Stress tables, permissible limits and the loading instrument
1The lesson in plain words
Your loading manual and loading instrument show whether shear force and bending moment stay within permissible limits at every station, in port and at sea.
The loading manual
The approved loading manual describes the loading conditions the ship was designed for, the permissible still water SF and BM, and local limits such as tank top loads in t/m². Older or smaller ships may use stress tables: you multiply each weight by a factor for its compartment, add them up, and compare the result with a permitted value.
Stations and limits
Limits are set at reading points, usually at transverse bulkheads and amidships. Use the positive or negative SF limit and the hogging or sagging BM limit that matches the result. Compare magnitudes to find the percentage of the limit used. Many ships have higher harbour limits for calm water in port and lower seagoing limits, because waves add to the still water values at sea.
The loading instrument
A loading instrument is an approved computer program that calculates SF and BM at the reading points, and usually stability too. IACS UR S1 generally requires a loading manual from 65 m and a loading instrument for Category I ships from 100 m; check your ship’s applicable class requirements. SOLAS XII/11 requires one on bulk carriers of 150 m and over. Stability software is approved under IACS UR L5.
Trust but check
Enter every tank, hold and constant correctly: one missing ballast tank changes everything. Run the approved test conditions from its manual and compare the results; class checks this at annual surveys. Watch the units: many programs show kN and kN·m. A green final condition is not enough; check every intermediate stage of the sequence.
Over the limit
If any station exceeds 100 % of its limit, change the plan before you continue: alter the sequence, the pour sizes or the ballast. Never sail with seagoing limits exceeded just because the harbour limits were met. Record the final values in the cargo or stability records.
2Step by step
Checking stresses before and during loading
- 1
Run the program’s approved test condition and compare it with the manual.
Shows the program and hull data still match the approved values.
- 2
Enter and verify the actual planned departure condition: every hold, tank and constant.
Missing items are the commonest cause of a wrong result.
- 3
Check SF and BM percentages at every reading point against the seagoing limits.
The departure condition must be fit for sea, not just for the berth.
- 4
Check each intermediate cargo and ballast stage against approved harbour limits where applicable; otherwise use seagoing limits.
Peak stresses often occur part-way through, not at the end.
- 5
If any value exceeds 100 %, re-plan the sequence, pour sizes or ballast.
Changing the plan is cheaper than cracked plating.
- 6
During loading, compare actual draughts and quantities with the plan and re-run.
Shore figures and ballast rates drift from the plan.
3Key data
- Result
- % of permissible SF and BM at each reading point
- Example
- 18,400 t ÷ 23,000 t limit = 80 %
- Harbour vs sea
- Harbour limits higher; seagoing limits apply on departure
- IACS UR S1
- Generally: manual ≥65 m; instrument for Category I ≥100 m.
- SOLAS XII/11
- Loading instrument on bulk carriers ≥ 150 m
- IACS UR L5
- Approval of onboard stability software
4Key points
- 1
At each reading point, use the matching signed SF limit and hogging or sagging BM limit.
- 2
Check every intermediate stage, not only the final condition.
- 3
Departure must meet seagoing limits; harbour limits apply only in port.
- 4
Verify the loading instrument with its approved test conditions.
5Frequent mistakes
Wrong: If the final condition is within limits, the sequence is safe.
Right: Peaks often occur part-way through loading or discharge. Check every stage of the sequence.
Wrong: Within harbour limits means fit to sail.
Right: Harbour limits allow for calm water only. Before departure every station must be within the lower seagoing limits.
Wrong: The loading instrument cannot be wrong.
Right: It is only as good as its inputs and data. Check tank entries, units and the approved test conditions.
6Practice case & exam keys
At the final stage the loading computer shows a BM of 2,185,000 t·m at frame 120. The seagoing limit there is 1,900,000 t·m and the harbour limit 2,470,000 t·m. Departure is in two hours. What do you do?
Show the answer
Frame 120 uses 115 % of its seagoing limit and 88.5 % of its harbour limit. This station passes the harbour BM check only. Do not depart. Re-plan cargo or ballast; check SF and BM at every station and correction stage against approved harbour limits where applicable, otherwise seagoing limits. Continue only if all applicable limits are met throughout. Verify actual inputs and record the revised departure condition, meeting every seagoing limit, before sailing.
Exam keys
- % used = magnitude of result ÷ magnitude of matching limit × 100.
- Use approved harbour limits where applicable in port; seagoing limits for departure and passage.
- UR S1: loading manual and instrument; SOLAS XII/11: bulk carriers ≥ 150 m.
- Run the approved test condition; check intermediate stages.
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