Propulsion improving devices (PIDs) have enjoyed much interest in the maritime community in recent years. Saving potential of up to 6% on sea trials have been reported. But not for all vessels/designs PIDs help to improve efficiency – in some cases they can make things even worse. A DNV GL Maritime Advisory project for a bulk carrier showed that a good hull is the best PID.
“In the end it depends on the actual hull, especially details of the aft body lines, the propeller, the propeller loading etc. whether it makes sense to apply PIDs,” says Volker Bertram, Senior Project Manager for Technology, Knowledge and Governance at DNV GL – Maritime.
This was confirmed in a recent DNV GL Maritime Advisory project for a bulk carrier. The starting point was a professional design for a bulk carrier for operation in US waters. Particu-lar draft restrictions and the very full hull shape made this project challenging. The optimisa-tion process targeted for minimum yearly fuel consumption. The ship owner supplied records of actual operational data which were condensed to a handful of representative speed-draft combinations. The objective was then to reduce the combined fuel consumptions for these operational states, considering their time share in yearly operation.
Close to 40,000 hull variants were investigated in total, resulting ultimately in reducing the predicted fuel consumption in calm-water conditions by ~25%. These unusually high savings were confirmed in independent model tests. The client tasked the model basin to investigate also whether a PID could further improve the design. But “the results show no savings by the PID, which very much speaks for your very good hull optimisation,” the customer said. “We will not further consider any PID […] and will keep the well-designed aftbody undisturbed by any interference.”
The key lessons are that hull optimization is a powerful tool for ship design, also for bulk carriers and tankers, and that a good hull is the best PID.