← Back to the map · Blog

Weather routing: letting the computer find the way

Polars, isochrones, thresholds from your own boat: how Solano works out a track between two points — and what it cannot do.

Published 14 September 2026 · 9 min read

Contents

  1. Routing and Route mode: two different questions
  2. The polar: what your boat can actually do
  3. Isochrones: how the search works
  4. Land: the coastline mask
  5. Where the wind comes from
  6. Your settings: engine, night, thresholds, manoeuvres
  7. Comparing models and departure times
  8. Reading the result: the summary and the passage table
  9. What routing does not know

1. Routing and Route mode: two different questions

Solano has two tools that both deal with a passage, and they are easy to confuse. They actually answer opposite questions.

🗺️
Route mode starts from a track you have drawn and tells you what weather you will meet along it, hour by hour. The track is the input, the weather is the answer.
🧭
Routing starts from two points and works out the track from the wind and your boat. The weather is the input, the track is the answer.

Neither replaces the other. You draw a track by hand when you already know the way — a traffic lane, a channel, a habit. You run a routing when the question is precisely "which way?", and when one tack more or less changes the arrival time.

You can move from one to the other: in Route mode, the 🧭 Route this track button takes the points you placed — the first becomes the start, the last the destination, the rest marks to be left in order — and opens Routing with the same departure time.

Routing needs an account: it needs a boat, and therefore a polar.

2. The polar: what your boat can actually do

Routing without a polar is meaningless. The polar is the table that gives, for every wind angle and strength, the speed your boat holds. It is what makes the search bear away to go faster rather than aim straight at the mark.

You enter it in your boat's details (⛵ My boats menu), in three ways:

1
The grid
An angle × strength table you fill in by hand. It comes pre-filled with a generic cruiser polar, which gives you a starting point rather than a blank page.
2
A CSV file
Polars produced by racing software export as CSV. Solano reads the file, shows you what it understood, and waits for your confirmation before replacing anything.
3
Nothing at all
With no polar entered, the search uses a generic 11 m cruiser. It works, but it is not your boat: arrival times are indicative only.
Worth knowing: a builder's polar is measured on a new boat, well canvassed, with a trained crew. Yours is probably slower. Many sailors enter the builder's figures then shave 5 to 10 % off — the routing becomes markedly more accurate.

3. Isochrones: how the search works

The principle is over a century old and it is simple. Starting from the departure point, the search advances in time steps. At each step, from every position already reached, it tries every heading and works out — from the polar and the wind at that moment — how far each one gets.

All the points reachable after one hour form a curve: the isochrone, the line of positions of equal elapsed time. Then it starts again from that curve, and so on until the destination is reached.

The front would grow without limit if everything were kept. At each step Solano therefore keeps only one position per angular sector as seen from the start: the one furthest towards the goal. That pruning is what makes the search feasible without changing the result.

Tacks are written nowhere

This is the least intuitive part of routing, and it is what gives it its value. No line of code says "tack here". The search only knows headings and a polar.

It is simply that when the goal lies dead upwind, the polar gives almost no speed on the direct heading and a decent speed at 45° — so the best path runs through two tacks, and the tacks emerge in the result. The same happens downwind, where gybing and sailing at 150° is often faster than running dead before the wind.

The search itself is fast: a few hundred milliseconds for a crossing. What takes time is downloading the wind.

4. Land: the coastline mask

A routing that ignores land cuts straight through Corsica. Solano therefore carries a worldwide coastline and refuses any step that would cross it — rounding a headland, like tacking, emerges from that simple refusal.

This coastline is deliberately coarse. It is there to stop you crossing an island, not to thread you between two rocks. It knows nothing of shoals, cardinal marks, channels or restricted areas. A routing is always checked against a nautical chart before it is followed.

There is no notion of draught in the calculation either: a routing may well take you where your keel will not go.

5. Where the wind comes from

Route mode queries the weather at a handful of points. Routing needs the wind everywhere in the area and at every hour: it evaluates tens of thousands of positions, none of them known in advance. It needs a full field, not point samples.

Solano downloads those fields from two providers, which you can compare:

🇫🇷
Météo-France — ARPEGE Europe, ARPEGE Global, AROME France. AROME is the finest but only sees a few days ahead; ARPEGE reaches further.
🇪🇺
ECMWF — the European model as open data, global coverage, long lead times. Useful outside the Météo-France area, and as a second opinion.

Lead times are hourly over the first six hours — the ones that decide whether you leave — then every three hours. The window downloaded is cut to the passage: Solano estimates the likely duration and fetches only what is needed, rather than a fixed three-day window.

The first run is the slow one (about thirty seconds on a crossing): it downloads. Running it again straight away with another polar, another departure or different settings is almost free — a published model run never changes, so it is kept. Coming back the next day will download again: they are no longer the same runs.

6. Your settings: engine, night, thresholds, manoeuvres

The Advanced settings of the Routing panel describe the way you sail. They are collapsed by default — the defaults suit ordinary passage-making.

The engine

Allowed or not, with its speed and the sailing speed below which you start it. In light airs the search then switches to engine and heads straight for the mark — under engine, speed no longer depends on heading, so aiming at the destination is always shortest.

Night performance

A percentage applied between sunset and sunrise. Few crews set the kite at 3 a.m.; at 85 % the search takes that into account and may choose to cross an area by day rather than by night.

Safety thresholds

Three ceilings that forbid a step rather than penalise it:

When a gust threshold is requested but the field could not be downloaded, Solano says so on the track concerned rather than letting you believe the threshold applies. A silently inactive safety limit would be worse than none at all.

Time lost per manoeuvre

Every tack costs a few minutes. Without this setting the search happily proposes twenty-tack routes that no crew will ever sail. A few minutes per manoeuvre is enough to make the result realistic.

7. Comparing models and departure times

A routing is never a truth: it is the consequence of a forecast. Solano therefore computes one track per model selected and overlays them on the map.

Two models giving the same route is good news. Two models diverging — one passing north of the island, the other south — tell you the decision is not ripe, and that is information at least as useful as the track itself.

Multiple departures

The Compare several departures box re-runs the same calculation at regular intervals: "I plan to leave at noon, but what would 4 p.m., 8 p.m. or midnight give?". Results are grouped by departure time.

It costs little: the wind does not depend on the hour you choose to leave, so it is downloaded once for all the departures.

Courses with marks

Clicking the map extends the course with each new point: the last one clicked is always the destination, the earlier ones become marks to be left in order. Enough to route a race, or simply to force a waypoint. Each mark does add a segment to the calculation, though — a twenty-mark course is paid for in time.

8. Reading the result: the summary and the passage table

Each computed track gives a summary: arrival time, duration, distance, maximum wind and sea met, and a bar splitting the passage between engine, close-hauled, reaching and running. It is often that bar which decides between two routes of similar length — ten hours hard on the wind is not ten hours reaching.

The ⛶ Passage table button opens the detail: one row per calculation step, with everything describing that step in columns.

Two wind references, not to be confused. TWD is a true bearing: where the wind comes from relative to north, independently of your boat. TWA is an angle off the bow: it changes when you bear away, with the wind unchanged. AWA and AWS describe the apparent wind, the one you will read at the masthead — downwind it drops, upwind it builds.

Optional columns (sky, rain, pressure, sea, swell) are switched on with the pills. Each column group carries the name of the model feeding it: the wind and sea that constrained the calculation do not come from the same place as the rain or pressure, sampled afterwards along the track.

Once you have chosen a track it can be saved as an ordinary route — without simplification, at the calculation step, so that no tack disappears. You then find it in Route mode and in "My routes", and can attach it to a passage in the logbook or freeze its forecasts for use at sea.

9. What routing does not know

A computed track looks like an answer. Here is what it does not contain:

🌀
Current. The isochrone engine does not account for it. In a tidal race, a strait or an area of large tidal range, the computed route can be plainly wrong — correcting for it is up to you.
🪨
Hazards. Coarse coastline, no draught, no beacons, no restricted areas. A routing is transferred onto a nautical chart.
🌊
Sea state, unless you ask for it. With no sea threshold set, the swell shown describes the passage but did not constrain it.
👥
The crew. Tiredness, seasickness, the wish not to arrive at 3 a.m. in an unfamiliar harbour. The search optimises a duration, not a good passage.

Routing is one more opinion, to be weighed against the official marine bulletins and your own reading of the situation. It is excellent at revealing that an option you had not considered saves six hours. It is not there to decide for you.

Going further

The detail of the algorithm — time step, pruning, thresholds — is in The Solano method. To study the weather along a track you already have in mind, see Route mode: the weather along your passage, and to judge how much to trust the models involved, The stability score.

Disclaimer: Solano is not a maritime safety service. Navigation decisions remain the responsibility of the skipper, who must take account of their experience, their equipment and the official weather bulletins.
Open Solano →