Hi, I have not used isochronal Routing for some time mostly because of leave to stb/port. I recently used an isochronal routing to "double check" the grid result and the times were VERY different  (4h for a Fastnet). I tried to isolate the source of the difference and it looks like the isochronal algorithm is calculating unrealistic values upwind. I have than used a very simple example route without being able to figure out the reasons. If needed I have all data of my example available.
Best 
Andreas