A vessel must have strength to with stand the stresses placed on it by large seas, cargo weights, and cargo distribution that is sometimes less than optimum. The first and important requirement in the construction of a vessel is the design. Designs are reviewed by ABS and approved if they measure up to the classification society's standards of safe design. Materials must meet approved specifications, and the construction standards of the building yard must be such that the completed vessel is free of discrepancies that may be do to poor workmanship.
A ship's hull has some longitudinal flexibility built into it; otherwise it would break. But the amount of flexibility is limited by the strength ofthe structure. There must be a compromise between flexibility and strength to produce a ship that is safe under the most adverse conditions that can be expected at sea. Also, loading must be such that the safety of the ship is not compromised.
Consider stresses placed on a vessel the following terms should be known:
Load: the total force in pounds or tons, acting upon a structure.
Stress: the force per unit area, pounds or tons per square inch, acting on a structure. Stress may be compressive (main deck of a sagged ship), tensile (main deck of a hogged ship), or shearing (tendency of one part to slide over another when two forces act in parallel and opposite directions vertically at the boundary between an empty hold and one that is heavily loaded, or horizontally at a butt strap at a point of maximum bending moment)
Strain: a distortion caused by stress.
