Strength of ships

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.

When ships are drydocked, and when they are being built, they are supported on regularly spaced posts on their bottoms.

Primary strength calculations generally consider the midships cross section of the ship.

Primary strength loads calculations usually total up the ships weight and buoyancy along the hull, dividing the hull into manageable lengthwise sections such as one compartment, arbitrary ten foot segments, or some such manageable subdivision.

For a ship in motion, additional bending moment is added to that value to account for waves it may encounter.

Standard formulas for wave height and length are used, which take ship size into account.

This diagram shows the key structural elements of a ship's main hull (excluding the bow, stern, and deckhouse).

However, they provide a detailed starting point for analyzing a given ship's structure and whether it meets industry common standards or not.

Shipbuilders today use steels which have good corrosion resistance when exposed to seawater and which do not get brittle at low temperatures (below freezing).

This steel has a yield strength of at least 34,000 psi (230 MPa), possesses an ultimate tensile strength of 58,000 to 71,000 psi (400 to 490 MPa), and must elongate at least 19% in an 8-inch (200 mm) long specimen before fracturing and 22% in a 2-inch (50 mm) long specimen.

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.

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.

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.