Here you can see, this top section, the face is lying on the helicoidal line, the expanded line is
straight and the face is matching with it and the back is here, so this forms a part of a
helicoidal surface. This sort of section shape is known as aerofoil section where you can see
the leading edge is not really sharp, it is rounded and the trailing edge is sharp- that is how
propeller blades are made- the leading edge has a curvature, but the trailing edge is sharp.
There is another way of knowing for which is the leading edge- the one that is sharp is the
trailing edge, the one that is blunt is the leading edge and that is the one that will be meeting a
water first, so you can also decide which way the propeller rotates by just looking at the
propeller if it has the aerofoil section.