By Kevin Sorbello, Chief Engineer
Voith Turbo announced their first Voith Linear Jet (VLJ) order by Turbine Transfers UK in February of this year. The Voith Linear Jet drives will be installed on a 19 meter BMT Nigel Gee designed Wind Support Vessel, used to transport personnel and equipment to and from offshore wind turbines. The VLJ is a departure from the water jet drives installed on the other 20 vessels (fast catamarans) operated by Turbine Transfers, a wholly owned subsidiary of Holyhead Towing Company.
The VLJ combines the increased Bollard pull (BP) capability of a ducted propeller (e.g. Kort or Rice speed/thrust nozzle) with the high speed capability of a conventional water jet drive.
To understand the significance of this development, it is important to understand a little about propeller design. Most fixed pitch propellers are designed for an optimum water speed around 20 knots. However, because some of the energy produced by the propeller is allowed to escape tangentially from the propeller, it is less efficient than a propeller that has been surrounded by a airfoil shroud, or “nozzle.”
Originally designed to reduce propeller-wash erosion, nozzles installed around propeller blades were found to increase the thrust imparted by the propeller at slower speeds. In 1934 Luigi Stipa and Ludwig Kort (an aeronautical engineer from Hanover) demonstrated that surrounding the propeller with an airfoil-shaped shroud could accelerate the inflow velocity of water. As water is circulated, there is an inward force applied that produces a positive thrust; up to 40% of the thrust is developed by the nozzle and transferred to the hull. The small clearance between the propeller and the nozzle reduced tip vortex, which also increased the overall efficiency of the system. These “accelerating” nozzles were found to increase Bollard pull by 30% and can result in up to 22% fuel savings.
When the airfoils face the other direction, they are “decelerating” nozzles, which reduce cavitation and vibration by increasing pressure and reducing water-flow velocity, offering quieter operation. Figure 1 provides an illustration of accelerating and decelerating profiles.
Maritime Research Institute Netherlands (MARIN) conducted extensive research on ducted propellers and established a series of numbered standard airfoil shapes. The most common shapes are the MARIN 19A and 37. They also designed a 22, which had a longer shroud in proportion to its propeller diameter, which increased its efficiency, but gave it poor backing characteristics. The 19A is used on vessels requiring more efficiency in the forward direction, whereas the 37 provides a more balanced profile. Since Kort was the first to produce these “nozzles”, ducted propellers are commonly known as “Kort Nozzles.”
However, Rice developed different profiles that increased the efficiency of the airfoils which produces a nozzle with a 17:1 drag coefficient reduction. Rice also developed a “thrust” nozzle that maximizes thrust characteristics over lower speeds forward and astern.
The differences in operating characteristics between the Rice and Kort nozzles can be seen in the following tables showing Kort to Rice Speed nozzle, and Kort to Rice Thrust nozzle performance:
Regardless of which type of ducted propeller, it is clear that ducted propellers produce high efficiency when the speed of the vessel is between 10 and 15 knots, and competition between makers of ducted propellers is indicative of the role they play in producing power plants that produce greater BP for the same input horsepower. However, the drag induced by the nozzles and fastening mechanisms increases along with the speed of the vessel until the drag offsets the gains of the nozzle design. The nozzles designed for higher speeds have reduced backing efficiencies and increased astern maneuvering issues. This makes ducted propellers ideal for slower vessels that require high thrust, such as tug boats, but not suitable for high speed resupply and support vessels.
High speed resupply and support vessels reach speeds up to 40 knots. Open propellers are best suited for speeds up to 20 knots, but some highly skewed propeller blade designs can handle higher speeds. Propellers designed for lower cavitation at such speeds have poor thrust and higher power demands at lower operating speeds, and still face the reduced efficiency associated with open propellers. Water Jet drives offer high speed capability by designing the water impeller to be operated at very high speeds and incorporating them into the body of the vessel, reducing the drag associated with propellers and struts that project out from the hull. They are better suited for shallow water operation, and their location ensures no air cavitation because the “propeller/impeller” is always submerged. The water passes through impellers and stators which increase the velocity of the waterflow. The resulting jet stream of water produces the thrust that can be directed without backpressure from rudder interference. The problem with these jet drives is that they work best when operating at high speed, and are less efficient at lower speeds, because they produce less thrust at lower speeds than even their open propeller counterparts.
The Voith Linear Jet drive, however, is a hybrid of the ducted propeller and the water jet drive, having a highly skewed propeller, similar to a jet drive’s impeller, within a shroud containing stators, also similar to a water jet. According to Voith, “It consists of a specially shaped jet, a rotor and a stator. The prototype has a rotor diameter of two meters, a jet length of three meters and an input power of 6.0 MW.” This gives their drive the thrust characteristics of a ducted propeller, with the speed characteristics of a water jet drive.
Like many technological developments, the VLJ seems less revolutionary and more evolutionary, but Voith was the only manufacturer to connect what seems like logical steps in the evolution of their drive. This is a significant improvement over conventional water jet drives because it provides greater BP and maneuverability at lower speeds, while not sacrificing the upper speed requirements of its intended service. Therefore Voith’s claim that “for the same installed power the VLJ is expected to provide a bollard pull approximately 50% higher than that of a waterjet and in excess of 30% higher than conventional propellers” withstands scrutiny; these numbers are the same for any ducted propeller.
A Voith press release stated, “The Voith Linear Jet takes away some of the disadvantages of traditional waterjets, without compromising the vessel’s top and sprint speed ability. The considerable higher efficiency at low-end cruising speeds will provide a positive design spiral for light weight craft. Smaller fuel tanks for the same range, reduce vessel displacement resulting in increased hull efficiency with ensuing increased range and top speed” and “this gives a VLJ equipped ship a considerable range advantage against its waterjet sister vessel. Typical VLJ applications will be any ship with a mixed operating profile between low speed cruising and sprint speeds like Navy and Coastguard vessels and Yachts. Ferries operating at sustained speeds around 30 knots employing high-speed or medium speed Diesel or LNG engines will also benefit of this new propulsion option.” Thus, Voith’s claim that their VLJ is capable of providing the speed characteristics of a jet drive, when the drives are designed to remain immersed to avoid cavitation and close to the hull to reduce the drag profile, is also validated (like its predecessors, the new vessel is a fast catamaran hull design).
Bollard pull is important for the Windfarm Support Vessel operator in docking operations because a higher BP allows personnel transfers in higher sea states. The largest advantage of the VLJ over waterjet drives is that the increased bollard pull does not require more powerful engines, and with a torque curve similar to that of a waterjet, the torque limits of the engine do not impact the available bollard pull which can be the case with fixed pitch propellers. The VLJ is therefore an interesting step forward in the evolution of ship propulsion.
Experience with these drives may turn up unexpected issues, but the hybrid design is sound and worthy of testing. Towards this end, Voith and BMT are conducting “extensive model tests” to optimize the VLJ design and validate its projected performance. The first VLJ900 unit is in production and the vessel it will power is expected to be completed early next year.