AROSA AQUA – an innovative propulsion system for a new generation of river cruise ships
Due to the stringent requirements of the owner, AROSA Flussschifffahrt GmbH, in terms of sound levels in the cabins and reduced energy consumption, newbuilding 514 from NEPTUN Werft GmbH was designed with a new propulsion concept in the stern.
The direct diesel-driven azimuthing SCHOTTEL Pump-Jet SPJ of type 82 RD, approved for use as an independent propulsion system and installed in the bow, has already found its way into other new river cruise ships. Due to its compact design and extremely low structure-borne noise levels, and its outstanding ability to generate virtually constant thrust at any speed, it is encountering great interest in this market segment and has already managed to boost its market share considerably.
The main propulsion system, consisting of 4 SCHOTTEL Twin Propellers of type STP 200 with highly elastic bearings, driven by 4 VOLVO PENTA D 12 diesel engines with a power rating of 331 kW at 1800 r.p.m., is unique and is making its inland shipping debut in this vessel. The next newbuilding, 515, also for AROSA Flussschiff GmbH, is being fitted with the same propulsion concept. Meanwhile, a river cruise ship from another renowned shipping company has also entered service with four STP 200 propulsion systems.
Dividing the propulsive power between four propulsion units has the following advantages
Reduced draught of 1.5 m
Since the overall propulsive power is divided between four propulsion units, the propellers can be designed to be highly efficient with a diameter of 1050 mm and a power rating per unit of 331 kW. It is possible to implement this small propeller diameter without compromising on efficiency thanks to the SCHOTTEL Twin Propeller technology.
Since river cruise ships on European rivers are operated with a wide range of water levels, it is particularly important to ensure that vessel operation can continue even when levels are low. The lower the draught that can be implemented, the greater the range of areas in which the vessels can be operated. The bottom line is that lower draught means more money in the coffers of the owner.
Extremely low structure-borne noise levels
The extremely low structure-borne noise levels result on the one hand from the hydrodynamic design of the STP 200, and from the unique elastic bearing with conical elements on the other. These give the propulsion system a natural frequency that is sufficiently far beneath the excitation frequencies generated by the propellers. Furthermore, the considerable clearance between the propeller and the structure of the vessel means that very little structure-borne noise enters the vessel. Since the draught is 1.5 m and the propeller diameter is only 1050 mm, the clearance to both the structure of the vessel and to the base line is highly favourable. The disadvantage of the propulsion concepts commonly used until now, with two main propulsion units and a typical rated power of approx. 2 x 800 kW for this vessel size, was that the propeller diameters could not fall below 1400 mm, even in the case of Twin Propeller systems, to ensure that the high propulsive power of 800 kW could still be transferred to the water via two propellers. For this reason, a draught of about 1700 mm was necessary. The clearance to the hull of the vessel nonetheless had to be kept extremely small so that the propulsion system – which was very large relative to the draught – could still be fitted beneath the vessel. It was frequently necessary to position the propellers on the base line.
Significant increase in efficiency
The smaller propeller diameter of 1050 mm means that the stern of the vessel requires very little tunneling. The efficiency of this stern is thus significantly greater than that of a highly tunneled stern with two 800 kW propulsion systems. The thrust-deduction coefficients are surprisingly low for the propulsion system with 4 x STP 200, thanks to the generous clearance between the propellers and the hull, with the result that almost the entire thrust generated by the system can be converted into propulsive power. Moreover, the propellers with a propulsive power of 331 kW are less loaded than the twin units with 800 kW; despite the smaller propeller diameter, this results in a higher specific unit efficiency than in the case of the twin units.
Same steering philosophy as for a twin-unit system
Since captains of vessels come and go, one particular development goal of the new propulsion system with four STP 200s was to ensure that there is no difference for the captain between steering the four-unit system and a twin-unit system. From a steering point of view, the port pair and the starboard pair are each assigned to a single joystick so that the bridge only varies in minor details from that for a twin-unit system.
Engine utilization similar to that of diesel-electric propulsion systems
The four propulsion systems can be activated and deactivated as often as required. Propulsion systems that have been shut down windmill in the current and continue to be steered by the azimuth steering in accordance with the angle set by the steering lever in order to keep the resistance as low as possible. It is thus now possible at last to eliminate the part-throttle problem, typical of twin diesel engine systems, that leads to sooting of the engines, higher energy consumption, increased maintenance and poor emission values! During operation in canals or downstream, propulsion of the vessel is still redundantly ensured by means of either the two inner units or the two outer units. This means that the two active propulsion units can still be operated at a high load and the diesel engines can be run with a relatively high degree of efficiency. The result is that virtually the same advantages are achieved in terms of diesel engine utilization as with a diesel-electric propulsion concept, but without the conversion loss.
9-fold redundancy and thus extremely enhanced operational reliability
Since all four SCHOTTEL STP 200 main propulsion units are each designed with redundant steering, this results in 8-fold steering redundancy. Given that the SCHOTTEL SPJ 82 RD in the bow of the vessel is also to be considered as an independent propulsion unit, the overall result for the entire vessel is thus 9-fold redundancy, providing unprecedented operational reliability.