I recently installed a complete Raymarine navigation package, which includes an E80, 4kW radar, ST60+ instruments, and an S1 autopilot. The radar overlay-onto-chart feature is one of the main reasons I upgraded my whole navigation system. Frustratingly, the radar overlay doesn't always align with the chart, even though I've tried calibrating numerous times. What is going on here?
The problem you described is easily solved with a gyroscopic sensor (or Gyro for short). Currently your radar overlay misalignment happens because your magnetic flux compass is not able to compensate for your boats pitch and yaw. A gyro unit does just that and allows for a significantly better synchronization between chart and radar on your E80. Raymarine sells a Gyroplus II unit (Raymarine product #E12101) that can easily be added to your existing S1 autopilot.