## 6 Conclusions  
Erosion of propellers and hulls is a critical issue in the shipping industry, significantly reducing the lifespan of components in seawater. Offering erosion-resistant materials to control and reduce erosion can increase the lifespan of these components and reduce repair and maintenance costs. In this regard, the effects of materials (Al and brass) and bio-inspired sawtooth riblets on cavitation-induced erosion morphology, depth, and cross-sectional area have been probed through experimental methods to provide scientific and comparable insights into erosion. These aims were achieved by subjecting Al and brass surfaces, with and without riblets, to 500 single cavitation bubbles, with a wall distance of \(\gamma=1.8\), followed by capturing optical microscopic images and depth diagrams of erosion.  
Figure 6: Intercomparison of the bio-inspired sawtooth pattern and proposed riblets. (**a**) Bio-inspired sawtooth riblets. (**b**) Proposed class I riblets (netted-spherical pattern). (**c**) Proposed class II riblets (netted-cubical pattern). (**d**) Proposed class III riblets (netted-star pattern).  
The results indicate that for both Al and brass with flat and ribleted surfaces, the erosion morphology resembles a rounded cone with a circular cross-section. However, the height of the cone axis (erosion depth) and the cross-sectional area of the erosion depend on the material and the presence of riblets. Initially, erosion begins with a large circular cross-sectional area, and as the depth of erosion increases, the cross-sectional radius decreases. In other words, the increasing depth of erosion along the cone axis results from multiple bubble collapses and the subsequent multiple impulses of water hammers. This process decreases the cross-sectional area of erosion due to the increasing wall distance of the bubble as the depth of the eroded region grows after repeated collapses.  
In terms of material properties, brass provides 21.6% more erosion resistance compared to that of Al due to its mechanical stability. Regarding the riblets, employing Al with riblets reduces the depth and width of erosion by 3.8% and 18.3%, respectively, compared to using Al with a flat surface. In contrast, employing brass with riblets decreases the depth of erosion by 7.9% and the width by 27.4% compared to the results for brass without riblets. Concerning riblet geometry, the width of erosion perpendicular to the riblet lines decreases more than the erosion parallel to the riblet lines due to the separation effects of water vortices. The summary of results reveals that materials with durable mechanical stability, when provided with riblets, can significantly decrease erosion. However, the probability of chemical reactions in a cathodic environment and the exploration of other potential riblet patterns should be examined to provide a comprehensive evaluation of corrosion and to achieve maximum erosion intensity reduction.  
Conceptualization, E.K. and O.e.M.; Methodology, E.K., U.L. and O.e.M.; Software, S.R.; Validation, S.R.; Formal analysis, U.L.; Investigation, E.K. and S.R.; Writing--original draft, E.K. and S.R.; Writing--review & editing, E.K., S.R. and U.L.; Supervision, E.K.; Project administration, E.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no competing financial interests or personal relationships that could have influenced the work reported in this paper.