"Old" and New Technology for Sailboat Navigation
How does GPS accuracy affect Apparent and True Wind Direction?
When using the wireless SailTimer Wind Instrument™, you have lots of apps to choose from. The list is here for iOS and Android, and the latest addition is NMEA Sail, shown in these screenshots (more on this below). Whether on a keelboat or even a sailing dinghy with no 12v battery, you can
now display the full range of navigation functions from wind gauges to chartplotting. For example, you can use the excellent Navionics worldwide charts with tide levels in the SailTimer tacking app on iOS (and soon on Android). This patented app can learn the polar plots for your boat, and is the only product that accounts for tacking distances to determine your optimal tacks. When receiving transmissions from the wireless Wind Instrument (above right), as the wind changes the app updates your optimal tacks automatically.
We are at an interesting point in history where data is often still sent to marine electronics or apps using NMEA formats from wiring, while increasingly using wireless connections to (a) avoid the need to run wires through the boat, (b) use tablets and smartphones. We received a customer support enquiry recently which raised some interesting examples of "old" and new...
When the Wind Instrument (photo above right) is sending data on Bluetooth to a tablet or smartphone, some apps allow you to see the raw data such as in the screen above (center) from the NMEA Sail app. NMEA Sail shows your key navigation information on one easy-to-view screen (above left), and can also receive parameters like depth, compass heading, and boat speed on wifi with the NMEA 0183 or Signal K formats. Don't worry, there is no need to read the raw data -- but it is handy to be able to see it streaming by, and to understand the general operation.
The Wind Instrument can send all combinations of True/Apparent wind speed & direction, wind direction/angle, and in reference to magnetic north/true north. You can see all of these combinations on the menus in the Android app store listing for the SailTimer Wind Gauge™ app. But interestingly, there is no Apparent Wind Direction sentence in the NMEA 0183 format. This is an industry standard for marine electronics data that we like and that is still widely used. But presumably when the format was going through updates in the 1990s, it was not anticipated that a masthead anemometer would ever have a built-in digital compass. So there is no NMEA 0183 sentence for Apparent Wind Direction. When the Wind Instrument is sending data to other apps, we therefore use a special sentence for Apparent Wind Direction. And when a user has the SailTimer Air Link™ to receive the Bluetooth transmissions from the masthead and wire into onboard electronics, the Air Link can't send Apparent Wind Direction, but instead sends True Wind Direction and a sentence for Wind Angle (both Apparent and True) that marine electronics can understand.
An NMEA Sail user in Europe sent in a screenshot like the ones above, wondering why different numbers were appearing for Apparent Wind Direction (AWD) and True Wind Direction (TWD). Basically, TWD is what the wind is actually doing if you look at a wind map, or if you are standing on a dock and feel it moving around you. But when you are sailing along (or put your arm out the window when driving down the highway), your own movement influences what you feel, so the wind measured then is AWD. That is the main reason that there are two different meausures for TWD and AWD, and why they may have different values. When the Wind Instrument is providing AWD data, that is straight from the internal compass, but TWD calculations also rely on GPS data.
When you are not moving, theoretically both AWD and TWD will be the same. However, GPS is only accurate to within about 10+ meters on the face of the earth. That is a circle with a diameter of 20+ meters. So within that circle if you are not moving or not moving fast, a GPS can’t tell exactly where you are, or if you are actually moving. So the GPS may estimate or interpolate from your previous heading and speed, and give you a speed of up to about 2 knots and a heading direction, even if you are standing still. So in practice, TWD and AWD often don’t have exactly the same the same readings when standing still.
New Quick-Release Nylon Wing Nuts for Mounting Rod
There are a range of handy Accessories available for the Wind Instrument and Air Link. We have just added a new nylon wing nut ($5) which is very handy for quick release when trailering a centerboard sailboat, and for temporarily mounting your Wind Instrument at home. It catches a tiny bit more wind than the normal lock nut, but it can be loosened with no tools. This is the metric M6 size for the mounting rod, which is very hard to find in a non-magnetic material like nylon (so that it doesn't affect the digital compass in the Wind Instrument). We can also provide a stainless steel version on request (which is also non-magnetic); it is stronger, but a bit heavier.