A fast, matrix-free implicit method based on the Lower-Upper Symmetric Gauss-Seidel (LU-SGS) method was developed to solve the three-dimensional incompressible Reynolds-averaged Navier-Stokes equations on multi-block structured grids. The method was applied to the simulations of a variety of flowfields around 3D complex underwater bodies with different appendages. The numerical procedure and results show that the method is efficient, reliable and robust for steady viscous flow simulations.