Four container vessels have broken in two during the last four decades. There may not be one single cause explaining these severe accidents. They all broke in moderate storms, but they did not break in extreme storms in terms of extreme wave heights. How could this happen? This paper addresses one possible contributing effect to all of these accidents, i.e. whipping, and how whipping contribute in different sea states.

Whipping, as a sudden hull girder vibration caused by bow flare impacts, can contribute significantly to increase the vertical hull girder bending moments of container vessels, which have high design speeds and pronounced bow flare. Model tests have been carried out based on modern container ship designs covering one Panamax vessel, one Post Panamax vessel and one Ultra Large Container Ship. The tests have been carried out primarily in head seas. The whipping contribution depends on the vessel speed, and the tests have been carried out using realistic speed in each sea state. Lower sea states are more frequent than higher sea states, and lower sea states are associated with higher speeds. Does this speed dependence give other dimensioning sea states when whipping is considered?

For all three vessel designs, it is not the highest sea states, which define the dimensioning wave moments when whipping is included. Actually, realistic encountered storms can produce the dimensioning wave bending with whipping. These sea states differ considerably from the sea states, which produce the maximum wave moment without whipping. It is also demonstrated how different trades affect the dimensioning wave bending with whipping.

The industry seems most concerned about the effect of whipping for the largest vessels. These model tests demonstrate that the dimensioning moment with whipping for the largest vessel is not the main concern. The tests suggest that the bow flare angle is most important, and these may be high for Post Panamax vessels. The speed is well known to be important, while size in terms of length is not particularly important from these tests.

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