Posts Tagged ‘rhumb line’

Passage planning – the great circle route

Tuesday, November 18th, 2008

When navigating over long distances it’s better to sail a great circle. Technically, these are circles on a sphere (Earth) whose planes pass through its centre. So, the equator is a great circle, and so is a circle through both poles. It follows therefore that any circle between those two is also a great circle. It is shorter to follow a great circle than to follow a straight line plotted on a chart.

Because it would be difficult to steer the constantly changing course that a great circle would demand, it is usually made up of a series of rhumb line courses between waypoints.

An alternative to this would be sailing a circle at, for example, latitude 30º north. This does not pass through the centre of the sphere (Earth) and is known as a small circle, as are any others like it.

Learn much more about Navigation and Passage Planning by investing in The Boating Bible Manual of Jeem the seamanSeamanship.

Passage planning – sailing the rhumb line

Monday, November 10th, 2008

Each year we see graphics of the Sydney-Hobart Yacht Race with the rhumb line shown. And it seems that the competitors should be trying to sail that course to optimise their chances. Tactics – covering your opposition or taking a flyer – and awareness of weather predictions can make skippers position their boats a significant distance from the rhumb line.

So how is rhumb line defined? A rhumb line is a course which intersects each meridian at the same angle. In practice this would mean sailing a steady course or following a constant compass bearing.

The Boating Bible Manual of Seamanship includes a disk on Navigation and Passage Planning.