Ketch and Yawl rigs both have a Mizzen mast. The main (no pun intended) difference is that a Ketch has the Mizzen mast stepped forward of the rubber post (without regard to where the steering station might be), and the Yawl has it's Mizzen mast stepped Aft of the rudder post. On equal length hulls, a Ketch rig would usually have a shorter main boom than a Sloop or Yawl rig. It makes for a smaller (and easier to handle) mainsail. Ask a Marine Architect about the differences in center of lateral effort, etc.