Free vibrations of fluid-solid structures are governed by unsymmetric eigenvalue problems. A common approach which works fine for weakly coupled systems is to project the problem to a space spanned by modes of the uncoupled system. For strongly coupled systems however the approximation properties are not satisfactory. This paper reports on a framework for taking advantage of the structure of the unsymmetric eigenvalue problem allowing for a variational characterization of its eigenvalues, and structure preserving iterative projection methods. We further cover an adjusted automated multi-level sub-structuring method for huge fluidsolid structures. The efficiency of the method is demonstrated by the free vibrations of a structure completely filled with water.where Ω s and Ω f denotes the region occupied by the structure and the fluid, respectively, Γ O the outer boundary of the structure, Γ I the interface between the fluid and the structure, and n the outward pointing normal of Ω s on Γ I . u refers to the solid displacement, p to the fluid pressure, ω indicates the eigenfrequency, σ(u) the linearized stress tensor, and ρ s and ρ f the density of the solid and the fluid, respectively.Discretizing by finite elements yields the unsymmetric matrix eigenvalue problem [5,7,8]