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    RL and GC thoughts
    From: Lars Bergman
    Date: 2010 Feb 2, 14:27 -0800

    Dear all, in recent days there have been some discussions about rhumbline versus great circle tracks, regarding an intended voyage from New Zealand to the Panama Canal.

    It is well known that when following an RL track you keep a constant "heading", as opposed to the GC track where you (ideally) would adjust your course all the time.

    But you could look at it from another angle. Although very idealised and theoretical, I find it amusing to have a helmsman's perspective:

    To follow a GC track, you direct the vessel's head to the initial course, then you lash the helm amidships. You don't need to touch your helm at all during the voyage. The compass however will be continuously changing (except at some special cases).

    To follow an RL track, on the other hand, you need to continuously change your helm (except in special cases) to get the desired ever-changing rate of turn in order to keep a constant course on the compass.

    The above seems rather contradictory to the well-known characteristics of the different tracks, but it is a quite amusing brain excercise.

    ---

    Another not often mentioned fact is that on the pole side of a GC track there is another track that has the same length as the RL track. As long as your track is somewhere between the RL and that "opposite RL", your distance travelled is less than the RL distance, although it may look that you are far far off track when looking at the chart.

    Lars
    59N 18E


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