When sailboats tack, standard GPS units from all manufacturers seem to assume that the boat is heading off course. That is why the ETA often goes blank when you are on a tack. There is no GPS chartplotter from any other manufacturer that accounts for the zig-zag tacking that sailboats do, to properly calculate your arrival time.

GPS units often encourage sailors to calculate their arrival time and speed towards the waypoint using Velocity Made Good (VMG), which gives totally erroneous results for sailboats on tacks.  For further information, see this page or download our article from the May-June 2009 issue of GAM magazine (836k PDF).