When a ship crash into something such as another ship
Ship collision is a type of maritime incident, a violent encounter involving moving ships. While the standard definition of collision involves more than one moving ship, and an engagement between a ship and a motionless object is formally known as "allision", in practice the word "collision" is usually used to describe also the situation where a moving ship hits a stationary ship or a fixed object such as a bridge. 
Ship collisions are of particular importance in marine accidents. Some reasons for the latter are:
As sea lanes are getting more congested and ship speeds higher, there is a significant possibility that a ship may experience an important accident during her lifetime. Higher speeds may cause larger operational loads, like slamming, or excessively severe loads, for example during a collision. Denser sea routes increase the probability of an accident—in particular a collision—involving ships or ships and shore or offshore structures.
Almost 27% of ship collisions occur near coasts and 22% at narrow channels. This is usually due to disregarding best practices and regulations by navigation officers and masters. In addition, the IMO guidelines for voyage planning are not always followed. Violations are usually associated with not maintaining a safe speed, overtaking, or miscommunication with the pilot.
Collisions with wildlife.
Large whales and species such as sea turtles or whale sharks often suffer lethal wounds from collisions with ships ("vessel strikes"). There are programs in development and implementation phases aimed at reducing vessel speed in critical waterways, both voluntarily and by regulation, which aim to protect endangered whales.
References.
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