Dear David, Generally we use True Wind Angle for sailing downwind, and Apparent Wind Angle for when sailing upwind. For the autopilot, these two options involve rapid calculations from the wind instruments, plus if there are waves, and the mast is moving a lot, it can change the data which inputs AWA and TWA, and if those numbers move a lot, then the autopilot may end up working overtime trying to respond to every wind change, when in fact there might not have been a wind change. In this case, sometimes it’s better to use compass mode, and the boat just goes reasonably straight at the target course. However, if the wind shifts while you are on compass mode, then the sails are less than optimally efficient, since they will be set incorrectly for whatever the current condition is.
Question submitted by David, Chesnut Hill, USA