ON THE MANOEUVRING PREDICTION OF POD DRIVEN SHIPS
Michael D Woodward (University of Newcastle, UK)
David Clarke (University of Newcastle, UK)
Mehmet Atlar (University of Newcastle, UK)
Existing semi-empirical manoeuvring derivative prediction methods are modified to account for the hull-form changes required when fitting podded propulsion units to ships. Also, empirical methods are proposed for the prediction of the hydrodynamic stabilising and control forces of the pod units. The estimation tools are validated by comparison with captive and free-running model test results for four podded ship designs. Suitable preliminary design assessment methods are identified and applied, demonstrating excellent agreement between the estimated and the measured values.
The use of integrated electric pod propulsion units has become a familiar sight in recent years. Since their commercial introduction, now more than a decade ago, the choice of pods available and the range of their application have grown steadily. In response to this 'home-grown' technology the European Community has sponsored several large scale research and technological development projects to investigate and exploit the development of pods and their application. Optimal design and implementation of azimuthing pods for the safe and efficient propulsion of ships (OPTIPOD) concluded three years of research at the end of January 2003 . The OPTIPOD project investigated all aspects of pod driven ship design bringing together 14 European partner organisations covering the broad range of expertise needed for such a study. The University of Newcastle lead the work-package dedicated to the safety issues of pod driven ships including the analysis of manoeuvring characteristics and course stability. The work included the development of semi-empirical derivative estimation tools; CFD analysis; captive model testing; free-running model tests; full scale trials; simulation studies; redundancy analysis; safety and risk assessments. The work was culminated with an optimisation report and assessment of the compliance and relevance of the IMO manoeuvring criteria for pod driven vessels.
The subject of this paper concentrates on the development of the semi-empirical tools and their validation using the four OPTIPOD designs as case study, namely: a ropax; cargo ship; cruise ship; supply ship. The proposed equations are compared with pre-existing equations to establish improvement by comparison with the captive model test results. Suitable post analysis methods are proposed and applied and the conclusion compared with the free-running model test results.
2. OPTIPOD SHIP DESIGNS
The viability of the OPTIPOD technology was demonstrated through the implementation of the four case study designs. The application and mission profile of each design are quite varied, encompassing many of the ship types attractive for the introduction of pods. Each design has further variations and a final optimised version.
2.1 OPTIPOD Ropax
The OPTIPOD Ropax is 194 metres in length and displaces approximately 22,300 cubic metres. The ship has a design speed of 29 knots and is driven by two puller type pod units. Tests were conducted on both Ropax hull driven by conventional twin-screws and by twin pods with a variety of configurations including various skeg arrangements, pod positions, control flaps and also the inclusion of a ducktail [2-5]. The final optimised form includes a bulbous bow, large central skeg and the ducktail as shown in Fig. 1.
2.2 OPTIPOD Cruise Ship
The OPTIPOD Cruise liner is 289 metres in length and displaces approximately 46,300 cubic metres. The ship has a design speed of 24 knots and is also driven by two puller type pod units. Tests were conducted on both the initial version and an optimised design [6, 7]. In general, much of the hull-form optimisation concentrated on the forward body and bulb and resulted in the form shown in Fig. 2.
2.3 OPTIPOD Cargo Ship
The OPTIPOD Cargo ship is 160 metres in length and displaces approximately 30,000 cubic metres. The ship has a design speed of 15 knots and is driven by a single pod unit. Tests were conducted on a variety of skeg and pod arrangement including puller and pusher variations and fins on both the pod and the hull [8, 9]. The final hull-form includes a bulbous skeg optimised for a single puller type pod unit and is shown in Fig. 3.
Fig. 1 OPTIPOD Ropax
Fig. 2 OPTIPOD Cruise Liner
Fig. 3 OPTIPOD Cargo Ship
Fig. 4 OPTIPOD Supply Ship
2.4 OPTIPOD Supply Ship
The OPTIPOD Supply ship is 89 metres in length and displaces approximately 9,000 cubic metres. The ship has a design speed of 15 knots and is driven by two puller type pod units. Captive model tests were conducted in parallel with CFD analysis and simulations were compared and validated by comparison with full-scale trials of similar pod drive ships . The final hull-form has a conventional bow and a large central plate skeg as shown in Fig. 4.