Upwind indicates your speed towards a distant mark without requiring you to know the exact position of that mark or how far away it is. It does this by taking a bearing to a mark with the magnetic compass in your device and then using the built-in GPS to determine the component of your speed in the direction of the mark. This is displayed as a number and as a graphical percentage of your total speed, enabling you to make course corrections with the intention of improving your speed towards the mark. The sailing term for this value is "Velocity Made Good".
Upwind automates the decision that every racing sailor must make on any upwind leg, "Is it better to head up to close the distance, or bear away to improve my speed?" Cruising sailors may also use Upwind to monitor their progress towards a diffuse headland, a lead of deep water, or another poorly-charted feature. Upwind helps you to avoid unnecessary pinching and steer the most efficient course to wind, and it may provide useful information about leeway due to currents and tides.
Of course, navigation software can provide you with this information, but only if you know the exact position of your destination. Often times your chart will not provide this and you will not have opportunity to sail to your destination in advance to determine its exact position.
Upwind can give you live feedback as your speed changes due to course corrections and sail trim, or else you can lock the bearing to leave your hands free for steering or other tasks. Upwind also has a pause button that allows you to freeze the display so that you can show it to crewmates.
Upwind displays a compass with a lubber line, your course and speed over ground, your latitude and longitude, and the bearing to your destination in degrees, both magnetic and true.
Upwind requires a device with a GPS and a magnetometer, such as an iPhone 3gs or later and the iPad 2 Wi-Fi+3G or later. iPod touches may not have the required hardware. Please be sure you have a supported iPhone or iPad before buying Upwind.
While Upwind is operating, it frequently accesses the GPS and magnetometer in your device, which may deplete your battery more quickly than usual. Please plug your device in to a charger while using Upwind if possible, or plan your battery usage accordingly.
Upwind should not be used for navigation. The prudent mariner will rely upon many redundant sources of information and proper preparation, competency and vigilance to ensure safety. Relying upon Upwind without reference to proper navigational resources can result in serious injury or death.