Investigation of the Use of Direct Laser Writing Riblets on a Container Ship Propeller

An extended study has been conducted to evaluate the drag-reducing potential of laser-fabricated riblet structures on container-ship propellers. Building on earlier work with foil-applied riblet foil, which demonstrated a 2.0 efficiency points, this investigation has applied direct ultrashort-pulse laser writing to produce trapezoidal riblets (30-70µm pitch) directly on curved metallic blades. The hydrodynamic results demonstrated that the laser-engraved riblets are the superior application method, achieving a maximum efficiency gain of 2.2 points compared to the smooth baseline, slightly surpassing the foil application. This enhancement is attributed to the customization capability of the laser process, which allows for locally optimized riblet dimensions tailored to flow conditions. Scalability of the process was assessed through a 0.5 m² prototype engraving, requiring 15 days of machining at 0.2 cm²/min and yielding energy consumption of 625 kWh (0.29 €/kWh). A techno-economic analysis predicts a four-year payback based on 7 700 h/year operation and 600 €/t fuel. These results demonstrate that direct laser riblet engraving is a viable retrofit for small to medium vessels and, with further process optimization, offers a pathway to broader maritime fuel-saving applications.