In English idiom, the term “rudderless ship” is applied to a variety of situations, all of them bad. Royal Caribbean’s largest ships are without rudders, and it’s all good.
By eliminating the rudder as well as traditional fixed propellers, RCL not only improved maneuverability of its giant vessels, but also cut fuel consumption by 10-15 percent, which translates into both significant cost savings and reduction of emissions.
Propulsion claims more than half of the energy used by RCL ships, and there’s a self-set goal of making each new ship at least 15 percent more efficient than the last. Among the methods already put into play are new coatings and configuration of ships’ hulls, and an integrated electronic system driven by industry-first software that calculates the best operational choices for the most efficient use of fuel.
Since 1999, with the launch of Voyager of the Seas – then the largest cruise ship in the world – RCL has also cut a substantial chunk out of its ships’ operating costs by using a massive propulsion device trade-named the Azipod.
Produced by Swedish-Swiss engineering concern ABB, the Azipod is a fixed-pitch propeller mounted on a gondola or pod that can be turned in a complete circle – enabling the ship to turn 360 degrees without moving forward or back. The pod contains the electric motor that drives the propeller. Because it can turn to any point of the compass, the need for a rudder is eliminated.
“The pod improves efficiency, and it improves the maneuverability significantly,” says Capt. Patrik Dahlgren, vice president of marine operations for Celebrity Cruises’ and fleet optimization for all RCL brands. “When you build really large cruise ships like when we started with the Voyager class, there were two Azipods and one fixipod,” which provides thrust but doesn’t rotate.
The fixipod was left off of subsequent newbuilds, and the Azipods continued to be mounted in pairs, with the redundancy providing backup propulsion if one fails.
“If you lose one shaft or one propeller on the traditional ship, it’s very difficult to maneuver,” Dahlgren explains. “If you lose one Azipod versus two, you can still maneuver fairly well. You can still sustain most of the itineraries even with one pod.”
The newer, bigger ships also require larger bow thrusters – essentially holes that run crosswise through the front of the hull and house a propeller that can be used to push the bow to one side or the other.
Additional energy savings, though small, were realized when RCL started putting a grid or screen over bow thrusters’ openings, allowing the flow of water along the hull to pass with less drag.
“It’s about one to two percent, but it does improve efficiency,” Dahlgren says.
Pods also save energy in part because they’re shaped for hydrodynamic efficiency and because they’re self-contained, allowing designers to tuck them up into the stern.”
“So the flow of water around the stern completely changes the dynamics there, and that’s kind of the big savings compared to conventional propulsion,” Dahlgren says. “It’s quite incredible.”