Podded Propulsor / Z-Drive
Propulsion of a ship has traditionally been accomplished by using one or several rotating propellers and steering has been accomplished by making use of one or more rudders in connection with a propeller. A marine vessel normally comprises a hull, a rudder support structure attached to the hull, a rudder attached to the rudder support structure in a manner allowing pivotal movement of the rudder relative to the rudder support structure, a drive train including a propeller shaft mounted in the hull and extending from the hull, and a propeller unit attached to the propeller shaft and positioned forward of the rudder device.
So called Z-Drive configurations, with fully directional propellers, eliminated rudders altogether. They are designed to independently rotate 360 degrees. Combined with a thruster in the bow, they can give vessels unmatched maneuverability. In such drives the joint connection is usually protected in the length of shafting between the motor and the gearing with a bellows preventing penetration of water and dirt. The bellows in turn is at least partly surrounded by a bell-shaped lug which starts out from the upper housing part of the Z-drive in order to reduce the danger of damage. Z-drive modular propulsion units are advantageous where draft minimization is important and simplify construction and maintenance.
With an electric drive removing the need for long propeller shafts, propulsion options now include pod mounted propulsors in either fixed or steerable mountings. Rotatable pods offer the advantage of applying thrust at any angle through the entire 360 degree field of motion, thus increasing the ship's maneuverability. Podded drive systems have been installed on several classes of large commercial vessels. They are also planned to be used on several new foreign warship designs and on the US Coast Guard's next Great Lakes icebreaker. Pods are not limited to just the stern of the ship, but can also be placed around the hull to increase survivability and maneuverability.
By one estimate, podded propulsors are 14% more efficient compared to conventional propulsion: 6-8 % from a more efficient hull, 4-6 % due to reduced appendages, and 1-2% by tilting the propeller into the flow. Additionally, the azimuthing pods can turn 35o at full speed, and a full 180o at slow speed or stop. This contributes to unsurpassed maneuverability in littoral waters. Podded propulsion systems can offer advantages in outfitting and fuel costs compared with conventional systems,
However, there are technical issues which must be examined and potential obstacles to overcome. These include shock resistance, power density, signatures, and reliability. The capabilities of current pod designs to resist shock must be determined and future designs may need engineering modifications to be more survivable. Current pod designs lack sufficient power density to provide the high speed needed by many surface combatants. The current designs are large and heavy, and simply scaling them up to provide the power necessary for high speed will make them too large for practical installation in small and medium sized warships. Pod acoustic and magnetic signatures need to be investigated and tailored to meet the demanding needs of modern warship designs. Finally, there are reliability issues with current pod designs that must be addressed.
Such a steering and manoeuvering system for water-born vessels with two individually turnable propulsion units arranged at the stern portion of the vessel and mutually spaced athwartships. These units may be such as jet units, turnable so-called thrusters or turnable propeller units of the "AQUAMATIC", "Z-DRIVE" type or the like, (but not propeller units with a stationary propeller shaft and a separate rudder).
A first type of known steering and manoeuvering system for vessels with such double propulsion units includes an actuating turning device, which is actuable by a steering control, e.g. a lever or a wheel, and keeps the propulsion units parallel while being turned, and an actuating drive device, which is actuable by a power control for adjusting the propulsive power ahead or astern of the respective propulsion unit. The vessel can thus be steered for sailing ahead or astern in a desired starboard/port direction, as well as for executing a rotating or turning movement (one unit set for propulsion ahead and the other one for propulsion astern) which can be to advantage in manoeuvering in ports, harbours and other cramped situations. It is not possible to achieve pure athwartships movement with such a system, however. Athwartships movement namely requires that both propulsion units can be turned in opposite angular directions with opposing directions of propulsion.
In a second type of known steering and manuvering system, all steering functions are achieved by a single controlling joystick which is movable in two dimensions. When the joystick is moved forwards/backwards the combined propulsion effect is increased ahead or astern (by different adjustments of the turning angle) and when the joystick is moved sideways port/starboard steering action is achieved to port or starboard. The power of the propulsion units can be regulated at the same time by pulling out or depressing a knob on the joystick. Turning and athwartships movement of the vessel can be achieved for certain positions of the joystick.
Known propulsion systems for high speed craft display considerable limitations in performance at low speeds, at high speeds, or throughout the desired speed range. The simplest form of propulsion for marine craft, the submerged propeller, has many limitations and tends to have low efficiency characteristics at high speeds. An improved drive system, the `Z` (or stern) drive, introduced in the 1960's, provides improved efficiency at higher speeds for smaller craft. However, at very high speeds problems are experienced with this type of propeller and often a surface-piercing propeller must be fitted instead.
For high craft speeds surface-piercing propellers fitted either to a specialised surface drive system, or to a Z-drive, give the highest efficiencies. However, conventional surface-piercing propellers are extremely power-absorbing at low speeds. One reason for this is that because these propellers are designed to be run semi-immersed their diameter is large compared to a conventional propeller. Thus until the craft has achieved planing speed the propeller is normally excessively immersed such that the flow and torque requirement are excessively high. A second factor, which is less well understood, is that at low speeds and high power the blades are running at a high lift coefficient, the vapor cavity behind the blade is wide and the distance between the external surface of one blade cavity and the propulsive surface of the succeeding blade is small. Thus the blade is effectively pushing against a vapour bubble with an evident loss of thrust. These two factors in particular cause craft fitted with surface drives to have considerable difficulty getting onto the plane which means they have to be fitted with excessively powerful engines. As a result of the limitations imposed by these drives their usage remains restricted and their cost is high. Also, such propellers are normally mounted well behind the hull which renders them vulnerable to damage when manoevreing or at berth. In most cases, the propeller cannot be raised sufficiently to enable the craft to be beached.
Conventional tugboats have been designed with large-diameter, fixed-directional propellers for providing the desired levels of thrust. This approach has resulted in relatively deep drafts for harbor tugboats, often preventing their use in shallow inland waters. The fixed direction of thrust limited the tugboat to handling vessels only by pushing or pulling them parallel to the centerline of the tugboat's hull. Accordingly, not only could the tugboats not apply thrust in any direction, other than fore or aft, but they also lacked the necessary transverse stability to resist heeling, with a significant danger of capsizing if subjected to any transverse force. In ship handling and docking of large vessels, tugboats are typically tied alongside either parallel to or at fight angles to the vessel's centerline (this is the normal method in most U.S. ports), a rapid change in the application of tugboat thrust normal to the vessel's centerline cannot be achieved without completely reorienting the tugboat. This also imparts excessively high torque to the rudder. Such an operation also requires handling of lines by the boat's crew, and involves considerable time. In some instances, such an operation may become impossible because of insufficient space between the ship and the dock, or because of other vessels or restrictions in the vicinity. Extreme care must be exercised to ensure that the tugboat is not subjected to transverse loads by its own actions or by loads imposed by the vessel being assisted, through the towing hawser which could tip and capsize the tugboat.
Designs of tugboats have traditionally incorporated ship-shape forms for tug hulls, with bow and stem lines and having compound curvature with shell plating. Such forms necessitate high construction costs, whereas simple straight-framed sections with fully developable shell plating are much less expensive. In any event, numerous shipyards were developed specifically for efficiently constructing such high-cost traditional tugboats.
Another problem with conventional tugboats is that their general hull configuration provides relatively small and confining deck areas, thus restricting optimal location of towing winches and mooring devices, as well as efficient action of the crew in handling lines both fore and aft of the boat.
In addition to the fact that propeller thrust of prior art tugboats was unidirectional, the hull configuration of such tugboats was asymmetrical from bow to stern. Such a configuration imposed a unidirectional thrusting feature. Therefore, prior art tugboats have been greatly handicapped by being unable to achieve optimum performance in most operations without releasing and changing hawsers, lines, etc. to reorient the tugboat so that it could push in the desired direction and position.
While prior art tugboats traditionally have been considered to have good maneuverability, particularly when large rudders, flanking rudders, nozzles, etc. have been installed, the designs have typically been limited by the need to use multiple towing hawsers to maintain the desired orientation and position with respect to the vessel being assisted and by the inherent limitations on its effectiveness due to the limited transverse stability of the tugboat.
Moreover, tugboats have had increasing power levels of parallel propulsion machinery installed, partly to meet demands for high thrust levels in handling ships and barges, and partly to hold the tug in the proper position using opposing thrust and rudder action. While these problems have been undesirable, they have not been solved by resorting to a brute-force approach.
Some tugboat designers have implemented the use of single or plural omni-directional drives to improve the application of thrust in directions other than parallel to the centerline of the tug. While using these omni-directional drives provided certain directional advantages, problems are still encountered when the tug is tied to another vessel. Transverse stability becomes even more critical to the safety of the tug because it is now able to impose significant transverse forces on itself through the direction of propulsion thrust in a direction other than parallel to the centerline of the tug.
Azipods« are azimuthing electric podded propulsion units capable of unlimited 360 degree steering. Because of this the need for rudders is eliminated. The pods contain an AC electrical drive motor coupled to a short drive shaft connected to a fixed pitch propeller. This eliminates the need for any mechanical gearing. This choice of propulsion system is based on the performance of Azipod« propulsion systems on other icebreakers around the world. Additionally, ice model testing with Azipods« demonstrated a significant increase in maneuverability, in ice and open water, over the traditional fixed shaft, propeller and rudder arrangement. The Azipods« are installed in heavy structural foundations and are rotated using hydraulic motors and gearing. A slip ring is used to transmit power and data. The smooth transmission of torque provided by this AC/AC drive system and the elimination of rudders are beneficial in icebreaking operations.
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