A course that crosses all meridians at the same angle, appearing as a straight line on a Mercator chart.
In practice
For relatively short ocean passages at low to mid latitudes, the difference in distance between a rhumb line and a great circle is negligible, and the rhumb line's operational simplicity is preferred. However, on long transoceanic passages at higher latitudes, the great circle can save considerable mileage and fuel. The OOW on passage planning must therefore compare both options, selecting waypoints that balance distance saving against weather exposure, ice avoidance, and TSS compliance.
Regulatory detail & full definition
A rhumb line, or loxodrome, is a curve on the surface of the earth that crosses every meridian at the same angle, appearing as a straight line on a Mercator chart. This property makes rhumb-line navigation straightforward: a constant compass course will follow a rhumb line from departure to destination. Bowditch (American Practical Navigator) demonstrates the mathematical basis in Mercator sailing, where differences in meridional parts allow precise calculation of course and distance along the rhumb. IHO S-57 vector chart data and its successor S-101 both represent coastal features and shipping routes in a Mercator projection, preserving the straight-line characteristic of rhumb lines on screen.
For relatively short ocean passages at low to mid latitudes, the difference in distance between a rhumb line and a great circle is negligible, and the rhumb line's operational simplicity is preferred. However, on long transoceanic passages at higher latitudes, the great circle can save considerable mileage and fuel. The OOW on passage planning must therefore compare both options, selecting waypoints that balance distance saving against weather exposure, ice avoidance, and TSS compliance.