Fleet Supply Sensitivity: Test the Employment Cycle

Key takeaways

• Define available vessel time and the complete cycle represented in the model.
• Recalculate the ratio when cycle duration changes; do not apply a linear shortcut.
• Distinguish aggregate cycle equivalents from a feasible schedule of completed voyages.
• Use the result to identify timing assumptions that need evidence, not to prescribe operating changes.

The calculation below is an original fictional planning model. It contains no actual vessel performance, port timing, cargo intake or market-capacity estimate. Operational feasibility and vessel suitability remain separate assessments for the actual programme.

Define the service being measured

A fleet count describes a population. A service-capacity model asks what that population could accomplish under specified assumptions. To connect the two, define the time available and the cycle of work that consumes it.

The cycle boundary matters. A model covering only a selected sea passage cannot be treated as a complete employment cycle if the commercial question requires other activities before the ship can repeat the service. Identify what is included and excluded rather than using an ambiguous label such as voyage time.

Available time also needs a definition. Do not count the same period as both committed to existing work and available to a new programme. If availability varies between ships, an aggregate model may need a more detailed schedule before it can support a specific commitment.

The model can remain simple for teaching or strategic comparison. Its simplicity should be explicit, and its result should not be presented as a list of actual sailings.

Illustrative ratio sensitivity

ILLUSTRATIVE — invented fleet, time and cargo assumptions. Assume 10 identical model ships, each allocated 60 days to a fictional service. Total allocated vessel time is 600 vessel-days. Assume a standardised model cargo of 50,000 tonnes per complete cycle, solely for this arithmetic exercise.

Define aggregate cycle equivalents as total allocated vessel-days divided by assumed cycle duration. Model service quantity equals cycle equivalents multiplied by the invented cargo per cycle.

Increasing cycle duration from 20 to 24 days is a 20% increase in time. The aggregate service quantity falls by 16.67%, not by 20%, because the model divides by duration. The vessel count remains 10 throughout.

To restore 30 aggregate cycle equivalents at a 24-day cycle with 60 allocated days per model ship, the equation requires 12 ships: 12 × 60 ÷ 24 = 30. This is an arithmetic equivalence inside the fictional model, not a recommendation or evidence that additional ships are available.

Do not confuse cycle equivalents with completed voyages

The ratio pools allocated time across the model fleet. It can be useful for explaining the direction and scale of an assumption change. It does not ensure that each ship can complete the implied work inside a finite schedule.

A fractional cycle equivalent is an analytical quantity. It is not a partly completed cargo delivery that can automatically be counted as a finished service. Starting positions, existing commitments and the reporting cut-off can all matter to a real schedule.

The distinction is particularly important when the decision concerns delivery by a fixed date. An aggregate capacity measure may look adequate while the sequence of complete jobs fails to meet the actual programme. Check the schedule rather than rounding an aggregate result into a promise.

Illustrative finite-window check

ILLUSTRATIVE continuation — an additional restrictive scheduling assumption. Suppose all model ships start an entirely new cycle at the beginning of the same 60-day window, and only cycles fully completed before the window ends count. Ignore all other constraints for this demonstration.

At a 20-day cycle, each ship completes 3 cycles, so 10 ships complete 30 cycles. At a 24-day cycle, each ship completes only 2 full cycles within the window, so 10 ships complete 20 cycles, not the 25 aggregate cycle equivalents from the ratio model.

Even 12 ships at the 24-day cycle complete only 24 full cycles under this synchronized-start assumption. The earlier arithmetic restoration to 30 cycle equivalents therefore does not restore 30 completed cycles inside this particular finite window.

This difference does not make the ratio calculation wrong. It shows that it answers a different question. The complete-cycle model introduces a specific start condition and completion rule that the aggregate ratio did not contain.

Test a coherent cycle change

When adapting the method to a real planning question, explain why the assumed cycle changes. A longer employment period might reflect a changed route, an additional activity or a revised timing estimate. The actual cause determines which other inputs should be reviewed.

Avoid changing duration while silently keeping an incompatible cargo or service assumption. If the proposed alternative changes what the ship does, the comparison may require a different output measure as well as a new duration.

Do not use the sensitivity to prescribe faster operations or shorter safety-related activities. It identifies commercial consequences of assumptions; qualified operational assessment determines what is feasible. A desirable spreadsheet result is not evidence that the corresponding timing can be achieved.

Where the timing estimate is uncertain, retain several labelled cases. A single precise duration can create false confidence when the underlying programme remains conditional.

Separate fleet additions from utilisation assumptions

The example demonstrates that more model ships can offset a longer cycle in one aggregate formula. Real commercial planning needs to establish whether the additional ships exist within the relevant candidate set and can perform the required service during the period.

Do not count speculative additions with the same certainty as confirmed capacity. Similarly, do not assume that every ship in a broad fleet statistic is available to the programme. The fleet definition and the availability assessment remain necessary inputs.

The desk should identify whether the proposed response changes fleet stock, allocation of existing ships or the timing of employment. These are different mechanisms with different evidence requirements and costs.

Use sensitivity to direct the next enquiry

The most useful output is the timing assumption capable of changing the commercial conclusion. If a modest duration revision materially reduces modelled service, investigate the cycle boundary and supporting evidence before refining less consequential details.

If the decision concerns a fixed delivery programme, move from the aggregate model to a feasible schedule. If it concerns broad scenario comparison, retain the ratio with its limitations. Choose the model that answers the actual question rather than treating the most convenient calculation as universally sufficient.

Report both the result and what it does not establish. The analysis should help the desk ask better questions about availability and timing, while leaving actual commitments dependent on verified arrangements.

Common mistakes

Equating a duration increase with the same percentage capacity decrease. Recalculate the ratio rather than applying a linear assumption.

Calling cycle equivalents completed voyages. Test finite-window scheduling where completion dates matter.

Assuming extra model ships are commercially available. Verify the relevant candidate set and actual programme.

Using desired output to justify an operating assumption. Obtain appropriate feasibility assessment instead of working backwards from the preferred result.

Where Zeaclub fits

Zeaclub provides a multi-party shared workspace for owners, charterers, brokers and port agents.

Zeaclub offers digitalised procedures across the voyage workflow.

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