The Naval Architect: February 2020
Many years ago, a senior technical officer of Carnival group said that naval architects often do not have experience of harsh conditions at sea. As a result, designers often underestimate the forces that a cruise ship has to face. The rapid development of expedition cruising has put this issue firmly back on the table.
Finding an optimal solution between seakeeping qualities and other relevant matters usually requires a compromise between various aspects of the ship’s design, says Esa Jokioinen, sales and marketing director at Deltamarin, the Finnish consultant naval architects.
“There are restrictions for the number of passengers ships can take to some regions, like the Antarctica, which limits the size of expedition cruise ships. By the very nature of this business, these vessels often cruise in regions where the sea conditions can be demanding,” he tells The Naval Architect.
Deciding on an optimal solution for the design of the ship requires that technical, commercial and aesthetic considerations need to be discussed. “You also need to find out what the architect in charge of the ship’s exterior thinks of what is on the drawing board, which can complicate matters,” Jokioinen points out.
Inspiration from offshore services industry
The expedition cruise industry has followed the lead of the offshore services industry, which has pioneered the use of various wave piercing designs. These generally offer superior qualities over a traditional bow design. But the seakeeping qualities of the ship is a broader question that also includes other factors e.g. the hull form.
One of the wave-piercing designs that has been introduced in the cruise ship sphere and put almost immediately to test is the X-Bow, designed by the Ulstein Group in Norway and initially used in offshore service vessels.
However, Greg Mortimer was completed last year and is the first in a series of 7,400gt vessels for SunStone Ships, which feature the X-Bow. The ship was completed by the China Merchants Heavy Industry shipyard in China and soon after delivery ran into heavy seas.
Niels-Erik Lund, CEO of SunStone, says he spent a week onboard and experienced those conditions. “The biggest noticeable difference between Greg Mortimer and our second-hand fleet is the complete lack of slamming, vibration and noise from the ocean,” he tells The Naval Architect.
Beam-draft ratio, bow and stern focal
One of the key aspects that a designer needs to look at is the beam to draft ratio: a figure in the region of four can be regarded as fairly optimal.
However, the larger the ship, the more difficult it is to have a good beam to draft ratio. Usually a draft cannot exceed nine metres, which creates a ratio higher than five on the largest ships, says Markus Aarnio, one of the founders of the Finnish naval architect firm Foreship. On the other hand, these ships are already so large that seakeeping is not usually an issue.
A fine, raked bow may look good but an excess results in propensity for slamming. Moreover, it is not just the bow that requires attention, the stern of the vessel can cause slamming as well. “Flat stern is a problem in port, where slamming due to waves can cause problems,” Aarnio notes.
Once the design has been completed, model tests will follow. Sensors in the bow measure stress at various speeds and stabilisers are engaged to counter the waves that will come from different directions and at different periods.
This enables the design team to define the seakeeping characteristics for the vessel, which can help the master to decide what speed should be used in the conditions that prevail at a given time.
New damage stability rules require higher GM
Heavy seas can result in bow slamming and force the master to slow the ship down. If the speed needs to be reduced significantly, the stabilisers will no longer work and the ship may start to roll more. Changes in damage stability rules mean that in general ships need to have higher metacentric height (GM) than before.
The SOLAS 2020 rules that demand a higher metacentric height than the previous ones also mean that accelerations of the ship in adverse sea conditions become more pronounced, which again will make them more uncomfortable, Aarnio points out.
As the mainstream cruise industry grows, the largest ships are first employed in regions where their operators can fill them, usually starting with the Caribbean.
But then a phenomenon well established in container shipping starts to take place: smaller vessels are moved to other regions to make way for the bigger newcomers. As a result, while such events may not be very frequent, even very large cruise ships can face extreme weather conditions.