An inline thruster is an electric ring motor with a rotor and nozzle combined in a powerful propulsion system without shaft and gearbox. Four inline thrusters with a rotor diameter of 170 mm have been developed in the SVA for model tests. The models are studied in open water and cavitation tests. In addition, velocity measurements, dynamic flow monitoring and open water experiments at different azimuth angles and in off-design conditions were carried out for various industrial applications.

The forces and moments on the different elements of the inline thrusters were calculated in the model and at full-scale. The results of these calculations were the basis for the investigation of the Reynolds number effects and for the optimisation of the rotor and nozzle geometry.
Propulsion and maneuvering tests showed the applicability of inline thrusters for main propulsion system of ships [1], [2].

Thrusters of various designs are used to improve manoeuverability. The variations go from impeller, z-drives and retractable types to permanently installed channel thrusters. For the design of bow thrusters, details are needed on the basic parameters of the propeller and interaction with the channel in particular. For bow thrusters with propellers, different approximation methods were derived. At SVA, the method of Bladt and Wagner [3] is used for the design of thrusters. Investigations with thrusters can similarly be carried out as ducted propellers up to a certain size (relation of propeller diameter to length of the tunnel). The following figure shows a schematic diagram of the experimental setup.

For cavitation tests with propellers for thrusters, the cross-channel system 25A26 was developed [2].

The propeller is driven with the J25 dynamometer. The channel inner diameter is 209 mm. At the end of the channel a nozzle-like constriction is arranged. This nozzle causes a throttling effect that achieves high thrust loads of the propeller in the experiments without a working impeller. Smaller loads can be realised by increasing the velocity in the pipe by a working impeller. The velocity in the channel is determined by pressure measurements. The velocity measuring device is calibrated by the measurement of the velocity distribution in the jet of the outlet of the nozzle. Therefor the LDV-measuring device is used in the cavitation tunnel. The calibration is performed with and without propellers. The static pressure which is required for calculating the cavitation number is measured in the entrance of the channel. The entrance of the cross channel can be equipped with a vertical or inclined plate relative to the channel axis.

 

Context Related References / Research Projects

[1] Vollheim, R.: Modellversuche zur Entwicklung eines Bugstrahlruders Schiffbauforschung 18 1/2/1979
[2] Schröder, G.: Eine Einrichtung für Modellversuche an Propellern für Querstrahlruder Schiffbauforschung 23 3, 1984
[3] Bladt, K.-J.: Beitrag zur Auslegung von Querschubanlagen mit Propeller für Schiffe, www.jbladt.drupalgardens.com, 2013

The main feature of podded drives is the integration of a powerful electric drive in a hydrodynamically optimised pod beneath the ship which drives the propeller directly. The full power can be used in any azimuth direction (0 – 360).
The podded drive is a propulsion system comprised of the components propellers and pod housing. The development and optimisation of podded drives requires knowledge of the interaction between propeller and pod housing and about the influence of design parameters on the characteristics of the system.
SVA Potsdam has been working in the field of podded drives since 1995 [1], [2]. Among other studies, the applicability of podded drives, dynamic flow calculations, analyses of various podded drives as well as numerous model studies have been performed there [3], [4], [8]. Developments for podded drives were conducted in collaboration with various manufacturers [3], [9].
As part of a BMWi funded project “Integrated Ship Design for Ships with Podded Propulsion Systems”, aspects of ship design for ships with podded drives were examined. In the BMWi funded project “High-Speed Pod”, concepts for podded drives with application speeds up to 30 knots were developed and studied experimentally and computationally [5], [6]. The ideas and findings resulting from these projects were the basis for the development of High Efficiency Pods with HTS technology [9].
The manoeuvrability of propulsion systems with podded drives were examined as part of a BMBF funded project [7]. The arrangement of the pod at the stern of the ship, the use of fins, and the influence of the propeller assembly were analysed experimentally and computationally.
Standard podded drives with pull and push propeller variants have been developed and investigated experimentally [6], [8], [9]. The pod housings (gondolas) were particularly varied with regard to the gondola diameter and the transition from the propeller to the gondola in order to determine the influence of geometric parameters on the characteristics of podded drives (open water, propulsion, cavitation).
The development and the study of podded drives places high demands on measurement technology and experimental methods. For open water, propulsion and cavitation measurements, manoeuvring and speed measurement systems on podded drives have been developed, tested and used successfully. To validate the evaluation and forecasting methods for measurements with podded drives, CFD calculations were performed.
With the available azimuth measuring systems for podded drives, pull, push, twin, and counter-rotating propeller assemblies can be analysed. Among other things, the thrusts and moments of the propeller are measured in the shaft and the lateral and vertical forces can be measured at the housing.

Thrusters are propulsion systems used for ships and platforms where manoeuvrability is an important criterion. Thrusters can usually swivel 360° so that the system can act in any direction. The propeller works in front of or behind the housing (pull and push) which results in strong interactions between the propeller and the housing. Experimental and numerical investigations of thrusters are conducted because of these interactions and also special operating conditions such as bollard pull, manoeuvring, off-design, which are more demanding than conventional propellers. When analysing thrusters mostly open water propulsion and cavitation tests are carried out.

Special propulsion and measuring systems have been developed for the study of thrusters. The SVA owns propulsion and measuring systems for use in the towing tank and cavitation tunnel for testing thrusters with pushing or pulling propellers, twin and also contra rotating propellers. The systems allow the measurement of thrusts and moments of the propellers and the determination of housing resistance. The system forces and torques of the thruster are measured with 3- or 6-component balances.

Tests and calculations for thrusters are carried out in particular for propeller manufacturers (determination of the characteristics of thrusters, optimisation and development), shipyards and shipping companies (Propulsion). To convert the model test results to full-scale, the SVA has developed its own method for Reynolds number correction for thrusters. Systematic CFD calculations of thrusters were carried out in the R & D project “Correlation of Z-Drives with Ducted Propellers” [2], [4].

In recent years, aspects of DPability of thrusters were increasingly analysed. As part of the research project “Thruster for Dynamic Positioning” [5], [6] extensive studies on the influence of gondola and nozzle angle on the operating parameters of the thruster, the interaction of the thruster with the platform, and the mutual interaction of the thrusters were performed.

For the study of DP capability of platforms, special propulsion and measuring systems have been developed to ensure the variability of the inclination of the gondola housing and an unlimited pivoting of the thruster.

Twin propellers

Twin propellers work with a pull and a push propeller with the same rotational direction. The design of the rear propeller places increased demands on the calculation method. Twin propellers should be arranged such that the vortex of the pulling front propeller passes between the wings of the rear pushing propeller. By the contraction of the propeller wash of the pulling propeller, additional water from the sides reaches the pushing propeller.

If twin propellers are used on thusters (Schottel Twin Propeller (STP)), or Podded Drives (Siemens Schottel propulsor (SSP)), the housing must be hydrodynamically optimised and provided with guide fins. The twist in the propeller wash of the pulling propeller is partially recovered in this way and influences the inflow to the targeted pushing propeller.

The SVA‘s VORTEX propeller calculation method has been adapted for the design and optimisation of twin propellers [1]. The following photos show examples of the hydrodynamic model and the calculated inflow to a pushing propeller.

Contra-rotating Propeller

Contra-rotating propellers consist of two contra-rotating propellers. Through the reverse rotating propeller, rotational losses can be avoided because the rear propeller can use the rotational energy of the flow which is induced by the forward propeller. In addition, the load is distributed over the two propellers.

The SVA has measuring systems for the study of contra-rotating propeller systems in both the towing tank and cavitation tunnel. In recent years systematic studies of the effect on efficiency of the distance between the contra-rotating propellers and the layout of pods between the contra-running propellers have been carried out.

On behalf of REINTJES, the Fortjes^® pod propulsion system was developed [5], [6]. The propulsion system is used especially in planing and semi-planing yachts up to 40 m long and in a power range of up to 3000 kW. The SVA’s VORTEX method was and is used for the design of contra-rotating propellers. Gondola and shaft were completely newly designed to optimally take advantage of the contra-rotating propeller and the pod.

The SVA has a long and diverse experience in the field of propeller design and the design of complex propulsion systems. As a model basin and research institute, the SVA Potsdam has the unique advantage of being able to use the experience gained from basic and applied research directly for the design of propulsion systems.

Major parts of design programs have been developed in the SVA. This includes pre-processing for propeller definition and geometry modification and recalculation processes for propellers, ducted propellers, twin and contra-rotating propellers. Furthermore, mathematically based optimisation methods and post-processing for assessment of cavitation, pressure fluctuation predictions and strength calculations using FEM analysis, and interfaces for 3D modeling are included. All of these programs are contained within the program package of VORTEX. Other propeller manufacturers and classification societies use, among other things, this software for design and certification. The continuous development of design tools is supported through close contact with these propeller manufacturers and classification societies.

The SVA has had, among other things, significant contributions to the development of the twin propeller concept from SCHOTTEL and set milestones in the development of low-noise propellers for research, naval vessels and submarines. For large tug boats, ducted propellers are designed with more than 200 t thrust. In the development of ducted propellers with high static thrust demands in particular, the broad experience with extensive CFD calculations of propulsion systems on the ship can be made use of.

Designs of propellers and propulsion systems can be fully tested in the SVA in model scale, whereupon despite advanced calculation methods, model testing cannot be dispensed with. After propulsion or cavitation testing, the propeller design can be improved to meet the highest standards of practice.

To determine the behaviour of ship and propulsion system and the tuning of the engine, trial runs are accompanied by the SVA during which special full-scale measurements (power measurement, vibration, pressure fluctuations and acoustics measurements, cavitation observations, manoeuvring measurements) are conducted.