TX and MARIN worked very closely together on the Symphony of the Seas, a goliath that pushes all the
shipbuilding boundaries, and this meant a rigorous test campaign was crucial.
The figures are impressive, she is a staggering 228,021 GT, more than 362 m long, boasts 18 decks and has a capacity for 8,000 people. Given this shipbuilding feat, Loic outlines the major challenges that STX and MARIN faced together.
Highest speed possible Loic explains: “One particular challenge of Symphony and her predecessor Harmony of the Seas was the contractual obligations to achieve the highest speeds possible, whereby the yard is penalised if the ships sail below their expected speed. However, there has to be a very delicate balance between the efficiency demanded and the comfort levels on board. Vibrations and noise from the propeller could be a potential issue and lead to discomfort for the passengers.”
Several model and cavitation tests were carried out at MARIN and these led to a substantial improvement in ship speed, and at the same time, there was no detrimental impact on comfort levels. Following the tests and calculations, both Symphony and Harmony of the Seas have a different hull form and propeller compared to the former Oasis Class vessels - Oasis of the Seas and Allure of the Seas.
One key factor was a change in the shape of the bulbous bow. The tools to optimise the bulbous bow have changed with the times. Optimising with potential computations led to the ‘Goose Neck’ bulbous bow type (a very pronounced bulbous bow), which provides a gain in
wave resistance but not for the viscous component (due to the swirl created by the bulbous bow). Therefore, CFD computations were done to redesign the bulbous bow and these resulted in a more slender and longer bulbous bow.
Bulbous bow shape STX France also worked on the propeller design, conducting pressure measurements, which were compared to the first Oasis Class ships. The lateral thrusters and ducktails were also optimised.
The cavitation tests brought some major challenges, Loic explains. “We have three propellers of 20MW each and the central propeller is behind the central skeg, which means the wake impact is more pronounced, meaning that it has even more influence.” Eventually five different designs of the central propeller were tested and optimised. “The main dimensions of the propeller were changed, the blade areas, ratio etc.”
As to whether the industry can carry on supersizing vessels, Loic says the challenge is not so much about the scale of the cruise ships but the balance between efficiency versus comfort. “We already have quite some experience with enormous ships. The Queen Mary 2, which at 350 m, is longer than even the Oasis Class for example. We can build bigger ships. But the only thing is a larger ship means an enormous amount of power is placed on each propeller, so we are then back to the efficiency versus comfort level compromise again.”
Comfort vs. efficiency A key challenge and something addressed together with MARIN is the tip vortex. “Given the higher level of efficiency, there is a bigger tip vortex, but is this acceptable for comfort
levels?” At the moment, he admits, the industry doesn’t have the answer. “Our current knowledge is relying on experience. As cruise ships get larger, the power will of course increase. I think this will be the parameter in the end, not the size of the vessels.”
STX is also designing for ‘non-optimal’ conditions now, he adds. “Previously we would design a ship for ideal conditions, without waves and wind. But the current trend is to take into account conditions such as wind and waves. Our target is to optimise the hull form and propeller for perfect conditions and take into account waves, current etc. as well.”
On the day of the interview Loic was at MARIN’s testing site in Ede where cavitation tests were being performed. “Here we are looking at the resistance of the ship, plus the additional resistance for wave and wind, and the impact cavitation has on the propeller.” If a hull form can have reduced added resistance in waves this is very interesting for cruise ship owners, he adds. And already there has been quite some success, with the joint STX/MARIN team reducing the additional resistance by 25%. These findings will be taken into account for the next vessel, which will be delivered in October. The model tests also led to the decision to design a straight bow, which significantly reduces resistance.
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