The given article is dedicated to problems, bound with the solving of hydrodynamic problems by a boundary element method. The problem of hydroelastic interaction of a vessel shell with a contacting liquid (vibration of elastic design in a liquid) is esteemed. Thus the version is esteemed if there is a free surface of liquid. The problem is stated in ideas of a module-element method.
The vessel shell is modelled by spatial module-elements. The liquid is modelled by spatial boundary elements. In the article the special boundary element is offered. This boundary element has the form by an arbitrary spatial thin shell (copy the form of a wetted surface of a vessel). This boundary element has a title "module-boundary element" (MBE). In the article the mathematical model of a problem in general is given and the basic mathematical equations for a module-boundary element of liquid are given.