Standard GPS chartplotters do not determine tacking distances when calculating ETA (Estimated Time of Arrival). But if they don't know how far you are going to travel, how can they provide a correct ETA? Many appear to use VMG (Velocity Made Good) to the mark to assess your "velocity" when dividing your distance by velocity to get your ETA. But this is an erroneous and mathematically incorrect measure of velocity for sailors. Because ETA (and VMG) gets increasingly unreliable the longer you stay on a tack, some GPS chartplotters actually seem to be designed to blank out the ETA in this situation. Not a very good solution. Better to make the instrument correct, than to stop the driver from being able to see it.
Superficially, it seems like a clever idea to calculate your estimated time of arrival by using VMG to account for how much you are off-track. However, this method was originally designed for aircraft, powerboats and vehicles, not for sailboat tacks. ETA was used on LORAN devices 30 to 40 years ago, and does not seem to have been updated for sailboat navigation by the major GPS manufacturers since then. In the digital age, sailors deserve a better solution.
That is your take-away message. But if you want the full details, they are explained below.
VMG to the Mark
VMG to a waypoint or mark is what is typically displayed on standard GPS devices. (Sometimes also referred to more precisely as VMC: Velocity Made Good on Course to the mark.) When on a tack, this common measure of VMG provides erroneous readings and should not be used for navigation (as explained in more detail in this 2009 article - PDF, 855k). Velocity Made Good 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 the waypoint 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 your speed and heading remain constant. Even if you are sailing on the correct tack to a waypoint, standard GPS chartplotters view the tack as cross-track error.
You may see high-profile racing sailors in a TV interview or product promotion say that their main goal in a race is to get their best VMG. This is really a conceptual goal about getting the optimal balance minimizing the tacking distance and maximizing the boat speed (which is traditionally called The Helmsman's Dilemma). While that conceptual goal is at the core of sailboat navigation, technically it cannot be solved with VMG calculations or VMG parameters on a GPS. As a measure of velocity, VMG is flawed, and would give different velocities depending simply on whether you are doing longer or shorter tacks, even if your boat speed and heading are the same. So if you see a high-profile race or interview where experts say the goal is to get the best VMG, they are really referring to the conceptual goal: getting the optimal tacks, to solve The Helmsman's Dilemma.
Fortunately, the patented SailTimer™ technology solves this problem. We use a fundamentally different approach. We use actual tacking distances, and the boat speed on each wind angle (preferably from empirical polar data if available, or else polar targets), to determine the optimal tacks and Tacking Time to Destination.
VMG to the Wind
One of the traditional responses to the problems with VMG is that you should measure VMG relative to the wind direction, not relative to a waypoint. But there are a lot of assumptions that way. First, your VMG with this approach will only be stable if the wind direction is fixed. Every time the wind swings, the VMG (and ETA) will change, using this method. You are assessing your progress in relation to a moving target.
Say you are on the final upwind leg of a race. You want to know your relative speed towards the finish line. But what if the wind shifts, and is not directly behind the waypoint anymore? You may not have any indication that this is happening or how erroneous it is (especially when you are off the rhumb line on a tack). VMG to the Wind is then showing your relative speed in other directions that are not towards the finish line. Is that accounted for, if you are using this type of system? Our related article on optimal tacks and polar targets shows that it is much simpler to just measure the distances and speeds to the waypoints you actually want to go to. Your grandfather may have needed to use the wind to measure his VMG in the days before computers and GPS, but there are simpler, easier, more accurate methods now.
Products calculating VMG relative to the wind direction will also only work if they have an anemometer connected (which is probably why most GPSs use VMG to the mark rather than to the wind).
VMG measures are also only available while you are underway, and require you to continuously do "trial & error". They typically show a velocity but not the distance on a tack, when you should tack, or whether you are on the optimal tack. (With the SailTimer tacking results, whether still on the dock or under sail it is easy to calculate and display full information for both tacks.)
Plus, even though there is a big difference in the results with VMG to the mark and VMG to the wind, for some reason, the type of VMG being displayed is usually not provided. This is like having a speedometer in your car that doesn't tell you if it is measuring in miles per hour or kilometers per hour. (Although at least a speedometer measures consistently, unlike VMG to the mark. That is even more reason that the type of VMG should be specified.)
You would also need to consider whether VMG relative to the wind works in other directions when you are not sailing directly upwind. This question also applies to "tacking angles", which are used as a shortcut to display laylines in some products. Although tacking angles are symmetric to a mark directly upwind, it is not so obvious how to use them when the wind direction is at a different angle than the rhumb line to the mark. Using a single tacking angle is also inaccurate because the angle changes at different wind speeds (and when sailing downwind).
But here is the most important problem with VMG to the wind... The geometry in this method only tells you when you have crossed an imaginary line going through the waypoint or finish line perpendicular to the wind direction. That imaginary line extends for miles in each direction (infinitely, actually). So with VMG to the wind, there is no way to tell if you have the best tacking angle to the actual waypoint or finish line, or how long it will take to get there. It may be telling when you are going to cross an imaginary line miles away from your actual destination or finish line. Plus, if the wind is on a different angle, the imaginary line will have the wrong angle and wrong distance, giving you incorrect results.
In contrast, the SailTimer tacking results do not have all of these assumptions, confusion and flawed math. They are simple to understand and more accurate; they show the time, heading and distance to the exact location of your waypoint.
Navigation Technology for the Age of Wireless Devices and GPS
Many people find that their eyes glaze over and they never even read the instructions about technical procedures like VMG or what it means. The major manufacturers don't usually warn you about the problems with VMG for sailing in their instruction manual anyways. But if you view your tacking results in the SailTimer™ app, they are clear and meaningful. Your eyes don't glaze over. There are no mathematical complications that need to be summarized for the layman. You just see a simple visual display of your optimal tacks as a chart overlay and your Tacking Time to Destination (TTD ®). It is inherently obvious.
This is a revolutionary change in marine electronics for sailboat navigation. We don't even include VMG in our products. Our calculations work in all wind directions, not just upwind. We get information about your tacks from your actual tacking distances -- not from inferences about how fast you are moving into the wind (which is kind of a strange approach, considering that sailboats can't sail directly into the wind). In the age of GPS, when we can do calculations based on the precise position of your location and waypoints, those ancient, indirect methods are no longer necessary.
The SailTimer™ software is fast and simple. It calculates your exact tacking distances, handles the polar data for you, can even learn the polars for your unique vessel, and continuously updates your optimal tacks and your Tacking Time to Destination.