CNC 2919, 4 Blade Magnalium 7075 impeller for 4mm propeller shaft. Held on with a setscrew. Machine-cut to high tolerances, this prop is ready to use right out of the box. The Magnalium 7075 alloy makes this material upwards of 30% stronger than standard 7075 aluminum. Left-hand rotation. These spin counterclockwise when looking at them from the back of the boat.