Case study on the intact stability of wind-assisted merchant vessels – assessing the impact of different stability criteria and rotor sail operational characteristics on E-ship1’s maximum operational wind velocity for various rotor sail sizes

The size of Wind-Assisted Propulsion System (WAPS) installations, exploited to reduce fuel consumption and emissions, is growing. Ship stability is critical and typically ensured through additional stability criteria, although a harmonized regulatory approach is currently non-existent. This paper presents a limited-scope case study analysing selected criteria using the E-ship1, perhaps the first rotor sail equipped newbuild. Different rotor sizes visualize the effect of variations between the additional criteria and the impact of passive depowering. This study demonstrates that current criteria are unsatisfactory and that the effect of operational factors, not unilaterally considered, can be significant. The maximum wind velocity at which rotor sails may be operated varies significantly with criteria, and rotor sails’ impact on stability is not as significant as traditionally assumed, attributable to their passive depowering behaviour. Criteria optimisation and the use of type-specific criteria may therefore carry considerable benefits, enabling a safe increase of operational efficiency.