they go live, and the rate at which the required line tightens. It steps the attained line down in the implementation year, holds it there, tightens the required line year by year, and reports the ratio and the rating for each year of the plan horizon. The headline output is the year the ship is expected to cross back to a C rating, or a flag that it never does within the horizon.

The distinction from the CII 3-year corrective plan projector matters and is covered in full below. That tool solves the inverse problem: given a target of C by the end of a plan horizon, it returns the annual reduction the ship must achieve. This tool runs the problem forward: given an assumed improvement, it shows the path and the year of return. Use the 3-year tool to size the gap; use this one to test whether a specific measure package closes it, and when.

How to use this calculator

1. Select the ship type; this sets the MEPC.354(78) rating boundaries (d-vectors) used to place the ratio on the A to E scale.
2. Enter the attained CII at the trigger year, in gCO2 per capacity-mile, from the year that pulled the trigger.
3. Enter the required CII at the trigger year, from the CII required calculator or the DCS verifier’s statement.
4. Select the trigger year, the calendar year of the third consecutive D or the single E.
5. Enter the combined CII improvement the corrective measures deliver, as one non-overlapping percentage.
6. Select the implementation year, the first year the full measure set is operational.
7. Set the required-CII annual tightening rate; read the year-by-year table and the year of return to C.

The trajectory table shows, for each year, the attained CII held or stepped, the required CII tightened to that year, the ratio, and the resulting A to E letter. The four result cards give the year of return to C, the attained CII at the end of the plan, the final rating, and a feasibility verdict. The attained input is the verified operational figure the CII attained calculator builds from fuel and distance; the required input is the reference-line value reduced by the year’s Z factor, from the CII required calculator. The combined improvement is the non-overlapping figure from the SEEMP combined measures calculator, not the arithmetic sum of overlapping measures.

Formula, assumptions, and limits

Formula

The attained line is held at the trigger-year value until the implementation year, then cut once by the combined savings and held:

The required line tightens from the trigger year by the annual reduction factor:

The rating for the year is the ratio placed on the ship-type d-vector:

Derivation

The required CII is the IMO reference line $a \cdot \text{Capacity}^{-c}$ reduced by the year’s factor Z, so $\text{CII}_r(y) = a \cdot \text{Capacity}^{-c} (1 - Z_y)$. The reference line is fixed for the ship, so the year-on-year change is driven entirely by Z. This tool models that change as a constant rate $r$ from the trigger year, a simplification: the real Z is a published schedule, not a single rate. The attained line carries no regulation-driven trend; it moves only when the operator changes the ship, so the model holds it flat until the measures land, applies the single combined cut $s$, and holds again. The rating is the MEPC.354(78) boundary test: divide attained by required, locate on the d-vector.

Assumptions

• The combined savings $s$ are realized in full from the implementation year and persist unchanged for the rest of the horizon, with no ramp and no decay.
• The required line tightens at a single constant rate $r$. The true factor is a schedule (see Regulatory basis); a constant $r$ understates the 2027 to 2030 tightening.
• The reference line, and therefore the ship type and capacity, do not change over the horizon.
• The attained CII does not drift between the trigger year and the implementation year. A ship that worsens while the plan is prepared starts the recovery from a worse point than modeled.

Worked example

Take a bulk carrier triggered in 2024 with an attained CII of 6.50 gCO2 per capacity-mile and a required CII of 6.00 for that year. The starting ratio is 6.50 / 6.00 = 1.083, which is above the bulk carrier d3 of 1.06 and below d4 of 1.18, so the ship sits at D. The plan assumes a combined improvement of 12 percent landing in 2025, so from 2025 the attained line steps to 6.50 x (1 - 0.12) = 5.72 and holds. Project the required line at a constant 5 percent a year from the 2024 base of 6.00, and the path runs:

The ratio first falls to 1.06 or better in 2025, at 1.004, so the projected year of return to C is 2025, the implementation year itself. The table also makes the second lesson plain: the C does not hold. By 2027 the tightening line pushes the ratio back to 1.112 and the ship is a D again, because a flat post-implementation attained line cannot keep pace with a required line that keeps falling. A one-step improvement buys a return year, not durable compliance.

Two checks the reader can run by hand. The 2026 required value is 6.00 x 0.95^2 = 6.00 x 0.9025 = 5.415, and 5.72 / 5.415 = 1.056, the C boundary. Run the same path with the true Z schedule instead of the constant 5 percent and the required line falls faster after 2026, since Z steps by 2.625 points a year to 21.5 percent by 2030; the ratio crosses out of C a year sooner. The constant-rate result is the optimistic case.

Edge cases and limits

If the combined savings are too small or arrive too late, the ratio never reaches the C boundary within the horizon and the tool reports the plan as insufficient rather than inventing a return year. A savings figure entered as the arithmetic sum of overlapping measures, rather than the non-overlapping combined figure, overstates the cut and returns an optimistic year. The horizon is finite, so a measure package that returns the ship to C only in year seven or later reads as a failure here even though it succeeds later. Because the attained line is flat after implementation, the tool deliberately does not model the realistic case of a multi-year measure rollout or continued operational improvement; for that, project successive runs with the attained value re-based each year.

Regulatory basis

The corrective action plan obligation is MARPOL Annex VI Regulation 28, in force since 1 January 2023 under MEPC.328(76), with the SEEMP Part III framework in MEPC.347(78) as updated by MEPC.376(80). The required line and its coefficients are MEPC.337(76). The reduction factor Z is the schedule of MEPC.338(76) for 2023 to 2026 (5, 7, 9, 11 percent) extended by MEPC.400(83), adopted at MEPC 83 on 11 April 2025, for 2027 to 2030 (13.625, 16.25, 18.875, 21.5 percent), a 2.625-percentage-point annual step. The rating boundaries are the current G4 d-vectors of MEPC.354(78), the 2022 amendment revising the original MEPC.339(76) and the ro-ro values. Corrective-plan content guidance sits in the SEEMP guidelines circular issued with the 2021 amendments; confirm the live circular reference before citing a number.

Common errors

1. Treating $r$ as 2 percent a year. The post-2026 line does not tighten at 2 percent; Z steps by 2.625 percentage points a year from 2027 to 21.5 percent by 2030. A constant 2 percent badly understates the tightening and returns a return year that the real schedule will not honor.
2. Summing overlapping measure savings. Slow steaming and weather routing both attack speed-related consumption; their separate percentages do not add. Enter the non-overlapping combined figure from the SEEMP combined measures calculator.
3. Reading the return year as durable. The C reached at implementation can revert as the required line falls under a flat attained line, exactly as the worked example shows. The return year is the first crossing, not a standing grade.
4. Confusing this with the gap-sizing tool. This tool takes an assumed improvement; the 3-year corrective plan projector returns the improvement needed. Running this one with a guessed savings figure to find the requirement is the wrong tool for the question.

When to use this versus the 3-year corrective plan projector

The two remediation tools answer opposite questions and feed each other. The CII 3-year corrective plan projector is the gap-sizer: it takes the current attained and required CII and a plan horizon, and returns the annual CII reduction the ship must hit to reach the C boundary by the end of the plan. It is the right first step, because it tells the owner how big the problem is before any measure is chosen. This trajectory tool is the second step. Once the SEEMP combined measures calculator has turned a candidate measure package into one combined savings percentage, this page tests that package: it steps the attained line down by that figure, tightens the required line forward, and shows whether and when the ratio crosses back into C, plus what happens in the years after.

In practice the workflow runs gap, package, path. Size the gap with the 3-year tool, build a package whose combined savings meets or beats the required reduction, then run it here to confirm the year of return and to see how long the C holds against the tightening line. If the path here shows the ship sliding back to D within two years, the package is a one-year fix and the plan needs a second improvement step the SEEMP Part III document should already name. The 3-year tool sizes; this tool schedules.

Reading the rating bands

The ratio is placed on the bulk carrier d-vector, the current G4 boundaries of MEPC.354(78). These differ by ship type, so the same ratio is a different letter on a tanker or a ro-ro passenger ship; the values below are for a bulk carrier.

A bulk carrier crosses from C to D at a ratio of 1.06; a tanker not until 1.08, and a ro-ro passenger ship not until 1.14, because the CII rating calculator carries the full d-vector set per type. The bands are why a corrective trajectory drawn for one ship cannot be read across to another without re-fitting the boundaries, and why a charter clause that quotes a recovery ratio must name the ship type.

Further reading

• CII corrective action plan
• SEEMP I, II and III
• CII 3-year corrective plan projector
• CII rating calculator
• CII attained calculator
• CII required calculator

Frequently asked questions

What is a CII corrective trajectory?

It is the forward path of a ship's attained CII over its corrective action plan period, plotted year by year against the required CII line, which tightens annually, and the A to E rating boundaries. The trajectory takes an assumed combined improvement from the planned measures, steps the attained line down in the implementation year, and shows where the resulting ratio lands each year so an owner can see the year the ship is expected to return to a C rating.

Which year will my ship return to C?

The first year, at or after the implementation year, in which the attained-to-required ratio falls to the C boundary or better. For a bulk carrier that boundary is a ratio of 1.06, since the bulk carrier d-vector is 0.86, 0.94, 1.06, 1.18 under MEPC.354(78). The return year moves earlier as the combined improvement rises and the implementation year is pulled forward, and later as the required line tightens under it; a C reached one year is not guaranteed to hold the next, because the line keeps moving.

How fast does the required CII tighten?

The required line is the reference line reduced by an annual factor Z that grows on a fixed schedule, not a single constant rate. Z is 5 percent below the reference line in 2023, 7 percent in 2024, 9 percent in 2025, and 11 percent in 2026 under MEPC.338(76), then steps up by 2.625 percentage points a year to 13.625, 16.25, 18.875, and 21.5 percent across 2027 to 2030 under MEPC.400(83), adopted at MEPC 83 on 11 April 2025. A constant-rate projection understates the post-2026 tightening.