A hydrofoil impeller (1) comprises a central hub (4) which is connected to the shaft (2), and being in the form of a flat plate and being perpendicular to the central axis (x). The central hub has a number of groups of first bolt holes (5) arranged to form a pattern, the number of groups of first bolt holes corresponding to a number of blades (6) attached to the central hub. At least three blades (6) extend radially outwardly from the central hub (4), each blade (6) having a root portion (7) being in a form of a flat plate with a uniform thickness and having a group of second bolt holes (8) arranged in a corresponding pattern in relation to the pattern of the first bolt holes (5) so that the group of second bolt holes (8) can be aligned with the group of first bolt holes (5) and bolts (9) can be placed through the first and second bolt holes to form bolted joints. The number of holes in each group of first and second holes (5, 8) is at least five. The pattern in which the first holes (5) and second holes (8) are arranged in each of the respective groups of holes has a form of a closed curved shape.