Compute change of trim and draft variation using MCT 1 cm and LCF. Sign convention: +TM = trim by stern.
Enter weights (t) and longitudinal arms from midship (m, +aft). TM = Σ(w × x) Result is sent directly to the TM field in both tabs.
| Total Trim | — | — |
|---|---|---|
| ΔT at AP (stern) | — | |
| ΔT at FP (bow) | — | |
| TA1 (final aft draft) | — | |
| TF1 (final fwd draft) | — | |
Sign convention: Positive TM → trim by stern. ΔTA > 0 means aft draft increases; ΔTF < 0 means forward draft decreases.
Draft changes are distributed proportionally from the LCF:
ΔTA = t × la / Lpp
ΔTF = − t × lf / Lpp
where la = LCF→AP distance, lf = LCF→FP distance.
Trim is the longitudinal inclination of a vessel — the difference between the aft draft and the forward draft. A vessel trimmed by the stern has a deeper aft draft; trimmed by the head means deeper forward. Even keel means both drafts are equal. Precise trim control is essential for fuel economy, propeller immersion, maneuverability, structural loading, and compliance with load line regulations.
The LCF is the centroid of the waterplane area — the point about which the vessel trims when a moment is applied. It is not generally at midship; on most vessels it lies slightly aft of midship. This is why draft changes at the AP and FP are not equal in magnitude when trim is applied.
On this calculator, LCF is entered as a distance from midship, with positive values indicating aft. The distances from LCF to the two perpendiculars are:
Because la > lf when LCF is aft of midship, the aft draft changes more than the forward draft for a given trimming moment — a frequently overlooked practical point.
MCT 1 cm is the trimming moment required to change the vessel's trim by exactly one centimetre. It is a direct function of the vessel's displacement and geometry at any given draught, and is listed in the hydrostatic tables of the vessel's stability information booklet.
When MCT is not available from tables, it can be derived from first principles:
Once the trimming moment and MCT are known, the total change of trim follows directly:
The result is in centimetres. To obtain metres: divide by 100. Sign convention: a positive TM (moment acting aft of LCF) produces trim by stern (+t), so the aft draft increases and the forward draft decreases.
The total trim is not split equally between the two ends. It is distributed in proportion to each perpendicular's distance from the LCF:
Note that ΔTA + |ΔTF| = t (the total trim is preserved), but the individual values are only equal when the LCF coincides exactly with midship.
The final drafts, if initial values are supplied:
In practice, a trimming moment arises whenever a weight is shifted longitudinally or loaded/discharged off-centre. The net trimming moment about the LCF is:
where wi is the weight in tonnes and xi is the longitudinal arm measured from midship (positive aft). The builder sums up to five individual weight–arm pairs and writes the result into the TM field of whichever calculation tab is active.
The visual diagram shows the vessel's hull oriented with the bow (FP) on the left and the stern (AP) on the right — the conventional ship plan view in naval architecture. Key elements: