broad question… with many potential answers.
Here are the most likely:
- The wind does not match forecasts.
- The boat speed does not correspond to the polar: optimistic polar, navigation in “cruise” mode, dirty hull, worn sails, sea state… you can edit the polar or simply play with the 2 efficiency parameters.'
- do not forget the maneuver’s time at the exit and at the entrance of the harbor or anchorage; I typically add 1 hour.
- presence of current: did you load a current grib?