A method of solving rhumb line problems using the properties of a Mercator chart to find course and distance between positions.
In practice
For the OOW or navigator planning a passage on a Mercator chart, Mercator sailing gives exact answers for any latitude and distance, whereas parallel sailing and mid-latitude sailing are only approximations. In practice, ECDIS software performs all such calculations automatically, but understanding the underlying method remains a STCW-examined competency, ensuring officers can check electronic results manually. The technique is particularly important for verifying course and distance when plotting great-circle waypoints as intermediate rhumb-line legs, since each leg must be calculated accurately to approximate the great circle faithfully.
Regulatory detail & full definition
Mercator sailing is a method of solving rhumb-line navigation problems—finding the course to steer and the distance to travel between two positions—using the mathematical properties of the Mercator projection. It is based on the concept of meridional parts, which are tabulated values that allow the mathematically correct latitude scale of the Mercator chart to be incorporated into calculations. Bowditch (American Practical Navigator) devotes a full chapter to Mercator sailing, providing worked examples alongside the simpler mid-latitude sailing method, which is adequate for short passages at low latitudes.
For the OOW or navigator planning a passage on a Mercator chart, Mercator sailing gives exact answers for any latitude and distance, whereas parallel sailing and mid-latitude sailing are only approximations. In practice, ECDIS software performs all such calculations automatically, but understanding the underlying method remains a STCW-examined competency, ensuring officers can check electronic results manually. The technique is particularly important for verifying course and distance when plotting great-circle waypoints as intermediate rhumb-line legs, since each leg must be calculated accurately to approximate the great circle faithfully.