Steerable propulsion units - hydrodynamic issues and design consequences

The last three decades have shown a strong development in the market for steerable propulsion units. This paper addresses several main developments and places them in a historic perspective. The major objective of the paper is to present a review of issues relevant to steerable propulsor units. These issues are essentially of a hydrodynamic nature. Although it is thought that hydrodynamic issues often have a heavy impact on the design, the professional background of the authors rather than anything else prompts the choice for an emphasis on hydrodynamic aspects. Starting from the hydrodynamic aspects, we draw several conclusions towards the design and operations of vessels equipped with steerable propulsion units.
Steerable propulsion units refer here to those units that are able to actively deliver a steering moment by rotating the thrust vector through the rotation of the thruster. Such propulsion units may occur in different concepts. The most renowned example and one of the oldest products in this range is the steerable thruster unit (Figure 1).
Recently, since the early nineties, a distinct concept has made its way into the marine world. This new concept is referred to as podded propulsor (or in short: pods) and is distinguished from the original thruster in that its prime mover is an electric motor, situated in the hub underneath the strut, directly driving the propeller (Figure 2).
Apart from the steerable thruster and the pod, a number of other steerable propulsor units exist. One of the oldest is the Voith Schneider Cycloidal propeller (see Figure 3). This propeller is characterised by a number of foils rotating about a vertical axis, with a blade angle that depends on the blade position. The blade angle is controlled by a mechanical actuator mechanism, which essentially determines the thrust/torque ratio in every position.
A special type of waterjet that is worth mentioning is the Schottel Pumpjet, which distinguishes itself by the combination of intake, pump and nozzle in one rotatable unit (see Figure 4). The gain in space and the consequent flexibility in the ship design are obvious.
The paper first aims at providing some historic background to the development of steerable propulsion units. This is followed by a discussion on hydrodynamic issues and design consequences for perhaps the two most popular steerable propulsors: the steerable thruster and the podded propulsor.