Keeping off the land, to about ten metres

A route that cuts across a headland is no route at all. Yet a coarse coastline, like the one the app first used, draws the coast in broad strokes: it closes a narrow channel or forgets a small island, sending the route the long way round, or straight onto the rocks.

Routes now keep off the coastline of OpenStreetMap, drawn by its contributors to within about ten metres. Every leg is checked against it and dropped if it would cross it, so the isochrones flow around capes and islands the way a boat must: the narrow passages a boat can take are open, the islets it must round are there.

It covers the whole world, every island and channel with it, the Pacific across the date line included. A coastal margin, 0.2 NM by default, keeps each point of the route that far off the coast, and is lifted near the destination so that the route can reach a harbour.

From L'Aber Wrac'h to Camaret, the choice lies between the Chenal du Four, inside the rocks off Le Conquet, and the long way round outside Ushant. The routes take the channel, and the margin is the room you want from the coast: widen it, and if the channel closes to that much room, the route goes round outside. You see whether the inside passage is open to you before you commit to it.

The route knows the land, not the rocks and banks just off it, nor the buoys, the restricted areas or the traffic separation schemes. And the margin holds at each point of the route, not between them: there the route never crosses the coast but may pass closer. Widen it where the coast is foul, and check the passage on a chart.