This study aims to evaluate the magnetic adhesion performance of an automatic grinding device used in shipyards, where surface finishing remains largely manual despite its importance to weld quality and painting preparation. To improve operational stability and reduce dependence on manual labor, a finite element analysis (FEA) was conducted to examine the effects of magnet quantity and air gap distance between the magnets and ferromagnetic surfaces. Three design cases were analyzed with two, four, and six magnets, respectively, and the air gap was varied from 5 mm to 10 mm. The magnetic flux density and resulting attractive force were calculated based on Maxwell stress equations. Results showed that increasing the number of magnets improved the adhesion force, with Case III (six magnets) yielding the highest force of 329.12 N at a 5 mm gap. The required adhesion force was also theoretically derived using a free body diagram and energy-based approach, confirming that Case III meets the necessary force (308 N) for stable vertical surface operation. These findings suggest that optimal magnet configuration is critical for ensuring safe and consistent performance of automatic grinding systems in harsh shipyard environments.