|This article does not cite any sources. (December 2008) (Learn how and when to remove this template message)|
Springing as a nautical term refers to global vertical resonant hull girder vibration due to oscillating wave loads along the hull of the ship.
The hydrodynamic theory of springing is not yet fully understood due to the complex description of the surface waves and structure interaction. It is, however, well known that larger ships with longer resonant periods are more susceptible to this type of vibration. Ships of this type include very large crude carriers and bulk carriers, but possibly also container vessels. The container ships are more slender, have higher service speeds and have more pronounced bow flares. Container ships are also known to experience significant whipping (transient) vibrations from bow impacts. Blunt ships may also experience whipping especially with flat bottom impacts in the bow area. The bottom part of the bow however rarely exits from the water on such ships. Vibration from whipping may also increase the extreme loading of ships potentially resulting in vessels breaking in two in severe storms.
In the extreme cases springing may cause severe fatigue cracking of critical structural details, especially in moderate to rough head seas with low peak periods. Vibration is normally more easily excited by waves in ballast condition than in cargo condition. The converse may also be true since some ships experience more head wind and waves in ballast conditions, while other ships may experience more head wind and waves in cargo condition, thereby vibrating less overall.
The first experience with this phenomenon was related to fatigue cracking on 700 foot Great Lakes bulk carriers during the 1950s. Later 1000 foot Great Lakes bulk carriers experienced the same problems even after strength specifications increased. Ocean-going ships have not had this problem until recently, when high tensile strength steel was introduced as a common material in the whole ship to reduce initial costs. This makes the ships less stiff and the nominal stress level higher.
Today's ship specifications do not account for springing which may be the dominant fatigue factor for some vessels.