How isochrone routing works

Under sail, the shortest way is rarely the fastest. The wind changes from place to place and hour to hour, and a sailing boat’s speed depends on its angle to it. Isochrone routing is a method to find the fastest way through all of that — here is the idea, step by step.

1. The boat: a polar diagram

Everything starts with how fast your boat sails. A polar diagram gives the boat speed for every angle to the true wind, for each wind strength. Close to the wind the boat cannot sail at all; on a reach it is fastest; dead downwind it is slower again.

2. One step: where can I be in an hour?

From the start, the router tries every heading. For each one it looks up the forecast wind at that place and time, reads the boat speed from the polar, and sails for one time step. The end points form a closed line around the start: every place the boat can reach in exactly one hour. That line is an isochrone — Greek for “equal time”.

3. Step after step

The router then repeats this from every point on the first isochrone, using the forecast for one hour later. The second isochrone is defined by the points the boat can reach in two hours. Step by step, the isochrones spread outward from the start.

In these figures the wind is the same everywhere, so every isochrone has the same shape, just larger. With a real forecast they bend and stretch: they bulge out where there is more wind and lag behind in calms and headwinds.

4. Tracing back the route

Every point on an isochrone remembers the point it came from. As soon as a frontier reaches the destination, the router follows those links back to the start — and that chain is the fastest route. Tacks and gybes are not planned by hand: they appear wherever zig-zagging is quicker than the straight line.

5. What a route depends on

• The forecast. A route is only as good as the wind it was computed in. Different weather models disagree — comparing a route in two of them is a quick check of how certain the plan is. See the models.
• The polar. Real boats rarely sail their polar: reefing, sea state, a loaded cruising boat and a tired crew all cost speed. A slightly pessimistic polar gives more realistic times.
• The forecast’s end. Every forecast stops somewhere. Past that time there is nothing to route on, and 40kn Sail Router will not make anything up to fill the gap.
• Currents and land. A favourable current moves the whole frontier forward; a coastline cuts it off. Both change which way is fastest.