How to create polar curves from logged data? Tacsail uses as a reference the polar targets which are stored in the NMEA2.ini file located in the main Tacsail folder (see also the Tacsail Manual para 5.3.3 ). Therefore, if you want to update your targets you have to edit this Tacsail NMEA2.ini file. As a first step you copy the NMEA2.ini file from the Tacsail_2_4.exe folder to the Polars V1.0 folder. (overwriting the existing version which by default is installed along with this last program). During a sailing season you may have collected different polar log files. In fact by default Tacsail will create polar log files automatically together with your main log. Before editing al these different and separate polar data files it is more practical make one big text file out of this. Use the windows notepad editor to cut and paste all files in one big.txt file and call it e.g. All_dates_polars.txt. Start the Polars program and load this file by using the explorer tab on the left side. You will get following picture: By default the program will show all the data points. In this case if you select the Results - TAB on the left side you will note that there are thousands of data points. Underneath left the data for the Starboard Upwind quadrant which has 8807 data points. If you click one of the coloured buttons the data points will be filtered. Click on the blue button 6. Only 170 points are available. These represent all data points logged for wind strength values between 5 and 7 knots for the Starboard Upwind quadrant.
Similarly clicking the other buttons will filter out data for different wind strength ranges. Now based on the collected data you want to create your own targets for the new season. For each wind strength range you filter out the collected data points and try to establish a new polar curve and optimum sailing angles. Supposing that your data points cover the whole wind range from 0 to 35 knots this would mean filtering out 11 wind strength ranges ( coloured buttons) and editing 11 curves and for each curve 2 optimum sailing angles, one upwind and one downwind. We ll illustrate how this can be done by using the 12 knot data points. Click the brown button 12. Let us examine this picture. On the top left side you will see a tick-mark next to the option 12. This means that the full brown curve shown in the picture on the right is the target polar curve for the wind strength range of 11 to 13 knots taken from your actual NMEA2.ini file. In fact in this specific case the curve is the standard curve VPP curve from a Beneteau First 31.7. You can also see that many brown data points are well above this curve (meaning that they are lying more towards the outer circles). This effectively means that the boat for which these data points have been collected clearly is faster than the First 31.7 and the polar curve as shown is not a suitable target curve. Before we create a new curve and new targets let us first observe a few other things. Firstly, we see that the complete data set is slightly shifted to the left i.e. the values are all displaced approximately 5 degrees. This probably is due to a wind meter misalignment. For future logging this should be corrected by calibrating the MHU (Mast Head Unit). See the Tacsail manual on how to do this. For now we can correct this here and now by shifting the whole data set by 5 degrees. Enter this value in the bottom left text box marked ( ) Angle and click Apply.
Secondly, we eliminate the Boat speed values which are far below the expected averages either because they have been recorded during tacking and/or motoring straight into the wind. Select a value of 5 in the drop down list for SOW(kn). All data values below 5 knots will now be filtered out. Thirdly, the highest speed values are around 8 knots so we set the maximum knots in the left pane to 8 instead of 10. The picture should now look like this: Now click the Tab Results on the left and click the row Starboard upwind. This is the picture of the data set and the corresponding graph. If you didn t change the NMEA2.ini file after installation then the full brown line is the polar curve for First 31.7
Click on the checkbox Custom 12. The polar in dotted lines is now being displayed. By default this is the Polar for a First 35 (Farr). Click on the Edit Custom and Edit curves checkboxes in the settings tab. On the full curve you will see little brown squares, on the dotted line there will be small brown circles. These points are the angles at which you can drag your curve to adapt it to your data points. If the regression order in the display options is set to 1 then the black line shows for each angle the average boat speed. By clicking on the + and - buttons in the upper left corner you can shift this thirty degree range left or right. While doing so, observe in the results Tab the figure #Datapoints. Set the black curve such that the number of points is maximal. In this case this point is reached when you have 1757 points in the dataset. The black curve now covers the range of 29 to 59. The black square is the average for the whole range of angles and speeds so to say a sort of gravity point. You can experiment by changing the regression order to 2 or 3 to adapt the curve to a more realistic polar curve form. In fact the speeds obtained for angles smaller than around 35 in a normal keel boat are mostly due to inertial effects and/or tacking. To obtain realistic speed figures for these angles you would have to sail the boat while pinching to a more or lesser degree. In this picture however you can already distinguish the area where most points are concentrated. The polar curve fitting can now start. Move your mouse pointer over the brown circle at 45. It changes into a cross. Now press the right mouse button and the pointer will change in a 4 pointed cross. Pull outwards along the 45 line. The point will slowly follow the mouse pointer. Keep moving slightly and slowly. Depending on the speed of your PC the curve will follow more or less rapidly. It is best to untick the checkbox Smooth curves while dragging the points around. When finished, tick the box marked Smooth curves and de the polar curve will search for a best fit. While dragging the points on the curve the data table is continuously being updated. Look at the line 12 knots in the Custom data part. When satisfied, tick the Edit targets check box. Move it to the highest point of the curve. This will be the new Optimum upwind angle for 12 knot wind speed. The figure is updated in the data table.
Perform similarly now for Downwind range and repeat the actions for all wind ranges. Finally you save the custom polar data from the data table to the folder *.pol folder. In this case saved under the name Morten_12knots_Upwind_All_dates.pol. To update
NMEA2.ini after all ranges have been set, click on Copy Custom to VPP and then finally export VPP to NMEA2.ini. Copy NMEA2.ini back in the Tacsail folder and your new targets will be used by Tacsail. Remember, if you don t change values for a particular wind range then the default values for these ranges will have been copied from Custom to VPP and effectively this means that in this case the First 35 values will be used. To avoid this you can always edit the VPP directly and gradually refine your polar data step by step. Another safe way to work, is to copy first your actual VPP to Custom, edit the ranges you want, save the *.pol file under a significant name and then copy back Custom to VPP. Also keep in mind that *.pol files will always be loaded into the Custom data table. Always keep in mind that Tacsail will use the VPP data table in NMEA2.ini as the targets for its calculations. Final result for 12 knots upwind