Why does ETA go blank when tacking?
GPS chartplotters are actually designed to go blank in this situation: right when sailors need ETA the most! Why? It is because the standard GPS method of calculating the ETA is based on velocity made good (VMG). Superficially, this seems like a clever idea: calculate your estimated time of arrival accounting for how much you are off-track. Unfortunately for sailors, this seems to be the cause of the ETA going blank while tacking, on all brands of GPS. When tacking, VMG provides erroneous readings and should not be used for navigation. Even if your speed and heading remain constant, VMG progressively decreases the further you get from the rhumb line (the direct line to your destination). This is because a destination upwind becomes increasingly off your beam the longer you tack, until eventually you would be going away from it. VMG erroneously gives the illusion that you are heading increasingly off-course or slowing down, even if you are on the correct tack and your speed remains cons
