Why does ETA go blank when on a long tack?

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 (Estimated Time of Arrival) 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 on a tack, VMG provides erroneous readings and should not be used for navigation.  It is not a reliable measure of velocity.  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 constant.That seems to be why GPS manufacturers intentionally cause the ETA to blank out on longer tacks.

The Sailing GPS does not have this problem.  Finally: a simple, accurate GPS display for tacking sailboats.

But There are Also Problems with VMC (Velocity Made Good to Course)

Although VMG only accounts for progress towards the waypoint when heading to a waypoint directly upwind, VMC uses a formula that accounts for progress towards the waypoint in any wind direction. 

High-end racing teams use resources that are out of reach of normal consumers, like full-time tacticians and navigation systems costing more than some entire sailboats.  Traditionally they have assumed that VMC is a better measure than VMG, even though it too is at best a bastardized guesstimate of velocity.  It slows down all by itself as you continue on a tack, even if your actual velocity (speed over ground) remains constant.  Like VMG, VMC views sailing on a tack as cross-track error.  They both tell you that your progress towards the mark gets progressively worse for every extra second that you stay on a tack, which is not true.  They don't account for the fact that the optimal strategy for a sailboat is to tack back to the mark once it gets to the layline.  There is nothing about the layline in the vector calculations for VMC or VMG.  

The only difference between VMC and VMG is that the rhumb line for VMC is based on the line to the waypoint, and for VMG it is based on the wind direction.  High-end racing teams, going back to the 1970s (before PCs, chartplotters and GPS satellites even existed), have graphically represented VMC on a laminated sheet with target speeds from polar plots, to try to define the optimal tacking angle.  But that doesn't show you the distance on a tack, how long it will take, anything about the subsequent tack, or even how to know when you should tack. 

We may as well go back to fiddling with imprecise plastic gauges and slide-rules, if we are going to be trying to visually determine the optimal tacking angles from target tables on laminated sheets.  That method gets even more unwieldy if you are not heading straight upwind, so the polar plot needs to be rotated (and who knows what happens if you use a table of target speeds instead of a plot that can be rotated).  

But why go to all that trouble anyways, when there is a simple way to do this and more -- a way that ordinary sailors can use now too?  The patented software used in The Sailing GPS (and the SailTimer app) does not calculate an estimated "velocity" like VMC or VMG;  it uses your actual velocity (speed over ground) to calculate your Tacking Time to Destination.  We provide a fast and easy display of exact distance, heading and time on both tacks, which you can't get using polar target speeds on a laminated sheet (from the pre-digital era).  Time to put that laminated sheet of polar targets on the shelf with sextants, LORAN and celestial navigation.    

The Sailing GPS is the first GPS from any manufacturer that can learn the unique polar plots for your individual sailboat.  It uses these to calculate full tacking results for you, and gives you a quick and easy display.  These results can update automatically with changes in your location or the wind.

Even just entering the directions of the wind and your waypoint allows you to display the optimal tacks.  Simple. Then if you want to add additional information like LWL, wind speed, GPS locations or distance to the waypoint, additional details appear in the results.

For further details:  "Bad News for Velocity Made Good".
GAM on Yachting magazine, May-June issue, 2009 (836k PDF).