Polar files describe the expected speed of a given boat for various values of wind heading and speed. The data in a polar file is a collection of triples (ws, wa, bs) where ws is a windspeed, wa is a true wind angle, and bs is the expected boatspeed when sailed correctly at this windspeed and wind angle. BLUEWATER uses this data to interpolate (by cubic splines) the general function bs(ws,wa). This function is required by the routines which compute performance and routing. Each route must be assigned a boat type for which a polar file exists before any performance can be computed. The assigned boat type can be changed using the “route options” window. (Right-click on a route in the Route Manager or on the chart.) It is convenient to use boat type names such as “J120” or “Farr40” that correspond to actual designs, but it is not necessary.
Polar data is called that because it is typically presented as a series of curves of constant windspeed drawn on a polar chart. BLUEWATER RACING” allows a polar curve for zero windspeed giving non-zero boatspeed. This would, for example, describe a cruising yacht able to use its motor. You can build polar files for one boat in different configurations (cruising, racing, high seas, flat water, etc), but each route can have only one polar file at a time.
Polar data is stored in one file for each boat type. BLUEWATER RACING can read and write three formats:
- BWR’s own format, with the file extension “plr”.
- Expedition/Deckman format, with the file extension “pol”.
- Sail Online format, with the file extension “sol".
- Expedition/Deckman format in comma separated value form, extension “.csv”.
BWR will also load data from any file with any extension, and will assume the format is “.plr”.
Polar data should be stored in a file called “<boat type>.<fmt>”. Thus, the polar data for a J120 would be stored in “J120.plr”, and the Expedition format data for “mytype” would be stored in “mytype.pol”. Names are case sensitive.
Whenever a performance calculation occurs, the program attempts to load any required polar files by searching through the default polar directories. The list of polar directories can be modified from Tools→Preferences→Directories. The “Polars” entry should contain a list of folders to search whenever attempting to load a polar automatically. Folder names should be separated by whitespace. If a folder name contains whitespace, it should be enclosed in quotes.
If you want to load a polar file that is not in one of these default directories, specify its full location and file name in the route options.
1) Boat Type(Polar) = “J120”. BWR will look for a file named “J120.plr” (then J120.pol, J120.sol, J120.csv, J120.txt) in the default directories in order they are listed in the polar directory entry.
2) Boat Type (Polar) = “J120.sol”. BWR will look for J120.sol in the default directories.
3) Boat Type (Polar) = “C:\Users\Jeff\BWR\polars\J120.pol”. BWR will look for exactly that file.
Polar data for a few boat types is included with the program. You can get polar data from boat manufacturers or US Sailing.
Upwind and Downwind VMG Targets
Upwind and downwind targets specify what true wind angle the boat should be sailed at to achieve maximum VMG -- velocity made good in the upwind or downwind direction. Helmsman and trimmers attempt to sail this true wind angle. To make performance predictions, the program needs to know these targets, since it assumes the boat is being sailed according to them.
Expedition format polar files always explicitly encode the target values as part of the specification of the polar curves. BWR plr files may or may not contain explicit VMG target information. To include explicit VMG information, a curve is added labeled “upwind” or “downwind” (rather than being labeled by a windspeed number.) The entries on the line labeled “upwind” consist of windspeed/true wind angle pairs, where the TWA is the target to be sailed.
For .plr files without explicit upwind or downwind curves, and for Sail Online files, which don’t include target information, the program calculates the VMG targets from the polar curves.
The target lines are displayed in the polar manager as magenta lines. If explicit in the polar file, they are drawn solid and labeled with lowercase “upwind/downwind.” If calculated by BWR, they are drawn dashed and labeled with uppercase “UPWIND/DOWNWIND.”
The Polar Manager
You can view and edit polar files with the Polar manager. It lists all polars used by any routes in the races that have been loaded during the current session. Additional polar files can be opened and added to the list.
The polar can be edited visually by selected Edit mode in the viewing window. Then click on a control point and move it to adjust the curve. Additional control points can be added by clicking in the middle of a curve.
The polar values can be edited in the spreadsheet. Changes to the spreadsheet are not applied to the edited polar until Apply Changes is clicked. To add a new curve, choose the new windspeed (or “upwind”/”downwind”) and click Add Windspeed. At least three control points are required for a new windspeed.
To delete a curve, right-click on it in the treeview window.
Editing changes can be undone/redone through the tools dropdown menu, or with accelerator keys specific to the Windows or Mac platform.
Polar file format
The .plr and .pol formats are essentially the same, except that .plr allows curves labeled “upwind” and “downwind.” A polar file is a sequence of lines. Each curve is described on one line of the text file by a windspeed, followed by a sequence of pairs of True Wind Angle and boatspeed. (TWA in degrees, BSP in knots)
10 30 0 45 6 90 8.1 160 7 180 5
15 30 0 40 8 90 12 150 10 165 9 170 5
This gives two curves, one for windspeed 10, one for windspeed 15. Different curves may have different TWA points, and different numbers of points. The line for windspeed 10 specifies a boatspeed of zero knots at a true wind angle of 30 degrees, a boatspeed of six knots at TWA 45, 8.1 knots at 90, seven knots at 160 degrees, and five knots at 180 degrees.
There can be a zero windspeed curve with non-zero boatspeed, i.e., “when the wind is gone, the motor's on.”
Rules for the data in polar files:
1) One windspeed curve per line.
2) There must be a least 3 points per curve.
3) There must be at least one non-zero windspeed curve.
4) The minimum TWA is zero, and the maximum is 180.
If these rules are broken, the behavior of the program is undetermined.