Navigation guide

How does sailboat routing work?

Routing computes the fastest route given the weather and your boat's performance. Here are the inner workings — and the limits.

~6 min read · Intermediate

Ingredient no. 1: the polar

The speed polar describes the boat's theoretical speed for every combination of wind angle and wind strength. It's your sailboat's performance DNA. A route is never better than the polar it's given: an overly optimistic polar produces unsustainable routes, while a realistic polar yields credible estimates.

A polar: slow close-hauled (top), fast on a reach (sides), moderate running downwind (bottom).

How isochrones work

From the starting point, the algorithm computes how far the boat can travel in one time step (e.g. 1 h), in every direction, based on the forecast wind and the polar. This draws a curve: the isochrone. The operation is repeated from each point reached, over and over. Step by step, the network of isochrones reveals the fastest route to the destination.

Each curve = how far the boat can travel in one time step. The blue line is the optimal route.

Note: routing optimises a single objective (often time). It doesn't "know" your fatigue, your comfort threshold, or the areas you want to avoid — unless you tell it.

Departure windows: the real superpower

More than the route itself, it's often the choice of departure time that makes all the difference. By comparing the passage time for different departures over the coming days, you identify the best window: the one that dodges a blow, catches a favourable shift or a fair current.

Example — choosing the departure time

Departure Saturday 6 a.m. → 14 h at sea in 25 knots, built-up sea (uncomfortable).
Departure Sunday 4 a.m. → 11 h at sea in 12 knots, flat sea.

Same passage: routing ranks Sunday 4 a.m. first — faster AND more comfortable.

Criteria and safety

Good routing sets limits: maximum accepted wind, maximum sea state, avoiding coastal zones, sailing at night or not, using the engine below a certain wind threshold. These settings turn a "theoretically fast" route into one that is genuinely feasible for you and your crew.

Never switch off your brain

Routing relies on forecasts, and therefore on uncertainty. A few safeguards:

Compare several models: if the proposed routes diverge, the situation is uncertain.
Keep a safety margin and a bolt-hole port identified.
Re-route with each new run and cross-check it against what you actually observe on board.

SailMeteo offers routing with polars for typical boats, safety criteria and a ranking of departure windows over several days — right on the map.