The Rackham Aero is compatible with Bote’s Apex Pedal Drive system. This is a propeller system you pedal with your feet (like a bicycle). The Apex Pedal Drive mounts to the Rackham Aero through this hole. The Rackham Aero includes the Pedal Port to either install the Apex Pedal Drive, or seal up the hole by leaving in the clear plastic plate.