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The podded propulsor as shown in Fig. 10 a is a steerable propulsion unit comprising a propulsion motor fitted inside a submerged pod. The pod unit can be rotated to any horizontal angles and thus offering the vessel large turning force in any directions without the use of rudder. The steerable pod has also been applied to twin propellers and contra-rotating propellers (CRP). In the multi-propeller concept, two propellers are rotated axially in the same or reverse direction as shown in Fig. 10 b. An electric rim driven thruster (RDT) as shown in Fig. 10 c is a novel integrated motor propulsor consisting of blades mounted on a ring of an electric motor. This propulsor design eliminates the use of long and large in diameter pod.
Research on podded propulsor technology is associated with the development in large and fast vessels where their maneuvering potential are fully realized. The fast ship application for pod drives (FASTPOD) project is established to develop electric pod drives for commercial large and fast ships in an efficient and environmentally friendly manner. The project investigates into various high-speed vessels including a container ship (35 knots), a RoPax ferry (38 knots) and a trimaran cargo vessel (40 knots). Podded propulsors have also been widely used in icebreakers and vessels operating in ice areas due mainly to the introduction of double acting design. In heavy ice conditions, the bow of the vessel is optimized for open water, while the stern is designed to be able to break the ice. This design is feasible only when the conventional rudder is superseded by a steerable propulsion unit. The extensive use of podded propulsors is not merely due to their superiority in maneuverability, but higher propulsive efficiency due to lower ship resistance by approximately 4.5%. Basically, power prediction procedure for podded propulsors is virtually identical to procedure of conventional propellers. A discrepancy is the propulsive efficiency term associated with complexities of scale effect on propeller blades and steerable podded units. The procedure ‘_1978 propulsion prediction method for single screw ships’_ is therefore recommended to the initial prediction for vessels equipped with podded propulsors. An iterative numerical method for performance prediction of a podded propulsor is also developed in Ref. [75]. The approach is to analyze the flow domain around the asymmetric pod with strut and the propeller by applying a low-order boundary element method (BEM) and a vortex lattice method (VLM). By validating the results through the experimental measurements, the proposed method is able to predict the efficiency values of the propulsor with very satisfactory accuracy. An improvement in propulsive efficiency with respect to conventional shaft systems can be estimated up to 12% and has been claimed approximately 20% fuel saving by some operators. Nonetheless, by comparing with open water propellers, a further increase in efficiency of podded propulsors is limited due to resistance of the pod body and strut. The efficiency of podded units hence can be improved by utilizing CRP. The principle idea is to recover rotational energy left in slipstream of the forward propeller and the total delivered power is the sum of delivered power from the forward and rear propellers. An analysis of energy saving potential for a contra-rotating azimuth propulsor is given by Ref. [78]. The hydrodynamic performance is investigated on the basis of low order potential-based panel method. At design conditions, the tangential induced velocities generated by the CRP are smaller than those generated by conventional propellers. This indicates swirl energy in the slipstream can be retrieved more effectively. The CRP concept is also combined with a conventional shaft line driven propeller. This results in hybrid CRP podded propulsion which inherits merits of CRP, electric pod drives and conventional fixed pitch propellers. In this arrangement, an azimuth pod is located immediately after the coaxial line of a conventional propeller. Compare to the conventional twin-shaft propulsion system, the hybrid CRP podded propulsion achieves more than 13% energy saving as reported in Ref. [79]. However, the hydrodynamics gains are partially compensated by mechanical loss from highly complicated transmission components.
The electric RDT technology has been received much attention recently thanks to its more compact design and high efficiency. The resultant high performance is due to the use of permanent magnet radial field motors which obviate field power loss and provide higher torque when operating with low speed. In terms of hydrodynamic efficiency, the open water efficiency of RDT is higher than podded propulsors at all operating conditions. As shown in Fig. 11, the maximum open water efficiency of the RDT and the comparative pod drive is 67.2% and 64.3% respectively. By defining off-design operation at a 3% drop from the maximum pod efficiency, the RDT demonstrates almost double the off-design operation range compared to that of the pod drive, i.e. an increase in bandwidth from 0.28 to 0.54. This insensitivity to off-design operation enables higher performance when operating in heavier sea states. However, a major constraint of RDT applications is the availability of associated high-power components. The propulsion of large commercial vessels generally requires high power thrusters with multi-megawatt rating, while almost RDT products are currently below a megawatt level. Standardization of the technology into larger scale is therefore important to respond such high-power demands.
Podded propulsors, in their current forms, were introduced into the marine industry in the 1990s. They derive from the concept of azimuthing thrusters, which have been in common use for many years: the first application being in 1878. Indeed, many of the early design principles for podded propulsors were derived from azimuthing thruster practice. However, the demand from the marine industry for the growth in podded propulsor size occurred very rapidly during the latter half of the 1990s with units during that period rising from a few megawatts in size to the largest, which are currently in excess of 20 MW. Their principal applications in the early years were for the propulsion of ice breakers and then cruise ships, but subsequently they have found application with other ship types such as Ro/Pax ferries, tankers, cable layers, naval vessels, and research ships. Much of this rapid expansion was fueled by claims for enhanced propulsive efficiency and ship maneuverability: the latter attribute having been clearly demonstrated.
In outline terms, the mechanical system of a podded propulsor has normally comprised a short propulsion shaft on which an electric motor is mounted and supported on a system of rolling element radial and thrust bearings. Within the industry, some thought has been given to changing from rolling element thrust bearings to a twin bearing arrangement comprising a conventional thrust bearing and a separate journal bearing: thereby, splitting the duty of the single rolling element thrust bearing of reacting the propulsor thrust and shaft radial load over two bearings, each having a specific duty. The motor is likely to be either an a.c. machine or, in some cases of smaller units, a permanent magnet machine. Also mounted on the shaft line may be an exciter and shaft brake, together with an appropriate sealing system. The electrical power to drive the motor, some control functions, and monitoring equipment are supplied by an arrangement of electrical cables and leads. These are connected to the inboard ship system by a slip-ring assembly located near the pod’s slewing ring bearing at the interface between the propulsor and the ship’s hull. The podded propulsor’s internal machinery is supported within a structure comprising a nominally axisymmetric body suspended below the hull by an aerofoil-shaped fin. The propellers fitted to these units are currently of a fixed pitch design and are frequently of a built-up configuration in that the blades are detachable from the boss. As in the case of azimuthing thrusters, podded propulsors can be either tractor or pusher units and some designs have a system of tandem propellers mounted to the shaft: one propeller mounted at each end of the propulsor body.
While each manufacturer has variants about these basic forms, Fig. 15.5 shows a typical schematic layout for a tractor unit, this being the most common form at the present time.
Assuming that the podded propulsor is of the tractor type, in its twin-screw propulsion configuration it will operate in relatively clear water, which will be disturbed principally by the boundary layer development over the hull. This contrasts with a conventional twin-screw propulsion arrangement in which the incident propulsor wake field is disturbed by the shafting and its supporting brackets or, alternatively, for a pusher pod configuration, which operates in the boundary layer and velocity field generated by the pod body and strut. Consequently, the wake field presented to the propeller of a tractor podded propulsor, in the absence of any separation induced by the effects of poor hull design, should be rather better for the ahead free running mode of operation than would be the case for a conventional twin-screw ship.
Notwithstanding the benefits of an improved wake field, the siting of the propulsors in relation to the hull and their attitude relative to the ship’s buttocks and waterlines needs to be considered with care. If this is not adequately achieved, then propulsion efficiency penalties may be incurred because the propulsion efficiency has been found at model scale to be sensitive to relatively small changes in propulsor location with respect to the hull. The optimum pod azimuth angle for ahead free running has to be derived from detailed consideration of the flow streamlines over the afterbody of the ship, particularly if a range of operating conditions is anticipated for the ship. Similarly with the tilt angle; however, this may be approximated for initial design purposes as being half the angle of the ship’s buttocks relative to the baseline at the propulsor station. Table 15.1 illustrates a typical example of this sensitivity to pod attitude, in this case relating to the relative attitude of the propulsor for a cruise ship.
The computation of the propeller thrust and torque at or close to the zero azimuthing position can be satisfactorily accomplished using classical hydrodynamic lifting line, lifting surface, boundary element, and CFD methods. Similarly, estimates can be made of the other forces and moments about the propeller’s Cartesian reference frame. However, full-scale trial measurements conducted some years ago on cruise ships’ propellers with conventional A-bracket shafting arrangements suggested that, within the then current state of development of the propeller computational codes, a greater error bound should be allowed for when extending the calculation of these loadings in the other Cartesian directions.
When undertaking maneuvers including turns and stopping, both at sea and when in port, as well as when operating in poor weather, model tests have indicated that the hydrodynamic loadings can significantly increase (Ball and Carlton, 2006a,b). Moreover, the predictions of these loadings do not at present lend themselves to assessment by the normal classical methods of analysis but must be estimated from model- or full-scale data. Similarly, Reynolds Averaged Navier-Stokes (RANS) codes are currently not generally at the required state of development to confidently make quantitative predictions of the loads; nevertheless, they can give useful qualitative insights into the flow behavior and the various interactions involved.
The loadings developed by the pod are complex since the axisymmetric body and a part of the fin, or strut, need to be analyzed within the helicoidal propeller slipstream for a tractor unit. The remainder of the strut lies in a predominantly translational flow field and for analysis purposes must be treated as such. Furthermore, the interaction between the propeller and pod body is complex and this also needs to be taken into account. A different flow regime clearly exists in the analysis of pusher units since the propeller then operates in the wake of the strut, and pod body and the propeller-pod body interaction effects are significant. Notwithstanding these complexities, it is possible to make useful quantitative approximations using earlier empirical data, provided a proper distinction is made between those parts of the propulsor, which are subjected to translational flow and those which will operate within the propeller slipstream. In this context, the earlier work of Gutsch (1964) for inclined propellers can be put to good use provided that appropriate corrections are made. Alternatively, systematic model test data, albeit in a limited form, are now beginning to emerge in the technical literature, for example that contained in Frolova et al. (2006).
Yakolev (2009) has found that reasonable accuracy for loading estimations can be obtained by a combination of analytical methods and empirical relationships. In this methodology, the computation of the propulsor characteristics at large flow angles or extreme advance coefficients utilizes the Rayleigh approach to address the influence of separation. Additionally, the analysis of model test data results in the development of coefficients, which are then utilized in the procedure to give an empirically based calculation procedure.
The forces and moments in the three Cartesian directions need to be quantitatively estimated as accurately as possible, either by model test or by calculation for the full range of different operating conditions, since without such an assessment the reactive loads on the bearings cannot be properly estimated. Indeed, if these loading estimates are inadequate, then the necessary fatigue evaluations that are undertaken for the bearing materials will prove unreliable and this may then contribute to premature bearing failure. Fig. 15.6, by way of example, illustrates a typical variation in propeller blade thrust generated at two different azimuthing angles, 15 and 35 degrees, as a propeller rotates through one revolution. This should be contrasted with the nearly constant thrust and torque signature produced at a zero azimuthing angle.
Notwithstanding the implied reliance on empirical data from model tests, since full-scale data are difficult to obtain for podded propulsors, the scale effects relating to the pod-ship and pod-propeller interaction mechanisms are significant as shown in Ball and Carlton (2006a). Therefore, when measurements are made in a model facility, the experiment must be carefully designed to minimize these effects. However, research effort still needs to be expended in refining the analysis of scale effects in order to gain a fuller understanding of their influence both in terms of the propeller loading and also for ship propulsion studies; ITTC (2005). Nevertheless, the ITTC, Appendix A of ITTC (2005), have developed a draft procedure and guidance for the extrapolation of podded propulsor model tests. A general problem is the treatment of scaling of the pod housing drag where currently a number of methods exist. Sasaki et al. (2004) have shown that a considerable scatter exists between the various methods that have been proposed. However, this scatter does not necessarily imply a similar scatter in the final power prediction for the ship.
Computational fluid dynamic RANS methods potentially offer a means whereby scale effects can be considered free from the constraints imposed by model testing and institutional practices. Chicherin et al. (2004) have endeavored to draw conclusions from studies using RANS codes. First, they considered that the numerical analyses do not support the application of the conventional appendage scaling procedures for full-scale pod housing drag estimates. Second, the form factor concept is inappropriate for pod housing drag scaling and finally, the most suitable extrapolation parameter is the nondimensional resistance coefficient used as a correction to the drag of the complete pod unit.
When testing podded propulsor configurations at model scale, it is important that care is taken to correct the results for the presence of the gap effect between the propeller and the pod body. Corrections of this type have been known to be necessary for similar configurations with ducted propellers, pump jets, and azimuthing thrusters for many years, and their importance lies in the correct estimation of thrust. Similarly, there is also a gap effect between the top of the strut and the lower end plate of the model test set up. Within present knowledge, this is thought to be relatively small.
The pod body design is important in terms of minimizing boundary layer separation and vorticity development. Islam (2004) and Islam et al. (2005a,b,c, 2006a,b) in a series of papers have made a useful model test and analytical examination of the influence of the various pod body geometric features. In particular, the radius of the axisymmetric pod body needs to be minimized, but this is dependent on the electrodynamic design of the propulsion motor, which is, in turn, both motor speed and length dependent. Furthermore, the motor speed, along with the unit’s speed of advance and power absorption, governs the propeller efficiency. In terms of future development, high-temperature superconducting motor research offers a potential for significant reductions in motor diameter and hence, if realized, will facilitate pod body drag reductions.
Full-scale experience shows that for twin-screw propulsion systems, podded propulsors, when operating close to the zero azimuthing position, generally have a superior cavitation performance when compared to conventional propulsion alternatives. This implies that the propeller radiated hull surface pressures will be significantly reduced. Typically for a nonice classed, tractor podded propulsion system operating on a well-designed cruise ship hull form, the blade rate harmonic hull surface pressures can be maintained at around 0.5 kPa, with the higher blade rate harmonics generally being insignificant. Such a finding is compatible with the expected enhanced wake field in which the podded propeller operates. However, while not generally reaching these low levels, it should be recalled that the radiated hull surface pressures for conventionally designed ship afterbodies with shaft lines and A-brackets have improved significantly in recent years, typically returning values in the region of 1 kPa. Notwithstanding this benefit, it has been found that broadband excitation can tend to manifest itself more frequently with podded propulsor propelled ships. When significant azimuthing angles are encountered or large incidence buttock flows are encountered, then the effects of cavitation may be rather more significant and solutions to this may involve aspects of the propeller blades and the pod body.
When turning at speed in calm open sea conditions, a complex flow regime is generated near the propeller, which significantly alters the inflow velocity field. For a twin-screw ship undertaking a turn, the resultant forces and moments generated by the propellers located on the port and starboard sides of the hull are different. This difference depends upon whether the propeller is on the inside or outside of the turn and on the extent of the influence of the ship’s skeg on the transverse components of the global flow field in way of the propellers. Fig. 15.7 shows an example of these differences, measured at model scale on the starboard propeller, during turns to port and starboard when operating at constant shaft rotational speed. Analogous variations are seen for the other force and moment components generated by the propeller in these types of maneuver.
By implication, Fig. 15.7 underlines the importance of implementing a proper speed control regime for podded propulsion systems. It can be seen that if the shaft speed were not reduced during the turn to port, the starboard motor would be in danger of being overloaded if the shaft speed at the beginning of the maneuver was close to the normal service rating. Furthermore, in this context, accelerations and decelerations of the ship are also important. In this latter case, the rate of change of shaft speed during such a maneuver influences considerably the loadings generated by the propeller.
The thrust, torque, lateral forces, and moments also vary significantly throughout a turning maneuver. A typical result measured from the model test programs discussed by Carlton and Rattenbury (2006) and related to the work of Ball and Carlton (2006a,b) is shown in Fig. 15.8.
Fig. 15.8 relates to a maneuver, which changes the heading of the ship through 180 degrees. It can be seen that relative to the steady values recorded on the approach to the turn, as soon as the propulsors change their azimuthing angle the torque and thrust increase. Similarly, the fluctuating shaft bending moment measured on the shaft increases in amplitude and then decays to an extent during the turn but then maintains steady amplitude. In contrast, the thrust and torque maintain their enhanced amplitude throughout the turn and then when corrective helm is applied to return the ship on a reciprocal course these parameters then decay back to their normal ahead values. However, this is not the case for the bending moment amplitudes, which upon applying corrective helm then sharply increase before decaying back to their premaneuver values. When analyzing these data during constant shaft speed turns over several similar tests, it is seen that while the thrust and torque increase their premaneuver levels by between 10% and 50%, the maximum bending amplitudes are amplified by factors of between four and six times their original free running ahead values. In the case of zigzag maneuvers carried out under the same operating conditions, analogous characteristics are found.
These types of maneuvering examples, because of their potential to develop high-bearing loadings, suggest that careful thought should be applied to sea trial maneuvering procedures. Ships driven by podded propulsors generally exhibit a better maneuvering performance when compared to equivalent conventionally driven ships. The implications of this enhanced performance are discussed in Carlton and Rattenbury (2006) and, in particular, the advisability of employing an equivalence principle for ship maneuverability between podded propulsor and conventionally driven ships is discussed so as to minimize the risk of shaft bearing overload in the podded propulsors. Such a principle essentially suggests that if the maneuvering of a conventionally driven ship is satisfactory, then it should not be necessary to effectively demonstrate that a similar ship fitted with a podded propulsor has better maneuvering capabilities since this is already known. Consequently, the sea trials program should be adjusted to define turning rates and other conditions appropriate to the ship and podded propulsor configuration.
In the case of linearly executed stopping maneuvers, exploratory model tests have indicated that if a crash stop maneuver is executed with the podded propulsors in a fore and aft orientation, the bending moments generated can be limited to values consistent with the normal free running service speed; Ball and Carlton (2006a). Notwithstanding this, the thrust and torque loadings change significantly during the maneuver. If, however, the pods were permitted to take up a toe-out attitude, then the shaft forces and bending moments could be expected to significantly increase. In the case, for example, of a 25 degrees toe-out maneuver at constant shaft speed, the ratio of induced bending moment during the maneuver to the free running bending moment at the start of the maneuver could be as high as 11, with similar ratios being developed in the other in-plane loadings.
The effects of poor weather have also been similarly explored, from which the relative motions between the propeller and the seaway have been seen to increase the loadings, which have to be reacted to by the shaft bearings. For the model configuration tested by Ball and Carlton (2006a), the shaft bending moments in irregular waves were found to increase by up to a factor of 1.8 over the free running condition when encountering significant wave heights of 7 m at constant shaft speed. In those tests, the sea conditions, which gave rise to the greatest increase in shaft forces and moments, were those encountered in head quartering seas. Clearly, however, in the general case these loading factors will be ship motion and seaway dependent.
Different loading regimes occur during low-speed harbor maneuvering. It has been known for many years that if several azimuthing thrusters are deployed on the bottom of a marine structure in a dynamic positioning mode, when particular relative azimuthing angles of the thrusters occur, they will mutually interfere with each other. In some cases, if this mutual interference was severe, mechanical damage to the thruster shaft line components could result.
By simulating the underwater stern of a typical cruise ship with a deployment of propulsors and varying their relative azimuthing angles, Ball and Carlton (2006b) found that a number of good and bad operating conditions were identified for a twin-screw ship. These may be summarized as follows:
i. If the pods are at arbitrary azimuthing angles and the angle of one of the pods is chosen such that its efflux passes into the propeller disc of the other, then high fluctuating shaft bending moments and radial forces can be expected to occur on the latter pod. The magnitude of these shaft bending moments at model scale has been measured to reach values of up to 10 times the normal free running values at low ship speed and constant rotational speed. In contrast, the thrust and torque forces appear to be relatively unaffected. At full scale when interference has been encountered between podded propulsors, vibration levels up to 116 mm/s have been recorded at the tops of the pods in the vicinity of the slewing ring.
ii. If both pods are positioned such that they are thrusting in approximately the same thwart-ship line, then the trailing pod will suffer significant fluctuating loads. The maximum loadings will be experienced when the trailing pod is slightly off the common transverse axis: whether this relative azimuthing angle is forward or aft of the thwart-ship line will depend upon the direction of rotation of the propulsors.
iii. It has been found that a benign harbor maneuvering condition is when both podded propulsors are in a toe-out condition. At this condition, the mutual interference with respect to shaft loads is minimal.
iv. The control methodology of podded propulsors when undertaking dynamic positioning maneuvers requires careful consideration if unnecessary, and in some cases harmful, azimuthing activity is to be avoided.
It was also observed that the interference signatures created in conditions (i) and (ii) exhibited a finely tuned characteristic with respect to relative azimuthing angle.
In the case of a quadruple-screw ship, a poor operating condition was found to be when the podded propulsors on one side of the ship are both operating and are positioned such that the forward unit is in the fore and aft direction and the astern one is transverse. In this case, the efflux from the forward propeller is attracted toward the transversely oriented propeller, which then suffers strong fluctuating loads. This is because it is operating obliquely in the helical flow field generated by the fore and aft aligned propulsor. Similarly, when both propellers on one side of the ship are aligned in the fore and aft direction the efflux from the forward propeller, although relatively attenuated, is attracted toward the propeller in the astern location. Consequently, some benefit in minimizing these slight oblique flow characteristics can be achieved by azimuthing the astern propeller toward the location of the forward propeller.
The severity of mutual propulsor interference is load dependent in that at high speeds the effects are potentially considerably more harmful than at low speeds. Under many harbor maneuvering conditions, particularly in benign weather conditions, propulsor speeds are low and consequently the propeller’s effluxes possess low energy. Due to energy dissipation, this implies that the potential of the efflux to damage an adjacent unit does not extend too far from the location of a propulsor. Under these conditions, it is therefore unlikely that significant adverse loadings will be encountered if, for example, one propeller of a twin-screw ship is aligned fore-aft to give longitudinal control while the other is placed in a transverse alignment to control sideways movement.
A number of configurations for podded propulsors have been developed in recent years and these have included units with tandem propellers, rim-driven propulsors, and contrarotating versions. In recent times, a pump-jet variant has also been proposed by Bellevre et al. (2006).
In the case of contrarotating podded propulsors, these tend to be hybrid designs, which deploy a conventional propeller and stern-bearing arrangement for the ship with a tractor azimuthing podded propulsor located immediately astern of the conventional propeller; Fig. 2.9. In this way, when moving ahead on a straight course, the efficiency advantages of contrarotating propeller can be gained without the complexities entailed in the mechanical shafting arrangements. Moreover, such an arrangement has the added benefit of being able to distribute the power between the two propellers, either equally or favoring perhaps the conventional propeller, and in so doing gives a rather better cavitation environment for the absorption of the total propulsion power. These types of arrangements are potentially attractive to relatively fast ships such as Ro/Pax ships. Under turning conditions, since the podded propulsor acts as the rudder it must be ensured that the bearings in the podded propulsor can withstand the level of excitation generated from the periodic loadings generated by the leading propeller as well as the additional loads induced by the turning maneuver. Bushkovsky et al. (2004) examined the mutual propeller interaction comprising the periodic forces and the crash stop behavior of these types of configuration. In the case of the periodic forces, these were shown to be complex because of each propeller’s induced velocities in the disc of the other and, furthermore, the induced velocities were both spatially and temporally dependent. This placed considerable demands on the computational procedures by which the induced velocities were calculated and overcome, in this case by coupling the vortex sheets of the propeller blades and making an analytical estimate of the viscous wake behind the propeller. From this analysis, it was shown, perhaps rather unsurprisingly, that the periodic forces have a wider spectrum of harmonics than would be the case for a conventional single propeller. With regard to cavitation, it was considered important to avoid the podded propulsor of the contrarotating pair interacting with the conventional propeller’s hub vortex and blade generated cavitation when the podded propulsor turns for steering purposes. As such, this situation needs to be carefully considered at the design stage.
Rim-driven podded propulsors comprise a multiple blade row propeller with a permanent magnet, radial flux motor rotor located on the tips of the propellers, which then interacts with the motor’s stator, which is sited within a duct circumscribing the propeller. It is claimed that this arrangement yields the required thrust with a smaller pod when compared to the normal arrangement; it develops a higher efficiency, develops reduced unsteady hull surface pressures, and has improved cavitation performance. The concept has been model tested by Lea et al. (2002). Such an arrangement is not dissimilar to an electromagnetic tip-driven propeller designed and tested by Abu Sharkh et al. (2003). This 250-mm diameter, four-bladed Kaplan type propeller was tested over a wide range of advance and rotational speeds, with differing duct geometries and a limited variation of stator angles. It was also tested in sea water and the unit was benchmarked against a _K_ _a_ 4-70 propeller in a No. 37 duct. It was found that at bollard pull conditions the thrust was about 20% lower than the Wageningen series propeller and that the _K_ _T_ values reduced more rapidly as the advance speed increased. This discrepancy was attributed to the additional drag of the propeller ring and the thicker duct.
In recent years a number of large cruise ships have been fitted with electric podded propulsors. Pod propulsion has also been considered for large container ships and fast ro-ro passenger ships.
The advantages of electric podded propulsors are:
1. Lengths of propeller shafting within the hull are eliminated, thus providing more revenue earning space.
2. The hull form can be designed for minimum resistance.
3. The thrusters provide good propeller clearances and can be aligned with the local waterflow to provide a clean flow of water to the pulling propeller.
4. Lower noise and vibrations are claimed but it is reported that with some installations this has not always been the case.
5. There is improved maneuverability, which is a major advantage for cruise ships visiting a number of ports.
Various podded propulsor arrangements are installed; a smaller vessel may have a single unit providing both propulsion and steering, whereas a large ship like the _Queen Mary 2_ has two fixed podded propulsors, ahead of and further outboard, than the two rotating podded propulsors that provide the steering force. Each of these podded propulsors has a 6-meter-diameter four-bladed propeller.
For each podded propulsor a seating is welded into the ship’s bottom structure to distribute the loads from the pod over a wide area of the hull. The fixed propulsors are bolted directly to their seating whilst the steering units are carried by a slewing ring in their seatings. Power from the ship’s main machinery space is transferred to the motors in the steering pods via a slipring unit.
The general class of azimuthing propulsors includes both azimuthing thrusters and podded propulsors. Before considering these systems in greater detail and to avoid confusion, it is important to be clear on the definition of a podded propulsor as distinct from other forms of propulsion and azimuthing thrusters. A podded propulsor is defined as a propulsion or maneuvering device, which is external to the ship’s hull and houses a propeller powering capability. This distinguishes them from azimuthing thrusters, which have their propulsor powering machinery located within the ship’s hull and commonly drive the propeller through a system of shafting and spiral bevel gearing.
A number of configurations of podded propulsor have been developed in recent years and these have included units with tandem propellers, rim-driven propulsors and contra-rotating versions. More recently, a pump-jet variant has been proposed (Reference 12).
In the case of contra-rotating podded propulsors these tend to be hybrid designs which deploy a conventional propeller and stern bearing arrangement for the ship with a tractor azimuthing podded propulsor located immediately astern of the conventional propeller. In this way when moving ahead on a straight course the efficiency advantages of contra-rotating propeller can be gained without the complexities entailed in the mechanical shafting arrangements. Moreover, such an arrangement has added benefit of being able to distribute the power between the two propellers, either equally or favouring perhaps the conventional propeller, and in so doing give a rather better cavitation environment for the absorption of the total propulsion power. These types of arrangements are potentially attractive to relatively fast ships, for example Ro/Pax ships. Under turning conditions, since the podded propulsor acts as the rudder, care has to be exercised to ensure that the rolling element bearings in the podded propulsor can withstand the level of excitation generated from the periodic loadings by the leading propeller and the additional loads induced by the turning manoeuvre. Bushkovsky _et al_. (Reference 13) examined the mutual propeller interaction: the periodic forces and the crash stop behaviour of these types of configuration. In the case of the periodic forces these were shown to be complex because of each propeller's induced velocities in the disc of the other and furthermore, the induced velocities are both spatially and temporally dependent. This places considerable demands on the computational procedures by which the induced velocities are calculated and was overcome in this case by coupling the vortex sheets of the propeller blades and making an analytical estimate of the viscous wake behind the propeller. From this analysis it was shown, perhaps rather unsurprisingly, that the periodic forces have a wider spectrum of harmonics than would be the case for a conventional single propeller. With regard to cavitation, it was considered crucial to avoid the podded propulsor of the contra-rotating pair interacting with the conventional propeller's hub vortex and its blade cavitation when the podded propulsor turns for steering purposes and this situation needs to be carefully considered at the design stage.
Rim-driven podded propulsors comprise a multiple blade row propeller with a permanent magnet, radial flux motor rotor located on the tips of the propellers which interacts with the motor stator's which is sited within a duct circumscribing the propeller. It is claimed that this arrangement yields the required thrust with a smaller pod, when compared to the normal arrangement; it develops a higher efficiency, develops reduced unsteady hull surface pressures; and has improved cavitation performance. The concept has been model tested (Reference 14) and a 1.6 MW demonstrator unit was destined for sea trials during 2006. Such an arrangement is not dissimilar to an electromagnetic tip-driven propeller designed and tested by Abu Sharkh _et al_. (Reference 15). This 250 mm diameter, four-bladed Kaplan type propeller was tested over a wide range of advance and rotational speeds, with differing duct geometries and a limited variation of stator angles. It was also tested in sea water. The unit was benchmarked against a Ka 4-70 propeller in a No. 37 duct and it was found that at bollard pull conditions the thrust was about 20 per cent lower than the Wageningen series propeller and that the _K_ t values reduced more rapidly as the advance speed increased. This discrepancy was attributed to the additional drag of the propeller ring and the thicker duct.
A number of configurations for podded propulsors have been developed in recent years and these have included units with tandem propellers, rim-driven propulsors, and contrarotating versions. In recent times, a pump-jet variant has also been proposed by Bellevre et al. (2006).
In the case of contrarotating podded propulsors, these tend to be hybrid designs, which deploy a conventional propeller and stern-bearing arrangement for the ship with a tractor azimuthing podded propulsor located immediately astern of the conventional propeller; Fig. 2.9. In this way, when moving ahead on a straight course, the efficiency advantages of contrarotating propeller can be gained without the complexities entailed in the mechanical shafting arrangements. Moreover, such an arrangement has the added benefit of being able to distribute the power between the two propellers, either equally or favoring perhaps the conventional propeller, and in so doing gives a rather better cavitation environment for the absorption of the
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