A hydrofoil impeller for producing fluid flow in axial direction relative to a shaft rotating around its central axis (x) in an agitated tank , said impeller comprising - a central hub which is connected to the shaft , the central hub being in the form of a flat plate with a uniform thickness and being perpendicular to the central axis (x), the central hub having a number of groups of first bolt holes arranged to form a pattern, the number of groups of first bolt holes corresponding to a number of blades attached to the central hub, - at least three blades extending radially outwardly from the central hub , each blade having a root portion , said root portion being in a form of a flat plate with a uniform thickness, said root portion having a group of second bolt holes arranged in a corresponding pattern in relation to the pattern of the first bolt holes so that the group of second bolt holes can be aligned with the group of first bolt holes and bolts can be placed through the first and second bolt holes to form bolted joints, and wherein the number of holes in each group of first and second holes is at least five, wherein the pattern in which the first holes and second holes are arranged in each of the respective groups of holes is in a form of a closed curved shape, wherein each blade comprises a leading edge and a trailing edge in the direction of the rotation; the hydrofoil impeller being characterized in that the attachment of the blade at the root portion provided by the closed curve shaped pattern of bolted joints provided by said first and second bolt holes and bolts is located, in the direction of the rotation, in front of and near to the side of the leading edge of the blade and substantially behind the trailing edge of the preceding blade. - a central hub which is connected to the shaft , the central hub being in the form of a flat plate with a uniform thickness and being perpendicular to the central axis (x), the central hub having a number of groups of first bolt holes arranged to form a pattern, the number of groups of first bolt holes corresponding to a number of blades attached to the central hub, - at least three blades extending radially outwardly from the central hub , each blade having a root portion , said root portion being in a form of a flat plate with a uniform thickness, said root portion having a group of second bolt holes arranged in a corresponding pattern in relation to the pattern of the first bolt holes so that the group of second bolt holes can be aligned with the group of first bolt holes and bolts can be placed through the first and second bolt holes to form bolted joints, and wherein the number of holes in each group of first and second holes is at least five, wherein the pattern in which the first holes and second holes are arranged in each of the respective groups of holes is in a form of a closed curved shape, wherein each blade comprises a leading edge and a trailing edge in the direction of the rotation; the hydrofoil impeller being characterized in that the attachment of the blade at the root portion provided by the closed curve shaped pattern of bolted joints provided by said first and second bolt holes and bolts is located, in the direction of the rotation, in front of and near to the side of the leading edge of the blade and substantially behind the trailing edge of the preceding blade. <EOS>