All routing software relies on a "Boat Polar" file, which contains the speed of your boat at various TRUE wind angles and wind speeds. It's normally shown graphically as a set of polar curves, which make it seem complicated, but you just need to produce a grid of figures in a spreadsheet.
The example below has the True Wind Direction as the row headers, the True Wind Speed in Knots as the column headers and each cell contains the Boat Speed in Knots. So, with a True Wind Speed of 12 Knots at an Angle of 75°, the Boat Speed is 5.4 knots.
I started off using the polar diagrams published by Hallberg Rassy for our HR42F and extracted the boat speed for the wind speed and direction. It was a theoretical fantasy - we'd be reefing well before we achieved 8 knots on beam reach. I went through the matrix doing a reality check and changing the boat speed figures. It was tedious, but simple - I just asked myself questions like "if the wind was at 20 knots at 90 degrees, would we really be doing 7.9 knots?"
The result was a very conservative set of boat speeds. I think that I'd rather let the program think that I'm slower than I actually am - I don't want to be hammering the boat, ourselves and the engine trying to beat a storm front.
In practice, you need a Boat Polar that is a good approximation of your cruising speeds - there’s no need to spend loads of time getting a highly optimised Boat Polar because the inaccuracy of the weather forecast probably has more influence than the theoretical Boat Polar.
Automatically Generate a Boat Polar
The screenshot above shows the Boat Polar Plugin for OpenCPN. If your Boat Instruments (Wind Speed, Wind Direction and Boat Speed) are connected up to your NMEA network, then you can use the plugin to monitor your boat instruments and create a Boat Polar for you as you sail along.
I’ve never used it in this mode, but I guess that you do the following: Load your Boat Polar file or start with a blank. Set the Mode to “From NMEA”, the Source as “Speedo” and click the Record Start button. The plug in will start to record the data. At the end of your session, save your Boat Polar file in one of several formats.
Create a Boat Polar With a Spreadsheet
Our Raymarine Boat Instruments aren’t wired into the NMEA network, so I initially went through the following process:
1. I obtained the polar diagram published by Hallberg Rassy for our HR42F and, on a piece of paper, I wrote out a grid of numbers extracted from the diagram i.e. the boat speed for the wind speed and direction.
2. I entered the data into an excel spreadsheet. It has to be a particular format with the text “TWA\TWS” in the r1c1 cell. The first row should contain the True Wind Speed e.g. 0, 2, 4, 6, 8 …. The first column should contain the True Wind Direction e.g. 0, 15, 30, 45, 60… The remaining cells should contain the Actual Boat Speed. You can download Alba's Spreadsheet. (Not yet refined...)
3. I went through the matrix doing a reality check and changing the boat speed figures. It was tedious, but simple - I just asked myself questions like "if the wind was at 20 knots at 90 degrees, would we really be doing 7.9 knots?"
4. I then output the spreadsheet as a CSV file. You will end up with a *.csv file which is "comma-delimited" e.g.
5. qtVlm and other routing software expects the file to be “semicolon delimited”, so load the file into your favourite text editor e.g. Notepad, and do a global replace to replace all commas with semicolons (or edit it manually). You will end up with a file in the following format, which will read into qtVlm.
You can download Alba's CSV File. (Not yet refined...)
6. Refine your data. Whenever we’re sailing along with constant wind conditions, I make a note of the actual boat speed and build up a list like the following:
Tack, True Wind Speed, True Wind Direction, Boat Speed.
S, 12, 65, 7.1
S, 22, 110, 8.0
S, 8, 70, 5.0
I then use this data to refine my spreadsheet; re-produce my Boat Polar File and re-import it into qtVlm.
Create a Boat Polar Manually with OpenCPN
The spreadsheet method is a bit tedious, so I’ve started to use the Open CPN plugin to refine my Boat Polar. The Plugin has a “manual” mode, which allows you to load your existing Boat Polar data file directly into the plug-in grid (or start with a blank one). The boat polar curves are displayed at the left hand side. By double clicking in any cell in the grid you can edit the Boat Speed.
When you’ve finished your editing session, you save the Boat Polar in one of several file formats.
NOTE: The File Load function will load a comma separated csv file.
Loading a Boat Polar into qtVlm
qtVlm will accept XML data, POL format or semi-colon delimited format. BE CAREFUL because qtVlm copies your file to its own folder and deletes the original - take a backup of the file before you import your lovingly constructed file.