Torsional stress occurs due to what relative movement?

• A. Propeller thrust
• B. Weather deck moving faster than the keel
• C. Hull twisting due to wave impact
• D. Deck bending downward

Answer: (B)

Torsional stress arises when parts of the hull twist relative to each other around the ship’s longitudinal axis. If the weather deck moves faster than the keel, the upper portion of the hull and the lower portion try to rotate differently, creating a twisting moment that is felt as torsion along the structure. This direct relative motion between the deck and keel is the classic cause of torsional stress in this context.

The other scenarios describe bending or localized torque effects rather than the twisting of the hull due to differential vertical movement. For example, deck bending downward is a bending stress, while hull twisting from waves is a form of torsion but not as cleanly described as the deck vs keel speed difference, and propeller thrust mainly affects the shaft and propeller system rather than the hull’s torsional twist.