Great Circle and Rhumb Line Sailing
- Reece Smith-ward
- 6 days ago
- 3 min read
Every ocean passage begins with a simple question: which way should the ship actually go? On a flat chart the answer can look obvious, yet the Earth is a curved surface, and the shortest track between two distant ports is rarely the straight line you might first imagine. Understanding why is one of the more rewarding steps in any navigator's study, and it rests on two classic ideas: the rhumb line and the great circle.
This guide explains both in general, conceptual terms so you can build a clear mental picture before you meet the mathematics. It is written as study material, not as sailing directions, and it is meant to support your wider revision rather than to plan a real voyage.
What Is a Rhumb Line
A rhumb line is a track of constant course. It crosses every meridian at the same angle, so you can set one heading and hold it. This makes it simple to steer and easy to plot on the familiar Mercator chart, where it appears as a straight line. For shorter distances the convenience is hard to beat. The trade-off appears over long ocean crossings: because the rhumb line does not follow the curve of the Earth, it is usually not the shortest path between two far-apart positions.
What Is a Great Circle
A great circle is the shortest route across the curved surface of the Earth. Picture a globe with a piece of string pulled tight between two ports: the path it follows is an arc of a great circle. Because it is the shortest distance, it can save both time and fuel on long passages, which is why it matters so much on ocean routes. The catch is that a great circle does not keep a constant heading. As you travel along it, the course to steer changes gradually, so the track is a curve rather than a single fixed bearing.
Comparing the Two Tracks
It helps to think of the two as a trade between simplicity and efficiency. The rhumb line offers a steady, easily held course but a longer distance over great spans. The great circle offers the shortest distance but asks the navigator to adjust the heading along the way. In practice, a long great circle route is often broken into a series of shorter legs, each steered as a rhumb line, so the crew enjoys steady headings while still following the curve of the Earth closely. In broad terms, the further the passage reaches towards higher latitudes and across wide east-west distances, the more the two tracks tend to diverge.
Why the Difference Matters
Appreciating this contrast underpins passage planning, chartwork, and the wider study of navigation. It also helps explain the shapes you see on charts and the reasoning behind published ocean routes. As a student, the goal at this stage is not to memorise a formula but to hold a firm mental model: constant course versus shortest distance, and the balance a navigator strikes between them.
Studying With CalcDeck
CalcDeck includes great circle sailing among its library of navigation calculators, alongside tools such as course to steer, set and drift, and ETA. Working through examples like these can help you connect the concept to the numbers and check your own practice as you revise. CalcDeck is free to download with a one-time CalcDeck Pro upgrade, and it is available on the Apple App Store for iPhone and iPad. Its results are for training and reference and must be verified against approved instruments and official tables before any operational use.
Going Further With OOW
For those studying towards an Officer of the Watch qualification, OOW supports wider navigation study with calculators covering speed, course and distance, ETA, dead reckoning, position fixing, and set and drift, together with a reference library. OOW is a one-time purchase, available on the Apple App Store for iPhone and iPad, and also on Android through Google Play. It is intended for training, study, and revision, giving you a place to explore the ideas in this guide as part of a broader course of learning.
A Note on Safe Study
Remember that these apps are study, learning, and revision aids only. They are not for real-world navigation and not a substitute for approved training or equipment. Real navigation is always governed by official charts, publications, and proper training. Use this guide to understand and revise the ideas behind great circle and rhumb line sailing, and rely on those official sources and qualified instruction for any real passage.
