Redbopeep, Here are a couple clarifications from some of the information online at The Sailing GPS (tm)
and SailTimer Inc.
-- you raise some good questions. The Owner's Manual for The Sailing GPS is available from the order page, and contains a lot of additional details too.
The SailTimer Wind Vane actually uses a radio signal with 300-foot transmission range from the masthead. The data is received in a small Base Unit, where it is converted to NMEA format and then transmitted on to The Sailing GPS (via Bluetooth) or an iPad/iPhone (via wifi).
Each boat has different performance. Yes, and The Sailing GPS accounts for this. It can learn the "polar plots" for a unique vessel (i.e., the boat speed in all different wind directions and wind speeds). Then it can calculate your optimal tacking angles and Tacking Time to Destination (TTD). Even if you have polar plots from a manufacturer for your model of boat, this is more accurate, because it accounts for whether you have 6 anchors, a lot of cruising gear on board, and tack with a cup of tea in your hand -- whereas someone else with the same boat has new sails and keeps it empty for racing.
As Danblu notes, The Sailing GPS does not account for the current. But if you enter your boat length at the waterline, it will calculate your hullspeed for you, which it also uses in determining your optimal tacks and Tacking Time to Destination (TTD).
The Sailing GPS (and the SailTimer app for iPad from the same company) doesn't replace your decisions or take the fun out of sailing. But sometimes you want to know if you should head upwind more to shorten the distance (at a lower speed), or head off the wind more to get more speed (but a longer distance). Or, you may want to know if you are going to get home by dinnertime. The Tacking Time to Destination shows you this.
Similarly, ETA doesn't take the fun out of sailing; it is a useful thing to know. Unfortunately, ETA is not accurate when you are on a tack. ETA was designed for powerboats. Standard GPS units do not account for tacking distances. But using The Sailing GPS, Tacking Time to Destination _is_ an accurate way to know if you will get back to the harbour by nightfall, for example. Because it comes in a waterproof DryPak, it is even handy on a Sunfish, Albacore, Laser or other small boats, for knowing how long it is going to take to tack your way back to the dock.