Pods release space within the hull otherwise required for propulsion motors and give the naval architect fresh opportunities for creating the 'ultimate hullform'. The propeller can be placed more freely in the longitudinal plane and tilted so that it faces the up-flow in the stern. To some extent, too, clearances between the propeller tip and the hull can be optimised more easily than with a conventional arrangement.

The traditional balance of propulsive factors, such as effective wake and thrust deduction, together with naked hull resistance, can inject a new dimension into design work. Creating a more homogeneous inflow to the propeller also improves cavitation properties and underwrites higher speeds or a reduction in noise and vibration. In operation, pods reportedly require a lower overall power demand than traditional plant, thanks to fewer hull appendages (no rudders) and a better propeller location in the wake field. An enhanced hydrodynamic efficiency is accompanied by 'dramatically improved' manoeuvring performance through the ability to deliver full thrust in any direction.

Recently reported problems directly or indirectly involving pods serving cruise ships, however, suggest that design details may need refinement and that pod and aft hull configurations must be fully integrated and optimised. ABB Industry reached a financial settlement in December with Carnival over a malfunction in the propeller bearing system of one of the 14MW Azipods powering Paradise. Insufficient lubrication was reported, although there appeared to be 'no singular root cause'. Appropriate modifications were planned and preventative measures initiated for pods in service. Problems with an 'under-performing electric motor' in one of the 19.5MW Mermaid pods powering Celebrity Cruises' Millennium have also been reported, and a duck-tail has been retrofitted at Newport News to correct alleged serious vibrations.

One designer has acknowledged that the pod propulsor is not yet a mature product and its development potential remains considerable. Cross-industry acknowledgment that the full potential of pod propulsors remains to be tapped inspired the EU-funded OPTIPOD R&D project, which started in January last year and completes at end- 2002. (A report on this project appeared in The Naval Architect October 2000, page 48). The aim is to develop design guidelines for pod-propelled ships, focusing on cruise liners, ro-pax ferries, product tankers and offshore supply vessels. The overall findings will help in the development of new class rules for pod propulsors.