Motivation
Plastic propellers are lighter than their metal counterparts. The lower density compared to metal makes it easier for divers to replace underwater. The lighter weight and easier handling mean that repairs and maintenance can be carried out more efficiently and safely. In addition, plastics are inherently corrosion resistant and have excellent cavitation erosion resistance properties, making them ideal for use in marine environments.
Plastic impeller blades have a better ability to dampen oscillations and vibrations than metal blades. This results in reduced noise during operation. Lower noise emissions not only contribute to increased comfort on board but are also more environmentally friendly as they reduce noise pollution in maritime environments.
Use in the SRT Propulsion System
The SRT (SCHOTTEL RimThruster) is a drive that is installed in the bow or stern of ships to improve maneuverability. This type of drive is very often used in the yacht sector. The drive is installed transverse to the direction of travel and is not used as the main drive. It is particularly useful in narrow harbors or for complex maneuvers. This drive can also be operated in follow-the-sun DP mode. In this mode, the sun deck of a yacht is continuously adjusted to the course of the sun via the dynamic positioning system.
The SRT is an entirely electric drive system without a gearbox or drive shaft. The stator of the electric motor is located on the outer ring of the drive, while the impeller blades are attached to the inside of the rotor. The result is a space-saving and weight-reduced drive that converts the electrical energy directly into thrust - there is no noise emission from gears or other mechanical components.
Validation through simulation and testing
A new development for a metal replacement requires virtual, plastic-specific product development and a final physical test. Modeling and simulation were carried out with Altair Hypermesh, with the integrated Altair Optistruct as the solver. For the strength assessment, the third-party program S-Life Plastics from PART Engineering was accessed via the Altair Partner Program (APA) with the existing Altair licenses to enable a simplified strength assessment for plastics.
The aim was both the static strength assessment and the fatigue strength assessment under the influence of water and temperature with a varying, point-symmetrical load on the blade with a number of load cycles in the high service life range. S-Life Plastics provides the necessary results for the assessment here, carrying out both static and cyclic strength assessments with a simplified assessment based on a minimal amount of material data that is usually available. The utilization ratio was initially too high (Fig. 3), whereupon the blade geometry was adjusted and the radius in the critical area was increased (Fig. 2). A small change with a big impact. The renewed strength assessment showed that the design limits were no longer exceeded (Fig. 4).
After the calculation, a test was carried out to validate the fatigue strength assessment. In order to shorten the test duration, another simulation with an overload was done. With this overload, the blade was expected to show the first material damage in the test according to the assessment (Fig. 2).
In the test setup, the blade was subjected to alternating bending loads. The cylinders were positioned and aligned in such a way that the results were equivalent in terms of stress and load compared to the simulation. Before the bending test, the blade was examined for pre-damage.
After the blade had withstood the expected number of load cycles without damage despite overloading in the bending test, the overload was increased again. This overload was also applied for the required load cycles and passed without any visible damage.
After the visual assessment of possible material damage, the blade was subjected to several non-destructive testing procedures. Again, no damage could be detected. The simulation was therefore validated and also shows that the safety factors used in the simplified procedure are suitable for a low-risk design in the development process.
Conclusion
The successful development of plastic impeller blades is a step forward in marine propulsion technology. Plastic materials offer advantages in terms of weight, corrosion resistance, design flexibility, noise reduction and maintenance. These properties help to increase the efficiency and environmental friendliness of marine propulsion systems and support the maritime industry in overcoming current challenges in terms of sustainability and performance.
The results of the simulation showed convincingly which areas still have potential for optimization. The software-supported strength assessment offers sufficient safety even with increased loads to reduce the number of test cycles.
Altair's license system and the associated possibilities provide enormous support for engineering work. The results of the simulation and strength assessment with S-Life increase our confidence in new developments prior to validation through testing.
SCHOTTEL GmbH, founded in 1921 in Spay am Rhein, Germany, is a leading company in the development and production of propulsion systems for maritime applications. With a large portfolio of different propulsion systems, SCHOTTEL serves a wide range of vessel types, including tugs, ferries, yachts, offshore and research vessels.
Author: Benjamin Dyck, Product Development TPE, SCHOTTEL GmbH, Schottelstr. 1, 56281 Dörth, Germany
Co-Author: Sascha Pazour, Sales- & CAE-Engineer, PART Engineering GmbH, Bergisch Gladbach, Germany
