The paper reports the substantiation of possibility rotary pump efficiency increase in view of hydraulic losses decrease in a centrifugal wheel at the expense of convergence introduction in its inter-blade canal. Besides losses decrease in a pump the transition to a convergent flow in a centrifugal wheel allows using the model of ideal liquid flow that was impossible at liquid flow in a diffusion inter-blade canal of a wheel providing mainly for an empirical direction of investigations. The convergence introduction, as it was shown by experiments, exceeds pump efficiency by 19% and pres-sure by 74% at the decrease of pulsation and vibrations of a pump that lightens the work of bearings. Theoretical investigations and experimental data have shown that at the convergent inter-blade canal of a wheel at the pump energy efficiency increase there is a situation of a flow different from the flow in the diffusion inter-blade canal. In such a way, the coefficient of the influence of a finite number of blades exceeds a unit, the angle of flow deviation at the wheel outlet takes a negative value (an angle of flow exceeds a bladeexit angle) and a theoretical pressure turns out to be higher considerably of a theoretical pressure at an infinite number of blades.