Z-Drives and Podded Propulsion Units
The Excellent Maneuverability
Z-drives are used for vessels requiring a high degree of maneuverability, such as tugs, ferries, offshore support vessels and self-propelled barges.
They offer several advantages:
The need for long shaft-lines, rudders, stern thrusters, CP-propellers and reduction gears are eliminated - space and weight saving installation
Reduced engine room size due to flexible machinery arrangement - new possibilities in ship concept design
Increased cargo capacity - added revenue
High propulsive efficiency - less fuel costs
Excellent maneuvering characteristics and dynamic performance
Low noise and vibrations
Available in various units to (20 - 30) MW.
Layout and General Features
In comparison with the conventional systems the z-drives offers a unique package. The design of those propulsion units includes following functions:
A propeller system including a CPP (FPP) with or without nozzle
A power transmission including bearing, shaft and reduction gear
Since hydrodynamic characteristics of propeller the inflow velocity to the propeller is very important to have an axial velocity which can be provided by z-drives. A typical arrangement of z-drives is shown in next page. With a pulling design, the wake field into the propeller will be homogeneous, as there are no appendages in front of the propeller. For steering the vessel, it is used by small rudder at the rear of the vertical hydrofoil (stem), like a flap on an aircraft wing. This is namely so called "Pod Propulsion System".
What is our objective?
By developing some new and advanced z-drives and its optimization both cavitation and required propulsive power will be reduced compared with the conventional solution. First the hydrodynamic characteristics of only propeller will be analyzed at different conditions. Interaction between the existing appendages and propeller should be studied both in numerically and experimentally in a cavitation tunnel.
However, the pulling design will be only experience the homogeneous wake at a zero azimuthing angle of the pod, even in turning condition by installing a rudder flap on the aft part of the stem to be used primarily for high speed steering while the complete unit is turned at lower maneuvering in harbors.
In order to clarify the open water efficiency of the pulling propeller described above in relation to that of a conventional propeller-rudder arrangement, experimental test should be conducted in the cavitation tunnel with some propeller models at different configurations.
1) Propeller model alone
2) Propeller plus rudder (conventional PRS)
3) Propeller with pulling design
As above mentioned with having many advantages for z-drives, regarding to the hydrodynamic performance of the total system with minimize vibrations and high maneuverability would be the best propulsion package for small craft probably for bigger marine vessels in future.
The Boundary Element Method (BEM) is suitable and powerful for analyzing such configuration system. The panels topological and mapping of body should be put as a complete z-drive unit and determinate the potential and pressure distribution at all panels on the propeller, pod and gear box. Then, it should be extended to calculate the hydrodynamic characteristics of total system and interaction between the propeller and those appendages at different turning angle.