Please report any issues with this site to Lexicon Webmaster.
These two blade sailboat propellers, hinge in the center. When the propeller shaft is rotated by the engine in either forward or reverse, the inertia of the turning shaft causes the propeller blades to fly into the open position and produce thrust just like a fixed blade propeller. While sailing, when the prop shaft is not turning, the blades fold into a dynamic shape that eliminates most of the drag.
The most popular manufacturer of folding propellers is Martec. Their brochure: http://www.martec-props.com/pdf_files/folding_brochure.PDF