When the curved surface is circular, the resultant of hydrostatic force acting on the surface always passes through the centre of the circle. This is because, as the pressure forces are normal to the surface, all the lines which are normal to the surface of a circle pass through the center of the circle.
As the pressure forces form a concurrent force system, it can be taken as a single equivalent force at that point.
When magnitudes of the horizontal and vertical components of the resultant hydrostatic forces are given, then the line of action is given as

Here  is the angle made by the line of action,  is the horizontal force component of hydrostatic force and  is the vertical force component of hydrostatic force.