085/98

We will firstly clarify what we mean by "worst Case" wave period. The worst case scenario is a function of Significant Wave Height and Wave Period. It can be extremely difficult to visually estimate wave period, especially at different headings and speeds. That is why our table only refers to significant wave height. For each wave height band, the speed limit is determined for the worst case wave period but due to the above mentioned difficulties in observation the speed limit applied to all wave periods within that band. For a vessel that is classed to be able to trade in numerous locations around the world it is not possible to do a full analysis of the effect of wave period on the safe operation of each craft. This is because of the difficulty in taking into account all the local factors that may influence the vessel response, such as shoaling, adverse currents, thermoclines, etc. Thus, the Speed Wave height curve is based on fully developed seas. If, due to the local variations, normal wave steepness limits are exceeded it is the Masters responsibility to ensure the vessel is handled in a prudent manner. We would have assumed that the principles of prudent seamanship would not need to be reiterated, as it is one of the fundamental requirements of a Ship's Master. With regard to your final safety recommendation, we would state that due regard is already given to the effect of different wave periods in normal operation. We would also comment that our Rules and Procedures are being constantly monitored and updated and your comments are noted as part of this process. In particular, if we have regular occurrences of prudent seamanship not being observed we will examine the issuance of a Classification Circular or Casualty Note to all Owners of High Speed Craft classed with DNV.