With the boundary integral-equation method developed by the author, the wave-exciting forces on and wave-induced motions of a floating body in two-layer fluids are computed and their characteristics are discussed. In a two-layer fluid, for a prescribed frequency, there can be two incident waves with different wavelengths (the surface-wave mode with longer wavelength and the internal-wave mode with shorter wavelength). Investigation is focused on how the difference in the ratio of the fluid density and the vertical position of the interface may influence the motions of a body. The effect of the density ratio appears mainly in heave, changing the resonant frequency especially when the lower-fluid density is large and a two-layer fluid is virtually of single layer with shallow depth. The effect of the interface position can be identified in roll and sway more obviously than in heave, as a shift of the frequency of roll resonance and associated rapid variation in sway.
A sharp change in the fluid density at a certain water depth owing to variation in salinity and/or temperature may be observed in a lake, an estuary or Norwegian fjords. Another example of sharp density change is a thin layer of muddy water at the bottom of harbors or channels with relatively shallow water depth. These density changes may alter significantly hydrodynamic characteristics of a ship, but little knowledge on this is available up to date. In the examples mentioned above, the change in density is usually confined within a thin layer called the pycnocline, and above and below this pycnocline the fluid density is practically constant. In this case, it is possible to model the fluid as a twolayer fluid in which a density discontinuity exists at the interface between the upper (lighter) and lower (denser) layers.