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.
Loads on ship hulls
The hulls of ships are subjected to a number of loads.
Even when sitting at dockside or at anchor, the pressure of surrounding water displaced by the ship presses in on its hull. The weight of the hull, and of cargo and components within the ship bears down on the hull. Wind blows against the hull, and waves run into it. When a ship moves, there is additional hull drag, the force of propellers, water driven up against the bow. When a ship is loaded with cargo, it may have many times its own empty weight of cargo pushing down on the structure. In heavy seas, water flowing over or crashing down onto the weather deck applies (possibly immense) loads on the deck and transverse loads on the superstructure or other deck features such as coamings and hatches.
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.
Primary hull loads, strength, and bending
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 down in the center, known as sagging up in the center, known as hogging. This can be due to: hull, machinery, and cargo loads wave loads, with the worst cases of: sagging, due to a wave with length equal to the ship's length, and peaks at the bow and stern and a trough amidships hogging, due to a wave with length equal to the ship's length, and a peak amidships (right at the middle of the length)
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 for each section of plate or girder making up the hull, with being the moment of inertia of that section of material, being the width (horizontal dimension) of the section, being the height of the section (vertical dimension), being the area of the section and being the vertical distance of the center of that section from the neutral axis.
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 each loading condition, the displaced water weight or buoyancy is calculated for that hull section based on the displaced volume of water within that hull section. The weight of the hull is similarly calculated for that length, and the weight of equipment and systems. Cargo weight is then added in to that section depending on the loading conditions being checked.
The total still water bending moment is then calculated by integrating the difference between buoyancy and total weight along the length of the ship.
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. The worst possible waves are, as noted above, where either a wave crest or trough is located exactly amidships.
Those total bending loads, including still water bending moment and wave loads, are the forces that the overall hull primary beam has to be capable of withstanding.