Max duration, sweep angle, step angle

Expert

The essentials

The routing works with isochrones: an isochrone is the line joining all the points the boat can reach at the same moment, starting from its previous positions. The calculation builds one isochrone after another until it touches the target. Three Expert mode settings fix the frame.

SettingRangeDefaultWhat it does
“Max duration”1 to 15 d15 dstops the calculation after this sailing time
“Sweep angle”40° to 360°, by 10°220°width of the fan of headings tried at each step
“Step angle”Auto or 1° to 5°Autogap between two headings tried

If your “stepped” routing hugs the edge of an isochrone, increase the sweep angle (up to 360°). Elsewhere, keep the defaults.

Going further

Max duration. This is the sailing time, from the start of the routing, after which the calculation stops. If the target is not reached in time, the message is: “The routing stopped at its maximum duration (N d) without reaching the target. You can increase it in the Routing menu.” Reduce it to see when a routing decision has to be made. Example: from Brittany to the Caribbean, the best route can switch from a northern to a southern route depending on the duration. Run one routing at 15 days and another at 8 days: if the first days look alike, there is no hurry; if they diverge from the start, you must choose right away.

Sweep angle. At each step, from each point of the previous isochrone, the calculation tries headings spread over a fan centred on the direction of the target. The value is the total width of that fan: 220° is plenty in general, 360° covers every heading.

  • The wider the angle, the longer the calculation, but the less likely you are to miss the right route.
  • When the optimal route strays far from the direct route (light-wind areas, very fast boats that go the long way round to find wind), it can end up at the edge of the fan: the line then makes “stairs” along the isochrone. Remedy: widen the angle. On a fast enough machine, 360° settles the question.
  • Without Premium you cannot change this angle: add an intermediate target near the middle of the stairs.

Interactive diagram (requires JavaScript): a fan of headings around the direction of the target. Two sliders set the sweep angle and the step angle, and the diagram shows the number of headings tried per point.

Simplified model, to understand the principle; the app uses your boat's real polar and the weather forecast.

Step angle. The step is the increment between two headings tried in the fan. The smaller it is, the more precise and the longer the calculation.

  • In “Auto” (value 0), the calculation starts at 1° for the first isochrone, then adjusts the step between 1° and 5° so that each isochrone takes about two seconds: it raises the step if the device is slow.
  • A value from 1 to 5 forces that step, without adjustment.