The trend toward larger vessels also applies to passenger and cruise ships—examples being Cunard’s Queen Elizabeth 2, 963 feet long, and the even newer Queen Mary 2, 1,131 feet long. Royal Caribbean Cruises Ltd. and the Carnival Group are reportedly planning new cruise ships that are even larger.
So what do these trends imply for the safety of the world’s merchant fleet? Today’s vessels are larger and longer and of necessity traverse such hazardous seas as the treacherous waters of the Agulhas Current with greater frequency. Because of design economies, it appears that vessels are less able to withstand the impact of an extreme wave. At the same time, a large part of the global fleet is 20 years old or older. How will new and old vessels fare in encounters with extreme waves? Only time will tell.
Building double-hulled tankers should improve safety, as long as the interior spaces between hulls can be inspected and maintained. Access to some portions is confined and difficult, and if care is not taken, corrosion or structural problems could go undetected.
Heavy seas have been known to shake loose and claim the containers on container ships or damage them. One problem is a phenomenon known as parametric rolling, which occurs in heavy seas when the length of the ship is close to the wavelength. Depending on the speed of the vessel relative to the wave, the vessel will sometimes be in a trough and sometimes be supported on a crest. Container ships feature streamlined hulls designed for speed. Because of the load they carry, these ships have a wide flare on the bow and the stern to provide maximum deck surface. Consequently, the hull area presented to the sea varies, depending on whether the vessel is on a crest (where its broad, flat hull gives maximum stability and restoring force) or in a trough (where the restoring force is less). Then, if the vessel starts encountering waves at about twice its natural roll frequency, it will tend to roll. The restoring force brings the vessel back, but on a crest it over-compensates in effect. In the next trough, the restoring force is less, so the vessel rolls even more, and the rolls become larger and larger. It takes a drastic maneuver—an abrupt turn and speed change—to break this vicious cycle of dangerous rolling. The shipmaster must be quick to recognize the problem and take action, or the ship motion can