The domain where a submerged vertical cylindrical tank exists is divided into two regions. Assuming irrational motion and an ideal fluid the Laplace s equations in both regions are solved by the method of separation of variables. The integral equation resulting from matching the potentials at the interface is solved numerically by the Galerkin method. It is shown that the mathematical labor is greatly reduced by dropping the surface effects and the results are in good agreement with those obtained by including surface effects albeit at higher values of excitation frequencies. The case of protruding cylinder is presented to further illustrate the simplicity of the solution.