Advanced Routing Parameters
The default routing settings produce a fastest-time route. Real-world sailing requires constraints — wind limits, wave height restrictions, tacking penalties, and night sailing adjustments that make the router's output actually livable.
Wind Speed and Wave Height Limits
The routing algorithm's default goal is minimum passage time. Left unconstrained, it will happily send you through 45-knot winds if that produces a faster route — mathematically optimal, physically miserable. Wind speed limits let you tell the router what you're willing to sail through.
In the routing settings (Routing → Edit Routing, then the Constraints tab), you can set a maximum true wind speed. The algorithm treats any grid cell with wind above this threshold as a no-go zone — it routes around it, the same way it routes around land. A typical cruising setting might be 25-30 knots; a shorthanded crew might set 20 knots; a well-found racing boat might accept 35+ knots.
Wave height limits work the same way. If your GRIB data includes wave information (significant wave height), you can set a maximum. The router avoids areas where waves exceed your threshold. This is particularly valuable for crossings where wave height might not correlate directly with wind speed — residual swell from a distant storm, for example, can produce dangerous seas in otherwise moderate wind.
The combination of wind and wave limits transforms the router from a pure speed optimizer into a comfort and safety optimizer. The resulting route may be longer in distance and time, but it keeps you within conditions your crew and boat can handle. This is arguably the most important routing parameter for cruising sailors.
Set your wind speed limit to the maximum sustained wind you're comfortable sailing in — not the maximum your boat can theoretically handle. A 36-foot cruiser can survive 40 knots, but routing through 40 knots when 25 is available with a 12-hour detour is a poor trade-off.
What happens when the routing algorithm encounters wind above your set maximum?
Tacking and Gybing Penalties
In theory, the routing algorithm can change course at every time step. In practice, every tack or gybe costs time and energy — the boat slows during the maneuver, the crew has to work, and there's risk involved in heavy weather. Tacking and gybing penalties make the router account for these real costs.
In the routing constraints, you can set a tacking penalty and a gybing penalty as a time cost in minutes. A typical setting might be 5-10 minutes per tack for a cruising boat (accounting for the slowdown, crew preparation, and speed rebuilding) and 3-5 minutes per gybe. Racing sailors might use 1-2 minutes. Shorthanded crews might use 15-20 minutes, reflecting the real effort of tacking a large boat with two people.
The effect on routing is subtle but significant. Without penalties, the algorithm might recommend 15 tacks on a beat — technically faster but exhausting. With a 10-minute penalty per tack, the router weighs the time cost and often finds that a slightly longer but less-tacking route is better overall. It might recommend holding a close reach for longer before tacking once, rather than short-tacking up a wind corridor.
There's also a minimum time between tacks setting that prevents the router from recommending rapid-fire tacks. Setting this to 1-2 hours means the router won't produce legs shorter than that duration, giving you stable sailing periods between maneuvers.
For shorthanded sailing, set generous tacking penalties (15-20 minutes) and a minimum time between tacks of 2+ hours. This forces the router to produce routes with long, stable legs — much easier to manage with a small crew and autopilot.
Why would you set a tacking penalty in the routing parameters?
Night Sailing and Motoring Parameters
Sailing at night is slower for most crews — reduced sail area for caution, slower maneuvers, difficulty reading the wind. qtVlm lets you apply a night sailing efficiency reduction that scales down your polar performance during nighttime hours. Setting this to 80-90% tells the router that you sail 10-20% slower at night, which produces more realistic ETAs and sometimes different route choices.
The router needs to know what happens when the wind drops below a usable threshold. The minimum sailing speed parameter defines the point below which your boat is effectively parked — the polar says 2 knots but in reality, with waves and current, you're going nowhere. Below this speed, the router can switch to motoring if you configure a motor speed.
Setting a motor speed (typically 5-6 knots for a cruising sailboat) tells the router it has a fallback in light air. Instead of routing 100 miles around a calm zone, it might motor straight through it — which is often the right real-world decision. You can also set a maximum motoring time to prevent the router from producing a route that motors the entire passage.
These parameters collectively make the router's output match how you actually sail. Without them, the router assumes you're always at peak polar performance, never motor, and sail the same day and night. With them, the route reflects your real-world capabilities and preferences.
Be conservative with night efficiency settings. If you're genuinely uncomfortable sailing at night and will likely heave to or anchor, set night efficiency very low (50-60%) or use the 'avoid night sailing' option if available. A router that assumes you're making 80% speed at night when you're actually stationary produces dangerously optimistic ETAs.
What does the 'motor speed' parameter tell the routing algorithm?
Polar Adjustments and Wind Scaling
Published polar diagrams represent theoretical performance — a clean bottom, new sails, optimal trim, and flat water. Real boats rarely match their polars. qtVlm provides several ways to close the gap between theory and reality.
The simplest adjustment is global polar efficiency — a percentage applied to all polar speeds. Setting this to 90% tells the router your boat sails at 90% of its theoretical polar in all conditions. This is a blunt instrument but effective: it accounts for bottom growth, aging sails, conservative sail handling, and the general reality that cruising boats don't match racing polars. Most cruisers should start at 85-90% and adjust based on experience.
More sophisticated is the point-of-sail efficiency adjustment, which lets you scale performance differently for upwind, reaching, and downwind. If your boat has old headsails but a new main, you might set upwind efficiency to 85% and reaching to 95%. This produces more accurate routing because it captures how your specific boat under-performs in specific conditions.
GRIB wind scaling is a different kind of adjustment. If you believe the forecast model consistently over- or under-predicts wind speed (some sailors find GFS runs slightly high in certain areas), you can scale the GRIB wind values. A setting of 95% reduces all forecast winds by 5%. Use this cautiously — it's better to adjust your polar efficiency than to second-guess the forecast model, unless you have strong evidence of systematic bias.
Track your actual versus predicted performance over several passages. If you consistently arrive later than the router predicted, reduce your polar efficiency by 5% and try again. Over time, you'll converge on a polar efficiency setting that produces accurate ETAs for your specific boat and crew.
Why should most cruising sailors set polar efficiency below 100%?
Summary
Wind speed and wave height limits create exclusion zones — the router detours around conditions that exceed your thresholds, optimizing for safety alongside speed.
Tacking and gybing penalties (in minutes) account for the real cost of maneuvers, producing routes with fewer, longer legs — especially important for shorthanded crews.
Night sailing efficiency (80-90% typical) and motor speed settings make the route reflect how you actually sail, not theoretical peak performance.
Global polar efficiency (85-95% for cruisers) closes the gap between published polars and real-world boat performance for more accurate ETAs.
Adjust these parameters based on experience — track actual versus predicted performance and converge on settings that match your boat and crew.
Key Terms
- Wind speed limit
- A maximum true wind speed threshold — the router treats areas with wind above this value as impassable no-go zones
- Tacking penalty
- A time cost (in minutes) applied to each tack or gybe, making the router account for the real-world cost of maneuvers
- Polar efficiency
- A percentage scaling factor applied to theoretical polar speeds — 90% means the router assumes you sail at 90% of published performance
- Motor speed
- A fallback speed the router uses when wind drops below the minimum sailing threshold — typically 5-6 knots for cruising boats
- Night efficiency
- A polar performance reduction applied during nighttime hours — accounts for reduced sail area, slower maneuvers, and conservative sailing
- GRIB wind scaling
- A percentage applied to all forecast wind speeds to compensate for known model bias — use cautiously