Draft decides whether a ship can enter a port channel, whether its freeboard is still safe and how much more cargo it can lift. Trim decides how the ship sits in the water: down by the head or too deep at the stern both cost you. This guide shows how draft and trim change when cargo is loaded, with an example you can repeat in the draft and trim calculator.
Draft and trim terms
| Term | Meaning |
|---|---|
| Forward draft (dF) | Draft at the forward perpendicular |
| Aft draft (dA) | Draft at the aft perpendicular |
| Mean draft | (dF + dA) / 2, the draft midway between the perpendiculars |
| Trim | dA − dF; positive means trim by the stern, negative means trim by the head |
| TPC | Tonnes per centimetre of bodily sinkage |
| MTC | Moment to change trim 1 cm, in t·m/cm |
| LCF | Longitudinal centre of flotation; the ship trims about this point |
TPC, MTC and LCF change with draft and are read from the hydrostatic tables. The formulas below hold for weights that are small relative to displacement. For large changes, use the hydrostatics at the new condition or the loading computer.
The three core formulas
- Bodily sinkage, cm = w / TPC.
- Change of trim, cm = w × d / MTC, where d is the weight's distance from the LCF.
- Splitting the change of trim: change aft = change of trim × (LCF distance from the aft perpendicular / LBP); change forward = change of trim − change aft.
Weight forward of the LCF puts the bow down and lifts the stern. Weight aft of the LCF does the opposite.
Worked example: loading 300 t forward of the LCF
Illustrative ship, not a real vessel. LBP 140 m. Initial drafts dF 7.20 m and dA 7.80 m, so trim is 0.60 m by the stern and mean draft 7.50 m. From the hydrostatics: TPC 25.0 t/cm, MTC 180 t·m/cm, LCF 3.0 m aft of midships. We load 300 t of containers with their centre of gravity 40 m forward of midships.
- Distance from the LCF: d = 40 + 3 = 43 m forward of the LCF.
- Bodily sinkage = 300 / 25.0 = 12 cm.
- Change of trim = 300 × 43 / 180 = 71.7 cm by the head.
- The LCF is 70 − 3 = 67 m from the aft perpendicular and 73 m from the forward perpendicular.
- Change aft = 71.7 × 67 / 140 = 34.3 cm (stern rises).
- Change forward = 71.7 × 73 / 140 = 37.4 cm (bow goes down).
| Draft | Initial (m) | Sinkage (m) | Trim effect (m) | Final (m) |
|---|---|---|---|---|
| Forward | 7.20 | +0.12 | +0.374 | 7.69 |
| Aft | 7.80 | +0.12 | −0.343 | 7.58 |
| Trim | 0.60 by stern | −0.717 | 0.12 by head |
The ship that was trimmed by the stern now sits slightly by the head. On many ships that is unwanted, because it can affect steering and manoeuvring and reduces propeller immersion. The planner has two options:
- Move the weight to the LCF. The same 300 t placed directly above the LCF only sinks the ship 12 cm: dF 7.32 m and dA 7.92 m, trim still 0.60 m by the stern.
- Choose a position for a target trim. To finish on an even keel the ship needs 60 cm of trim change by the head. The distance is d = 60 × 180 / 300 = 36 m forward of the LCF, or 33 m forward of midships.
Mean of means and the hog and sag correction
A hull that is bent means the forward-aft mean no longer represents the true draft. If the midship draft (mean of port and starboard) is greater than the mean of dF and dA, the ship is sagging. If it is smaller, the ship is hogging.
Example draft survey readings, already corrected to the perpendiculars: dF 7.69 m, dA 7.58 m, midships 7.68 m.
- Forward-aft mean = (7.69 + 7.58) / 2 = 7.635 m. Midships is 4.5 cm deeper, so the ship is sagging.
- Mean of means = (dF + dA + 2 × dM) / 4 = (7.69 + 7.58 + 15.36) / 4 = 7.66 m.
- Mean of mean of means (quarter mean) = (dF + dA + 6 × dM) / 8 = (7.69 + 7.58 + 46.08) / 8 = 7.67 m.
The quarter mean is what a draft survey uses to read displacement from the hydrostatic tables. Try it in the draft survey calculator.
Reading draft marks
Draft marks are painted at the bow, stern and midships on both sides. On metric marks the figures are 10 cm high and 10 cm apart, and the bottom edge of each figure marks the draft it shows. On imperial marks the figures and the spacing are 6 inches. Points to remember:
- marks are not always exactly at the perpendiculars, especially forward, so readings need a correction;
- read both sides and average them to remove the effect of any list;
- in a swell, average the highest and lowest water level at the mark;
- harbour water density affects draft; fresh water makes the ship sit deeper.
Propeller immersion, UKC and air draft
Propeller immersion. In ballast the aft draft must be deep enough to immerse the propeller properly. Too shallow, and efficiency drops while vibration and the risk of propeller racing rise. The forward draft also needs to be deep enough to limit slamming. Minimum drafts are given in the stability booklet or the ship's instructions.
UKC. Under keel clearance is the distance from the keel to the channel bottom. The deepest draft, plus squat under way and allowance for tide, must meet the UKC required by the port authority or local harbour master.
Air draft. The distance from the waterline to the highest point of the ship. A light ship, or one trimmed by the stern, has more air draft at some points, which can limit passage under bridges or the reach of terminal cranes. Stern trim and tall deck stacks also affect the view from the bridge; see bridge visibility and line of sight.
Common mistakes when working out trim
- Measuring from midships instead of the LCF. In the example above, the 3 m difference changes the trim change by about 5 cm.
- Splitting the trim change equally forward and aft. An equal split is only right when the LCF is exactly at midships.
- Using TPC and MTC from the old draft after a large amount of cargo has been loaded.
- Mixing up the trim sign. Check whether the software treats trim by the stern as positive or negative.
- Forgetting the tanks. Burning fuel from tanks near the stern or bow also changes trim on passage, so arrival trim can differ from departure trim.
Trim in cargo planning
Planners rarely load a single weight. The final trim is the result of the whole plan: container blocks in each bay, fuel and ballast. Good practice is to aim for the modest stern trim the ship recommends, check drafts against UKC and the load line, then confirm that GM is still adequate; see metacentric height (GM) explained and ship stability basics for planners.
STOWAGE.ID Vessel Stowage calculates drafts and trim for every planned condition from shore as decision support. The vessel's class-approved loading computer and stability booklet remain the official reference.