State of the art propulsion concepts for yacht applications are nowadays typically designed as hybrid powertrains
A combination of a combustion engine and an electric motor offers many advantages in operations such as a pure electric silent drive mode or boosting, load balancing, etc.
Coupling solutions for hybrid mega yachts therefore need to fulﬁll various tasks. Located between the gearbox and the propeller shaft, they minimize the transfer of the structure-borne noise while transmitting high torque levels. In addition, they resist high misalignments due to the very soft mounted frames of the ship’s engine and gearbox.
The couplings have to sometimes deal with diﬃcult torsional vibration situations in the drivetrains due to the complexity of the operating modes for hybrid system configurations. They have to be lightweight and in the case of electric motors they often need electric insulation properties. To fulﬁll all these partly contradictory demands, innovative and tailor-made approaches are essential.
As an example, figure 1 shows a typical powertrain in a high-performance, hybrid mega yacht. In many cases, the engine is built on a double elastic mount and is coupled to a gearbox which is mounted either on to elastic or hard elastic. In hybrid applications, the gearbox is also connected to an electric generator or engine on a separate elastic mount. In many cases, the powertrain includes lightweight composite couplings (1-3) as well as individually adapted steel spring couplings and dampers (4).
The misalignment coupling between the gearbox and the propeller shaft (2) is most critical for the transfer of the structure-borne noise. From the shaft bearings, the noise may be transferred into the whole ships structure and into the environment. Therefore, the reduction of the structure-borne noise level at the bearing position is essential for a low noise level in the whole ship. To fulfill these demands, the Geislinger Silenco® coupling has been developed. The Geislinger Coupling helps to reduce the noise in the cabins as well as the noise reduction into the environment.
Silenco® couplings are made from carbon and glass fiber combined with elastomer layers. The segments are connected by steel bolts. Figure 2 shows a picture of the coupling. The length can vary depending on the necessary misalignment and the customer’s needs. Both sides can have a different design with elastomer layers and a different membrane to avoid resonance effects.
Due to this design, the coupling is a modular system and easily adaptable in length. Further components are available. Due to the different sizes, it can be used for a wide torque range. With a diameter of 630mm (SC63), the coupling can transmit a nominal torque of 20 kNm with a weight of only 42 kg. Versions with 1400mm diameter (SC140) can transmit a torque of over 300 kNm, e.g. between a super silent 4MW electric engine and the propeller shaft.
Powertrain acoustics need an excellent system understanding and therefore a holistic system approach. The reduction of the structure-borne noise needs to be combined with the reduction of the torsional vibration levels and gearbox excitations in order to achieve outstanding noise reduction. Therefore, the misalignment couplings are often combined with tailor-made steel spring couplings and dampers, see no. 4, figure 1. They mainly reduce the torsional vibrations, but also decrease the low frequency noise. Figure 3 shows the design of such a steel spring coupling.
The electric generators or engines can easily be integrated using e.g. Membrane-Misalignment-Couplings made from composite material with a composite shaft in between (see figure1, no. 2). The length of the shaft can be varied. In this case, the transfer of the higher frequency noise, mainly caused by the inverters, needs to be minimized.
This example shows the complexity of hybrid propulsion plants, especially under consideration of the noise reduction aspects. A deep understanding of the system behaviour and the different effects of the components, their interaction and the influence of the different operating conditions is key for the successful development of tailored powertrain solutions for such applications.
Further information can be found on the Geislinger homepage: geislinger.com