No matter what floating state the ship is in, there will always be longitudinal bending.
For the total longitudinal bending, the total longitudinal strength of the hull can be obtained:
Total longitudinal strength, that is, total longitudinal strength of ship hull, is a common term in ship engineering, which refers to the ability of ship hull structure to resist total longitudinal bending. The external force corresponding to the total longitudinal strength is the total longitudinal bending force, which is the gravity, buoyancy, wave hydrodynamic force and inertial force acting on the whole hull to produce the total longitudinal bending.
The gravity of the hull is uneven along the length of the ship. At the same time, the fore and aft shape of the ship is sharp and thin, the middle part is hypertrophy, and the boiling water volume discharged by each part is different, which leads to different buoyancy. In this way, the gravity and buoyancy of the ship along the captain's direction are unevenly distributed in the captain's direction, so the total longitudinal bending force is produced.
There are two situations in which the longitudinal bending of the hull occurs: one is that the stern of the hull is arched and the head and tail are drooping; The other is a depression in the middle of the hull; Head and tail upturned. The former state causes the longitudinal members of the deck to be pulled and the longitudinal members of the bottom to be compressed; The latter state is the opposite.
In the process of total longitudinal bending, the compression members in the hull often buckle due to excessive compression, which greatly reduces the ability of the hull to resist total longitudinal bending. It is an important content to analyze whether the compression members in the hull buckle and their residual ability to resist external forces after buckling.
When a ship sails in waves, it will be subjected to a large total longitudinal bending force, and when the wave crest is in the ship, it will strengthen the bending of the middle arch; When the trough is on the boat, it will strengthen the vertical bending. If the wavelength of the wave is equal to that of the captain, the above situation will be more intense. This is the most dangerous thing for sailing ships. When studying the longitudinal strength of the hull, the most dangerous situation should be considered.