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    Re: Gnomonic Charts - Usable Zones
    From: Jeremy C
    Date: 2018 Jan 20, 05:38 -0800

    Hello all,

    That is some fun stuff about GC sailings.  I can't add too much to the technical discussion other than to say how the professional mariners do it these days.  We generally learned how to do it by formula as found in Bowditch II.  While tables were available, they weren't emphasized.  Most ships also have Gnomic charts and some navigators use them to plot the track and then just pull off the Lat and Long (typically for every 5 degrees of Longitude) and re-plot them on the ocean charts and/or plotting sheets.  Of course in the computer age we don't do either of those things.  Even for paper charts we plug the start and end points into the computer and select "GC."  The program typically asks the longitude interval and then gives a list of the latitudes.  Last summer I did a GC track from Hirohima, to Japan to the Panama Canal.  Since it was summer, the apex wasn't too high, so we did a GC route.  Of course, as was pointed out earlier by Robin, this is really just a series of Rhumb lines.  However, with the new track following autopilots on new ships, you can use a direct GC route and the Iron Mike will follow the course to the 0.1 degree and maintain a small cross track error so the track is very close to a true GC.  ECDIS helps with this a great deal.  

    For composite sailings, the navigator will basically plug in the start and end waypoints and plot the GC track until the maximum latitude (as set by the Master) and will run a parallel sailing until the lower part of the GC track is found again.

    As Frank says, anticipated weather both long and short term, are considered in routing.  I would not take the same route in the Pacific in January as in July.  Besides the requirement to slow down in poor weather, thus negating the time saved by the shorter route, you also have to look at the risk of cargo and vessel damage in very heavy weather.  The owners aren't going to want to hear how you saved a few hundred miles (say a day) of time, but cost them an insurance claim because 20 containers went over the side as you were rolling through a F9 extra-tropical cyclone at N55.

    A good example of this is when I was a young 3rd mate, and I was on a ship exiting the English Channel.  There was a good storm south, so the brand new captain decided to do a GC from Bishop Rock to Baltimore, USA in November.  We ended up hitting every low rolling off the Polar Vortex and our barograph looked like an EKG.  We were nearly a week late and hit 5 storms before we could dive south near the Flemish Cap.  We would have done much better taking the "long way" by sailing to the Azores and cutting across at about 30N.

    These days companies often hire weather routing services that use all sorts of algorithums to try to predict the best routes.  Still, it's GIGO, so with marginal weather products in certain parts of the world, these services can literally steer you into a tempest.  

    Jeremy

       
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