Springing

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 exposed to this type of vibration. Examples of this include very large crude carriers and bulk carriers, but possibly also container vessels. The container ships are more slender, has higher service speeds and have more pronounced bow flare. These are also known to get significant whipping (transient) vibrations from bow impacts. Whipping may also occur on blunt ships especially in the cases of flat bottom impacts in the bow area. The bottom part of the bow however rarely exits from the water on such ships.

In the extreme cases it may cause severe fatigue cracking of critical structural details, especially in moderate to rough head seas with low peak periods. Vibration in ballast condition is normally more easily excited by waves than cargo condition. The trade may also matter, 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 is related to fatigue cracking on Great Lakes bulk carriers (700 feet) during the 1950s. Later 1000 feet Great Lakes bulk carriers experienced the same problems even if the strength requirements became stricter. Ocean-going ships have not had this problem until recently, when high tensile steel was introduced as a common material in the whole ship to reduce the initial costs. This makes the ships less stiff and the nominal stress level higher.

Today's ship rules does not account for this effect which may dominate the contribution to fatigue for some vessels. The vibration from whipping may also increase the extreme loading of ships potentially contributing to breaking vessels in two in harsh storms.