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Topic in naval architecture

The strength of ships is a topic of key interest to naval architects and shipbuilders. Ships which are built too strong are heavy, slow, and cost extra money to build and operate since they weigh more, whilst ships which are built too weakly suffer from minor hull damage and in some extreme cases catastrophic failure and sinking.

The total load on a particular section of a ship's hull is the sum total of all primary, secondary, and tertiary loads imposed on it from all factors. The typical test case for quick calculations is the middle of a hull bottom plate section between stiffeners, close to or at the midsection of the ship, somewhere midways between the keel and the side of the ship.

Modern ships are, almost without exception, built of steel. Generally this is fairly standard steel with yield strength of around and tensile strength or ultimate tensile strength (UTS) over
Shipbuilders today use steels which have good corrosion resistance when exposed to seawater and which do not get brittle at low temperatures (below freezing). Many ships endure cold winter storms, and the use of some older ship steels that were not tough enough at low temperature caused ships to crack in half and sink in the Atlantic during World War II. The benchmark steel grade is ABS A, specified by the American Bureau of Shipping. This steel has a yield strength of at least , possesses an ultimate tensile strength of , and must elongate at least 19% in an long specimen before fracturing and 22% in a 2 inch (50 mm) long specimen. A safety factor above the yield strength has to be applied, since steel regularly pushed to its yield strength will suffer from metal fatigue. Steels typically have a fatigue limit, below which any number of stress load cycles will not cause metal fatigue and cracks/failures. Ship design criteria generally assume that all normal loads on the ship, times a moderate safety factor, should be below the fatigue limit for the steel used in their construction. It is wise to assume that the ship will regularly operate fully loaded, in heavy weather and strong waves, and that it will encounter its maximum normal design operating conditions many times over its lifetime. Designing underneath the fatigue limit coincidentally and beneficially gives large (factor of up to 6 or more) total safety factors from normal maximum operating loads to ultimate tensile failure of the structure. But those large ultimate safety margins are not the intent: the intent is that the basic operational stress and strain on the ship, throughout its intended service life, should not cause serious fatigue cracks in the structure. Very few ships ever see ultimate load conditions anywhere near their gross failure limits. It is likely that, without fatigue concerns, ship strength requirements would be somewhat lower.

While it is possible to develop fairly accurate analyses of ship loads and responses by hand or using minimal computer aids such as spreadsheets, modern CAD computer programs are usually used today to generate much more detailed and powerful computer models of the structure. Finite element analysis tools are used to measure the behaviour in detail as loads are applied. These programs can handle much more complex bending and point load calculations than human engineers are able to in reasonable amounts of time. However, it is still important to be able to manually calculate rough behaviour of ship hulls. Engineers do not trust the output of computer programs without some general reality checking that the results are within the expected order of magnitude. Additionally, Preliminary designs may be started before enough information on a structure is available to perform a computer analysis.