Aux 2 Unit 16 Stress on hull Q5 Answer

With reference to longitudinal stresses in a vessel’s hull:
(a) state the cause of the stress;
(b) state the areas where the stress is a maximum;
(c) describe the structure that resists the stress.

Longitudinal stresses arise because the vessel acts as a beam supported by buoyancy and loaded by its own weight and cargo.
When the distribution of buoyancy and weight along the vessel’s length is unequal, a bending moment is produced in the hull.
This causes the ship to either:
- Hog – when the midship region is lifted by buoyancy (wave crest amidships).
- Sag – when the bow and stern are lifted while the midship is unsupported (wave trough amidships).
These bending moments create longitudinal tensile and compressive stresses along the hull girder.

(Explanation of unequal buoyancy/weight + hogging/sagging = 3 marks)

In hogging:
- The deck plating amidships is in tension, while the bottom shell plating is in compression.
In sagging:
- The deck plating amidships is in compression, while the bottom shell plating is in tension.
Therefore, maximum longitudinal stresses occur at the extreme fibres of the hull girder — i.e. the deck and bottom plating amidships.

(Identification of tension/compression zones = 3 marks)

The hull is designed to act as a longitudinal girder, and the following structures resist the bending stresses:

Deck plating and deck girders – resist tensile or compressive stresses at the upper part of the hull during hogging/sagging.
Keel and bottom shell plating – resist opposite stresses to the deck (compression in hogging, tension in sagging).
Longitudinal frames, stringers, and girders – connect the deck and bottom structures, providing continuity and stiffness along the vessel’s length.
Longitudinal bulkheads – contribute to the longitudinal strength, particularly in tankers and larger vessels.

(Each point with function explained = 1 mark each = 4 marks)