Loading and Adjusting Polars in qtVlm
Getting your boat's performance data into the router — and tuning it for real-world conditions
The Boat Settings Menu
Before qtVlm can route anything, it needs to know what boat it is routing for. Open Boat > Boat Settings from the main menu bar. This opens the boat configuration dialog where you enter basic information about your vessel: boat name, model, and hull parameters. The boat name is what appears in routing outputs and on the chart display, so use something you will recognize if you end up running multiple profiles.
The Boat Settings dialog has several tabs. The first tab covers basic identification and display options. You do not need to fill in every field for the router to work — the critical piece is the polar, which lives on its own dedicated tab. However, entering accurate hull length, beam, and draft values is worthwhile because qtVlm uses these for safety margin calculations when routing near shallow water or narrow channels.
If you are setting up qtVlm for the first time, start here. Get the boat name and basic dimensions entered, then move to the Polars tab. Everything else — NMEA connections, AIS settings, instrument displays — can wait. The router only needs two things to produce a result: a polar file and a GRIB weather file. The boat profile is where the polar lives.
Give your boat profile a descriptive name that includes the sail configuration — for example, Hallberg-Rassy 40 - Cruising or HR40 - Racing Sails. This becomes invaluable when you create multiple profiles for different setups.
Where in qtVlm do you access the boat configuration and polar import settings?
Importing Polar Files
Within the Boat Settings dialog, click the Polars tab. This is where you load the file that drives all routing calculations. Click the Import button and navigate to your polar file — qtVlm accepts both .pol and .csv formats. Remember that the file must use semicolons as separators; a file with comma separators will either fail to import or produce garbled results. If your file loads without errors, you will see a confirmation and the polar data will be associated with the current boat profile.
Once imported, qtVlm stores the polar data internally — you do not need to keep the original file in any particular location, though it is good practice to keep a backup. If you later need to update the polar (perhaps you obtained better data or want to switch to a different source), simply return to the Polars tab and import a new file. The old data is replaced entirely; qtVlm does not merge polar files.
A common stumbling point is file encoding and line endings. Polar files created on different operating systems may have different line ending characters. If an import fails silently — no error but no data appears — open the file in a text editor and verify it uses standard line breaks and contains no hidden characters or BOM (byte order mark) at the beginning. Saving the file as plain UTF-8 without BOM from Notepad++ or VS Code usually resolves these issues.
What file formats does qtVlm accept for polar imports?
Verifying with the Wind Polar Analysis View
After importing a polar file, you should always verify it visually before trusting it for routing. Go to Boat > Polars > Wind polar to open the Wind Polar analysis view. This displays your imported polar data as a graphical plot — the same circular diagram discussed in the polar theory lesson, rendered directly from the data qtVlm loaded. If the curves look reasonable and match what you expect from your boat, the import worked correctly.
Look for anomalies: curves that cross each other unexpectedly (a higher TWS curve showing lower speed than a lighter TWS curve at the same angle), sudden jumps or flat spots in the curves, or missing data at certain angles. These can indicate errors in the source data, formatting issues during import, or interpolation artifacts. A clean polar should show smooth, nested curves that generally expand outward as TWS increases, with each higher TWS curve equal to or outside the previous one.
The Wind Polar view also lets you see the optimal VMG angles — the points on each curve where upwind and downwind VMG is maximized. These are the angles the routing algorithm will use when calculating tacking and gybing legs. If these angles look wrong (for example, if the optimal upwind angle shows 60° instead of the expected 40-45°), double-check your source data. An error in the upwind angles will produce routes that sail too free upwind, adding significant time and distance to windward legs.
Compare the polar display in qtVlm against the original polar diagram from your source. If the shapes match, you are good. If anything looks different — especially in the critical upwind and downwind VMG zones — re-examine the source file for data entry errors.
How do you access the visual polar plot in qtVlm to verify your imported data?
Adjusting Polar Efficiency for Real-World Conditions
Theoretical polars assume everything is perfect — new sails, clean bottom, flat water, full racing crew. Real-world cruising is none of these things. qtVlm lets you apply a polar efficiency percentage that scales all boat speed values uniformly. Setting this to 85% means the router will use 85% of the polar's predicted speed at every TWA and TWS combination. This single adjustment is the most powerful tool for making your routing predictions match reality.
Here are practical guidelines for setting the efficiency factor. Aging sails with stretched shape, UV damage, or tired laminate cost roughly 10-15% off theoretical speed. Bottom growth — even light slime — adds 5-10% drag; heavy barnacle growth can cost 15% or more. Shorthanded crews who cannot constantly optimize trim, change sails at every wind shift, or fly asymmetric spinnakers should deduct another 10-20%. These factors stack: a cruiser with three-year-old sails, six months of bottom growth, and a couple sailing double-handed might realistically sail at 65-75% of the theoretical polar.
The key insight is that it is far better to route conservatively and arrive early than to route optimistically and arrive late — or worse, to make risky decisions based on unrealistic speed predictions. A routing that says you will make your destination in 18 hours at 85% polar but 22 hours at 70% polar gives you a realistic window to plan provisions, watches, and daylight arrivals. Start at 80% and adjust based on experience. After a few passages where you compare predicted vs. actual performance, you will converge on your true number.
Do not run routings at 100% polar efficiency unless you are racing with a competitive crew and fresh sails. Overestimating your speed leads to optimistic ETAs, which can cascade into dangerous decisions about weather windows, fuel reserves, and harbor arrivals in darkness.
A cruising couple with two-year-old sails and moderate bottom growth is sailing double-handed. What polar efficiency percentage is most realistic?
Multiple Boat Profiles and When to Use Them
qtVlm supports multiple boat profiles, and taking advantage of this feature will make your routing significantly more accurate. The idea is simple: your boat performs differently depending on its configuration, and a single polar cannot capture all configurations. Create separate profiles for your most common sailing setups — for example, one for full sail inventory (main, genoa, and spinnaker), one for cruising canvas (main and furling jib only), and one for heavy weather (reefed main and storm jib).
Each profile can have its own polar file and its own efficiency percentage. The cruising canvas profile might use the same base polar as the full sail profile but with a lower efficiency factor — say 75% instead of 85% — reflecting the fact that you are not flying an overlapping genoa or asymmetric in light air. The heavy weather profile might use a completely different polar that only covers the wind range above 20 knots with reduced sail areas, or it might use the same polar with a 60-70% efficiency factor.
When planning a passage, run the routing with the profile that matches how you will actually sail. If you are crossing the Bay of Biscay in November and plan to reef early and sail conservatively, use the heavy weather profile. If you are doing a summer coastal hop in settled trade winds with all your canvas available, use the full sail profile. You can even run the same route with multiple profiles to get a range of arrival times — this is useful for planning watch schedules and identifying which weather window is viable at your realistic performance level.
Run the same route with your optimistic and conservative profiles. If both show you making your destination before the weather deteriorates, you have a solid weather window. If only the optimistic profile gets you there in time, that is a signal to wait for better conditions.
Why would a sailor create multiple boat profiles in qtVlm?
Summary
Import polars through Boat > Boat Settings > Polars tab — qtVlm accepts .pol and .csv files with semicolon separators.
Always verify imported polars visually using Boat > Polars > Wind polar — look for smooth, nested curves without crossings or flat spots.
Apply a realistic efficiency percentage: deduct 10-15% for aging sails, 5-10% for bottom growth, and 10-20% for shorthanded sailing.
Most cruising boats perform at 70-85% of theoretical polar — start at 80% and adjust based on actual passage data.
Create multiple boat profiles for different sail configurations (full canvas, cruising, heavy weather) to get accurate routing for each scenario.
Conservative routing predictions are safer than optimistic ones — underestimating speed is a planning inconvenience; overestimating it can be dangerous.
Key Terms
- Polar efficiency percentage
- A scaling factor applied to all polar speeds to account for real-world performance losses — set in qtVlm's boat configuration
- Boat profile
- A saved configuration in qtVlm that includes boat identity, polar data, and efficiency settings — multiple profiles can be created for different setups
- Wind Polar analysis view
- The graphical polar display in qtVlm accessed through Boat > Polars > Wind polar, used to verify imported polar data
- VMG angles
- The optimal upwind and downwind sailing angles where velocity made good is maximized — derived from the polar and used by the routing algorithm
- BOM (Byte Order Mark)
- A hidden character at the beginning of some text files that can cause polar import failures — remove it by saving as plain UTF-8