If you select this option, SailGrib will calculate the sailing time along the great circle line. If the great circle intersects the earth, the routing will stop. This option helps assessing the risk of the proposed isochrones routing. If you save 5 minutes on a crossing of the Bay of Biscay sailing far away from the great circle line, it does not seem a good option.
In the following example, the gain following the isochrones routing is in the order of 4:40 minutes over fifty hours, the weather situation seems clear, the isochrones routing seems to be the best option.
