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.

The hulls of ships are subjected to a number of loads.

If the ship's structure, equipment, and cargo are distributed unevenly there may be large point loads into the structure, and if they are distributed differently from the distribution of buoyancy from displaced water then there are bending forces on the hull.

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

The primary strength, loads, and bending of a ship's hull are the loads that affect the whole hull, viewed from front to back and top to bottom. Though this could be considered to include overall transverse loads (from side to side within the ship), generally it is applied to longitudinal loads (from end to end) only. The hull, viewed as a single beam, can bend

This can be due to:

Primary hull bending loads are generally highest near the middle of the ship, and usually very minor past halfway to the bow or stern.

Primary strength calculations generally consider the midships cross section of the ship. These calculations treat the whole ships structure as a single beam, using the simplified Euler-Bernoulli beam equation to calculate the strength of the beam in longitudinal bending. The moment of inertia (technically, second moment of area) of the hull section is calculated by finding the neutral or central axis of the beam and then totaling up the quantity ${\displaystyle I_{y}={\frac {bh^{3}}{12}}+Ad^{2}}$ for each section of plate or girder making up the hull, with ${\displaystyle I_{y}}$ being the moment of inertia of that section of material, ${\displaystyle b}$ being the width (horizontal dimension) of the section, ${\displaystyle h}$ being the height of the section (vertical dimension), ${\displaystyle A}$ being the area of the section and ${\displaystyle d}$ being the vertical distance of the center of that section from the neutral axis.

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