Sailing the seven seas with Dolphin pods

In February this year the Italian shipbuilder T. Mariotti delivered the 754 passenger cruiseship Seven Seas Voyager to Radisson Seven Seas Cruises (RSSC). It is the world?s second all balcony-suite vessel, following another RSSC ship Seven Seas Mariner. More interesting from a technical standpoint though is that the 41,500g Seven Seas Voyager is the first vessel to be equipped with the Dolphin azimuthing podded propulsion system.

Hamburg-based STN Atlas Marine Electronics and Wärtsilä Propulsion (Lips) jointly developed this system. Designed for ships of all types and sizes, the Dolphin principal is typical of such systems integrating a powerful electric drive into a hydrodynamically optimised pod protruding below ship, resulting in a directly driven propeller. Vuyk Engineering assisted Wärtsilä with strength calculations and construction drawings of the pod housing.

Dolphin magic

Dolphin has been developed for power ranges typically between 3MW and 19MW. The two units fitted to the RSSC cruiseship are type DDP395 each delivering 7MW at 172 rev/min. They are fitted with FP propellers of 4.40m diameter. Wärtsilä also supplied two 1,200kW bow thrusters and a joystick system for the ship.

The diesel electric plant consists of four medium speed Wärtsilä 9L38A generators each with an output of 5,280 kW. The cruising speed is 21.5 knots.

The driver of the fixed pitch propeller is an air-cooled six-phase synchronous motor, located within the pod, which requires neither reduction gears nor a long shaft line. The pods also eliminate the need for rudders and stern thrusters while electrical and hydrodynamic efficiency offer a potential reduction in fuel consumption, exhaust emissions, noise and vibration levels.

STN Atlas Marine Electronics supplied a complete electric propulsion system with synchro-converters, control and joystick system, diesel alternators, and a high-voltage distribution assembly with propulsion and mains transformers.

Similarly, the vessel?s bridge has been equipped with a NACOS 45-4 integrated navigation command system together with a full range of internal and external communications facilities. The NACOS assembly, designed to Watch 1 IBS standards and featuring a radar-controlled track pilot, comprises two Atlas Radar pilots (S and X-band), a Conning Pilot, a Multipilot Navigation & Chart Monitoring console, and a Chart pilot ECDIS-type workstation for general planning and chart work.

Luxury

accommodation

Built at an estimated cost of $215 million, the 206.5m long vessel has an exceptionally high space ratio, with every guest enjoying a minimum of just over 300ft2 (28m2) of living space plus balcony. The nine decks devoted to passenger facilities contain 353 cabins, or suites as they are called onboard RSSC vessels, and four main dining venues including the 110-seat Le Cordon Bleu. Three of the restaurants are situated on deck 5 while the La Veranda restaurant is situated on deck 11. Since dining is an integral part of a cruise, the Voyager is one of the first ships to feature an open galley in one of its restaurants. With a six star rating, the newbuilding continues the high standards set by RSSC when it put its first ship, Radisson Diamond, into service some ten years ago.

Dolphin advantage

While the general principles of Dolphin are similar to other pod drives, i.e. Rolls Royce?s Mermaid, ABB?s Azipod and Siemens Schottel SSP, it nonetheless differs in some important respects from other systems. For example, whereas systems such as the ABB Azipod use cyclo converters, Dolphin employs synchro-converters. As STN Atlas sees things, the disadvantages of cyclo-converters are that they have more components as well as control and supply systems, resulting in higher overall costs.

Similarly, cyclo-converter type systems use permanently excited motors. While these can provide advantages in terms of overall dimensions and weight, it is a relatively costly technology. Based on conventional motors, Dolphin is more compact and simpler in concept. And with fewer components it is less expensive. As regards to reliability, only time will tell since all other pod manufacturers have had or are still experiencing serious reliability problems.

STN Atlas has calculated the propulsive performance of a Dolphin-driven vessel compared to a twin-screw ship with conventional fixed propellers is 6-10 per cent better. This is due mainly to the omission of the rudder, appendages, shafts and stern thrusters.

Lifting the Dophin pod propeller assembly into place on Seven Seas Voyager