Old and new technology: A SailTimer Wind Instrument™ on the 108-year-old s/v Stella. Thanks to Kai Reschke in Germany for the photo. Home port of Wieck on the Darß peninsula. You can see the entire boat with its folding tabernacle mast on the Photos and Video page of the updated website for the Wind Instrument. The hull length of 11.06 m increases to 14.95 m LOA with the impressive bowsprit. Built of oak in 1910 by the Jarling boatyard in Freest, Germany, for a fisherman on the isle of Hiddensee.
How to See Your Optimal Tacks in the SailTimer™ App
Example from User
The free SailTimer™ app provides unique tacking calculations on iOS, and we expect to be releasing a new Android version within a few weeks. There are 3 steps in this patented system. (1) The app can learn the "polar plot" for your individual boat (i.e. boat speed on all wind angles and speeds). You also have the option to manually edit the generic polar in a table. The app website provides instructions for using the polar learning and theoretical background. (2) The SailTimer app is also unique in evaluating distances for different possible tacks. (3) Using the polars and tacking distances, it then provides a quick and easy display of your optimal tacks as a chart overlay. Your TTD® (Tacking Time to Destination) is also always shown on the bottom center of the screen.
Below is a scenario sent in by a user who asked to be anonymous, showing how it works. He was on land when he set this waypoint, but that doesn't affect the logic...
If you are using the wireless SailTimer Wind Instrument™, as the wind changes your tacking results update automatically. But the app is free, and you can easily test out the tacking results by entering the wind conditions manually, as this user did. Under the Input button, on the Wind tab he set the wind from the East at 15 knots. With the Routes button he set a waypoint (which the Bearing tab showed to be 5.2 nautical miles from his current location at a compass bearing of 126 degrees). Once you have wind and a waypoint, just press Optimal Tacks at the bottom of the panel, and presto: your optimal tacking lines are displayed, showing where to sail to get to the waypoint in the least amount of time. Now you know why it is called SailTimer.
If you are sailing a route with multiple waypoints, as you pass one pin you can tap the Next button to move the tacking lines to the next waypoint. Or tap Previous to move the lines back to the earlier waypoint if you are not quite there yet.
In the screenshot above, the user had tapped on the blue waypoint pin to see a box with information about Tack 1 and Tack 2. The app recommended Tack 1 at a heading of 135 degrees (distance 5.1 miles) and Tack 2 at 45 degrees (distance 0.8 miles). Total distance 5.9 miles is shown on the bottom of the screen above with his TTD of 42 minutes.
But this user sails a one-design racing keelboat, and was expecting to sail there on one tack at a heading of 126 degrees (possibly based on polar targets available from the boat manufacturer, which are critiqued here). As noted above the direct rhumb line would be a distance of 5.2 nautical miles. So why would the SailTimer app calculate that two tacks with a longer distance would get there earlier? Read on to see how the calculations work, or skip to the last paragraph for the conclusion.
Since the wind is coming from the East (90 degrees), if he took one tack to get there with a heading of 126 degrees, his wind angle would be 36 degrees. That is pretty tight to the wind on most boats, lowering speed. A polar he was using around the time of these tests is shown below. The red circle shows where his boat speed would be, between 3.9 and 7.3 knots at 5.9 knots. The app would calculate that going 5.2 nautical miles in a straight line to the waypoint at 5.9 knots takes 5.2 / 5.9 = 0.88 hours.
But the tacks overlaid on the chart above get him there faster. At 5.1 nm the first tack in itself is nearly the same distance as the direct rhumb line. The wind angle on the first tack is 45 degrees, which according to the polar produces a faster boat speed of 7.3 knots. So the first tack will take 5.1 miles / 7.3 knots = 0.70 hours. The second tack is 0.8 nm at a wind angle of 45 degrees. From the polar above this also gives a faster boat speed of 7.3 knots. So the second tack will take 0.8 miles / 7.3 knots = 0.11 hours. The total time on both tacks is therefore 0.70 + 0.11 = 0.81 hours.
So sailing straight to the waypoint in one tack to reduce distance he would arrive in 0.88 hours, but by tacking with higher speeds even though it is a longer distance, he gets there in 0.81 hours. The difference is an improvement of 0.07 hours x 60 minutes = 4.2 minutes. So even though tacking covers more distance, in this case the app calculates that he'll get there 4 minutes and 12 seconds earlier with the optimal tacks.