Steering just one course all the way to your destination
If you steer your boat in one constant compass direction, you’re following a rhumb line. If your course is due north or south, you’ll also be following a meridian; if it’s due east or west, you’ll be following a parallel of latitude. In all other cases, you will be spiraling toward a pole.The glory of the rhumb line for the amateur navigator is that one compass course takes you all the way from one port to another. The penalty you pay for this convenience is extra mileage.The shortest distance between two points on the Earth’s surface is a great circle, and (with few exceptions) a rhumb line is not a great circle. Over short distances, the difference in distance is unimportant, but on ocean passages it can be significant. For example, the great-circle distance between San Francisco and Yokohama is 4,517 nautical miles. The rhumb-line distance is 4,723 miles, or another 206 miles.Of course, it would be impossible for a yacht to make the gradual and minute course corrections a true great-circle course would require. The practical method is to divide the course into straight legs of about 600 miles each, and change course more substantially at the end of each.The nearer your course takes you to the poles, the greater is the

A great circle course plots as a straight line on a gnomonic chart. A rhumb line plots straight on a Mercator chart, but if you follow the rhumb line from San Francisco to Yokohama you will travel substantially farther than you would on a great-circle route.

difference between rhumb-line and great-circle courses. The great-circle course also offers significant benefit if your course is mainly east or west. If you’re traveling mainly north or south, you might as well stay with a more convenient rhumb-line course.See also Ocean Voyaging.

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