This study investigates the corrosion additions associated with ship structures and analyzes their effects by assessing the ship hull girder's ultimate bending moments. The corrosion margins for each structural part of an oil tanker and a bulk carrier are compiled and presented. The details of ship particulars, structures of cross-sections, and material properties are also included. The stress distribution method is employed to obtain the ultimate longitudinal bending moments. As a result, the current paper presents changes in ultimate longitudinal bending moments under the degradation percentages of corrosion margins from 100 to 0%. Based on these results, the safety factors of ultimate bending moments combined with a proposal for reducing the magnitude of the corrosion margins are suggested.