"What was the method of calculation of longitudinal stress on your last vessel?"
This question clearly demonstrates the increasing expectations of shipping companies when it comes to combining theoretical knowledge with practical proficiency in cargo and stability software. In real-world operations, the correct assessment of longitudinal stress is crucial to maintaining the structural integrity of large container vessels—especially those exceeding 250 meters in length
1. What is Longitudinal Stress?
Longitudinal stress refers to the stress along the ship’s longitudinal axis caused by the combination of cargo weight, ballast water distribution, fuel consumption, and environmental forces such as waves.
There are two typical deformation conditions:
- Hogging: When the bow and stern are supported by wave crests, causing the midship to sag downward.
- Sagging: When the midship is supported by a wave crest, causing the bow and stern to drop below the midship level.
Excessive bending moment or mismanaged stress levels can result in structural deformation, cracking, or even failure of the hull girder.
2. Methods of Longitudinal Stress Calculation
Depending on the vessel design and available documentation, two main methods are commonly used in loading software:
a. Bonjean Table Method
This is the standard method for modern vessels. Bonjean curves provide sectional area and moments at various frame stations along the hull. These are used to compute:
- Still water bending moment
- Section modulus
- Resultant longitudinal stress:
Referenced Standards and Class Rules:
- IACS Recommendation No.34 – Longitudinal Strength Standard
- SOLAS Chapter II-1, Reg. 5-1 – Longitudinal Strength
- DNV-RU-SHIP Pt.3 Ch.1 Sec.4 [203]
b. S-Value (Simple Value) Method
Used mainly on older or converted vessels where Bonjean data is incomplete or unavailable. It applies pre-calculated simple section modulus values for approximate bending moment and stress estimation. Accuracy is limited and not preferred on modern tonnage.
3. Loading Software and Calculation Options
Popular loading computer systems such as CASP, SEACOS MASC3, ShipManager-88, and TSB Supercargo are capable of performing automatic longitudinal strength calculations, including:
- Still Water and Total Bending Moment
- Shear Forces
- Longitudinal Stress at reference stations
For example:
- In CASP, the vessel definition module allows selecting between Bonjean Table data and Simple S-Value data depending on the vessel’s loading manual.
These programs are aligned with class-approved limits and will issue warnings when stress values exceed structural tolerances defined by the builder or classification society.
Training Support at BCC-SMR Marine
At BCC-SMR Marine, we support deck officers and fleet operators by providing access to industry-standard loading software, comprehensive technical training, and expert guidance — all aligned with the latest IMO and Class requirements.
We offer hands-on access to major loading software widely used by leading shipping lines such as EVERGREEN and WAN HAI LINES, including:
- CASP
- SEACOS MASC3
- ShipManager-88
- TSB Supercargo
We also welcome deck officers from other companies to join our free training sessions on loading software. All programs are completely free of charge and aim to enhance safety, competence, and operational readiness across the maritime industry
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BBC – SMR Marine
Manager – Capt. CAN SON DUNG
+84 0921755555
