Sheer Strake
The Sheer Strake is the top strake (row) of side shell plating, running the length of the ship at the level where the side meets the strength (main) deck. Because it lies farthest from the neutral axis of the hull girder, it carries some of the highest stresses when the ship bends in a seaway.
For that reason the sheer strake is usually thicker than the plating below it, and its welded seam to the deck stringer is a critical, well-detailed joint. Cracks or heavy corrosion in the sheer strake are treated as serious structural defects.
The sheer strake and the deck stringer form the upper flange of the hull girder — the part working hardest as the ship hogs and sags. A defect here threatens the longitudinal strength of the whole ship, which is why it gets extra thickness and close survey attention.
Why is the sheer strake thicker than other side plating?
Because it is farthest from the hull neutral axis, it carries the highest bending stress; extra thickness keeps stresses and buckling within limits and adds a corrosion margin.