In the paper we study free vibrations of a longitudinally stiffened, viscous liquid-filled orthotropic cylindrical shell in ideal liquid. The Navier – Stokеs linearized equation is used to describe the motion of the internal viscous liquid, the motion of the external liquid is described by a wave equation written in the potential by perturbed velocity. Frequency equation of a longitudinally stiffened orthotropic, viscous liquid-contacting cylindrical shell is obtained on the basis of the Hamilton – Ostrogradsky principle of stationarity of action. Characteristic curves of dependence are constructed.