Trim is the difference between the forward and aft drafts, showing whether a ship floats deeper by the head or by the stern. It is set by the longitudinal position of weights relative to the centre of flotation, the point about which the ship trims.

MCTC is the moment to change trim one centimetre, a hydrostatic value in tonne-metres per centimetre. The change of trim in centimetres equals the trimming moment, the weight times its distance from the centre of flotation, divided by MCTC.

How is list angle calculated?

Moving or loading a weight off the centreline shifts the centre of gravity sideways. The list angle satisfies tan(list) equals the heeling arm divided by GM, where the heeling arm is weight times transverse distance divided by displacement. This tool uses that relation.

What is the centre of flotation?

The centre of flotation is the centroid of the waterplane, the longitudinal pivot about which a ship changes trim. A weight added exactly at the centre of flotation changes draft evenly with no trim, while the same weight further forward or aft produces trim.

Does this give the final drafts?

It gives the change of trim and the list from a single weight operation, not absolute drafts. Apply the trim change to your current drafts, splitting it forward and aft about the centre of flotation, and use the list to check angle of loll and deck-edge immersion limits.

What is the Ship Trim & List Calculator?

Compute change of trim from MCTC and a weight's longitudinal shift from the centre of flotation, plus the resulting list angle from a transverse shift and GM. Uses LBP, TPC, and MCTC ship parameters. Chief officers and cargo planners use it during port operations. It runs free in your browser on Gera Tools, with nothing uploaded.

Ship Trim & List Calculator

Name: Ship Trim & List Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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During cargo work the duty officer must know how each lift changes the ship’s attitude before committing to it. This calculator takes the standard hydrostatic parameters and a weight’s longitudinal and transverse offsets to return the change of trim and the resulting list — the two numbers that keep a vessel safe and within draft limits at the berth.

How it works

Trim change comes from the longitudinal trimming moment divided by MCTC, while list comes from the transverse heeling arm against GM:

trimming moment   = weight × longitudinal distance from CoF
change of trim    = trimming moment / MCTC        (centimetres)
heeling arm       = weight × transverse distance / displacement
tan(list)         = heeling arm / GM
list angle        = atan(heeling arm / GM)        (degrees)

A weight placed forward of the centre of flotation trims the ship by the head; aft of it, by the stern. A weight off the centreline heels the ship toward that side until the righting moment from GM balances the heeling moment.

Understanding MCTC and TPC

MCTC (Moment to Change Trim one Centimetre) is a hydrostatic property found in the ship’s stability booklet for the current displacement. It tells you how many tonne-metres of trimming moment produce one centimetre of change in trim. At light displacement MCTC is smaller, so the same weight movement produces a larger trim change than it would at full load.

TPC (Tonnes Per Centimetre immersion) is used when you add or remove weight rather than shift it. Adding weight at the centre of flotation sinks the ship bodily at one centimetre per TPC tonnes, with no trim change. This tool covers shifts and additions whose distance from the centre of flotation drives trim.

Worked example

Loading 200 tonnes at a point 30 m forward of the centre of flotation, with an MCTC of 180 tonne-metres per centimetre:

Trimming moment = 200 × 30 = 6,000 t·m
Change of trim = 6,000 / 180 ≈ 33 cm by the head

If that same parcel sits 4 m to port of the centreline on a vessel with 20,000 tonnes displacement and a GM of 1.2 m:

Heeling arm = (200 × 4) / 20,000 = 0.04 m
List angle = atan(0.04 / 1.2) ≈ 1.9° to port

A GM of only 0.4 m on the same ship would produce a list of atan(0.04 / 0.4) ≈ 5.7° — a significant difference that illustrates why a low GM is dangerous in port.

Distributing trim change to individual draft marks

The change of trim calculated above is the total change forward plus aft. To find what each draft mark reads after the operation, split the trim proportionally:

Change of aft draft = change of trim × (distance of aft mark from CoF) / LBP
Change of fwd draft = change of trim × (distance of fwd mark from CoF) / LBP

If the CoF is aft of amidships, a weight placed forward trims more at the forward mark than at the aft mark. Checking both against the permitted arrival and departure draft limits is a required step before signing the cargo plan.

Tips

Keep GM positive and realistic — a small GM produces a large list for the same offset, so verify stability data before trusting a tight angle. For multi-cargo operations, calculate the cumulative trim and list after each major lift rather than only at completion, and check that no intermediate condition exceeds the vessel’s stability criteria or maximum list angle under the Flag State’s requirements.