Defining Bad Weather
A merchant vessel is designed to sustain a wave height of 15 metres without damage. This does not mean that a wave height in excess of this value will result in immediate and catastrophic structural failure. Excessive wave height may result in stressing the steel of the shell plate above the yield point thus causing deflection between frames. Such “damage” manifests as undulation of the shell plate and is readily apparent in older vessels. It is similar to the undulation observed on most vessel tank-top plating even at the first guarantee dry-docking and may be more appropriately described as “fair wear and tear”.
The RealDistance.com routing engine renders all voyage videos using
Significant Wave Height data (SWH).
Essentially the SWH corresponds to the wave
height most readily observed by the human eye.
When considering SWH in relation to
vessel stress it should be noted that due to the statistical
distribution of wave heights in a seaway there is a 1 in 3,000 possibility
that the actual wave height encountered may be twice that of the SWH.
That means a Significant Wave Height of 7.5 m can
precipitate a wave height of 15 m every 3,000 waves. (
The forces exerted on cargo are significantly greater from rolling than pitching and so this must be considered when formulating a maximum wave height for beam seas. For the purposes of the specimen documents the following table of Limiting Parameters has been utilized. That is the bench-mark for what is to be considered bad weather has been clearly defined in terms of maximum permissible wave action on the hull at various bow angles. These parameters are set after discussion with the vessel operator.
A typical maximum wave height distribution is illustrated below.
