Sailometer was designed to provide the instrumentation needed to race sailboats. It basically automates many of the tasks we currently do manually and also provides some new metrics which might prove useful. Specifically the app displays:
Prior to every race you need to establish the close haul port and starboard tack angles for your boat. This is likely something you are already doing, the only difference now is that instead of writing it down on a pad or marking it on a compass you're going to press a button on an app. Once these tack angles have been defined for a particular race then you don't really need to touch the app anymore. It will switch between screens for the upwind and downwind legs automatically. Throughout the race the app assumes you're a good sailor and on the upwind leg that you are sailing as close to the wind as you can and you should therefore be achieving your tack angle. However if the boat's angle to the wind is greater or less than the predefined amount then that was due to a wind shift rather than your poor sailing. The app will use these "boat angle to the wind" changes to slowly update wind direction and thus accomodate wind shifts during the race.
You know the phrase "garbage in, garbage out" - well that's particularly true of this app. If you don't go through the necessary calibration processes of your phone or tablet prior to the race then the data displayed is basically worthless. Its not very involved and it only takes a couple of minutes, you just need to remember to do it. Specifically you need to:
First of all make sure that the compass is not close to any magnetic interference - don't forget this might include your phone case itself as many of them use a magnet on the cover. The Apple iPad uses a magnet to attach a pencil. Just make sure these kinds of magnetic interference are well out of the way. At the top of the "Config" tab is a row labelled "Compass". This will give you your phone's current compass heading either as magnetic or true north based, but for the purposes of calibration make sure this is always set to true north, you can change back to magnetic after calibration is over if that's what you want.
Now point your phone in a known direction and make sure the compass reading is within one or two degrees of that known heading. If you do not know any headings accurately then use the sun.
A great way to check if your compass is calibrated is to make sure the heading is consistent between the phone being horizontal and vertical. If you see a difference of more than a degree or two then your compass requires further calibration.