The loading instrument: approved software, honest inputs
1The lesson in plain words
The loading instrument calculates draughts, stability and stresses in seconds. It is only as good as its approval, its test conditions and the figures you type in.
What it is
A loading instrument is approved software, with its computer, that calculates draughts, trim, stability and, where fitted, hull shear forces and bending moments for a loading condition. It does not replace the approved stability booklet and loading manual: it uses approved ship data and must reproduce approved test results within accepted tolerances. The ship’s classification society approves it for that ship. Keep the approval and the user manual on board.
Four types
IACS UR L5, a stability-software requirement from the International Association of Classification Societies, defines four types. Type 1 calculates intact stability only. Type 2 calculates intact stability and checks damage stability against a limit curve, a required minimum GM or maximum KG. Type 3 calculates intact stability and damage stability with pre-programmed damage cases. Type 4 calculates damage stability for an actual flooding case, to support safe return to port.
Test conditions
Approval includes test conditions, loading conditions with known results. Run them after software or hardware changes. The master must verify at least one approved condition at each annual survey and retain results for class verification. At special survey, run all approved conditions in the surveyor’s presence. Results must meet approved tolerances. If they fail, stop using it and report the fault. Use an approved alternative covering all required stability and strength checks. If the instrument is mandatory, obtain flag/class acceptance before sailing without it. Demonstrate compliance before sailing; arrange repair.
Garbage in, garbage out
Check every input: lightship and constant; tank soundings and densities; free surface applied to slack tanks as the booklet says (actual or maximum); cargo weights and positions per hold. Use each field’s stated units. When converting volume to tonnes, use the actual liquid density in t/m³. One wrong digit can turn a failing condition into a passing one.
The draught check
Compare calculated draughts with corrected observations. First check readings, trim, hull deflection, water density and matching draught references. Once other causes are excluded, a small mean-draught difference may indicate unaccounted weight. TPC means tonnes per centimetre of immersion: difference in cm × TPC estimates extra tonnes. Calculated 8.62 m, observed 8.70 m: 8 cm × 25 t/cm suggests about 200 t extra. Reconcile the figures before sailing.
2Key data
- Type 1
- Intact stability only
- Type 2
- Intact, plus damage stability checked by a limit curve
- Type 3
- Intact, plus pre-programmed damage cases calculated
- Type 4
- Damage stability for an actual flooding case (return to port)
- Draught check
- Small draught difference (cm) × TPC ≈ unexplained weight (t)
3Key points
- 1
The instrument uses approved ship data and agrees within approved tolerances; it does not replace the booklet.
- 2
UR L5 Types: 1 intact; 2 + limit curve; 3 + pre-programmed damage; 4 actual flooding case.
- 3
Failed test: stop using it, report the fault; use an approved alternative covering all required checks or delay departure.
- 4
Check inputs: lightship, constant, soundings, densities, free surface, units.
- 5
Reconcile readings, density, trim, deflection and draught references first; remaining cm × TPC estimates unexplained tonnes.
4Frequent mistakes
Wrong: The computer says the condition passes, so the ship is safe.
Right: It passes only if the inputs are right. Check soundings, densities, weights and free surface, and compare calculated with observed draughts.
Wrong: After a software update the instrument is still approved, so carry on.
Right: Run the approved test conditions after any change. If the results do not match, do not use it until it is corrected.
5Practice case & exam keys
After loading, mean draught is 8.62 m calculated and 8.70 m observed. Readings, trim, hull deflection, water density and draught references have been checked and reconciled. TPC is 25 t/cm. The stability result passes with a small margin. What do you do?
Show the answer
Report it. The remaining 8 cm difference suggests about 8 × 25 = 200 t of unaccounted weight. Recheck tank soundings, cargo figures and the constant. If the weight is high up, real GM may be lower than computed and the small margin may be gone. Do not sail until the difference is explained and the condition recalculated.
Exam keys
- Approved for the ship; agrees within approved tolerances; does not replace the booklet.
- UR L5 Types 1–4: intact only; limit curve; pre-programmed damage; actual flooding case.
- Run test conditions after changes. After reconciling readings, density, trim, deflection and draught references, small mean-draught difference (cm) × TPC estimates unexplained tonnes.
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