When Do You Use a Great Circle Course?

Definition: Rhumb line course - a course that follows a single compass bearing. On a chart with a Mercator projection, a rhumb line course looks like a straight line.

When you are travelling north-to-south from anywhere, or east-to-west at the equator, a rhumb line course is precisely the same as a great circle route. Great circle routes become progressively more efficient than rhumb line courses the further away you are from the equator, and the more you are travelling east-west rather than north-south. This graphic shows rhumb line courses as dashed lines and great circle routes as solid lines.

If you are travelling from Port Hardy, BC, to Hawaii, the rhumb line course is little more than an hour of sailing time longer than the great circle route. Definitely not worth the effort of recalculating your great circle heading on a day by day basis. But if you are seeking to sail non-stop from Port Hardy to Tokyo, the great circle route can shave several days off your trip compared to the rhumb line course.

Sorting out the type of route to follow will be a part of the cruise planning you undertake before starting your trip.