A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2016 Oct 31, 14:07 -0700
>>Since the rhumb line is only 18.0 nm longer than a perfect great circle would it be worth doing especially if it takes you into high latitudes or other hazards?<<
I want to make a belated response to this. If one is doing this calculation to establish a great circle route to sail along, then no...the calculation in this example is not worth doing. But my goal was that someone should learn to use a technique that was useful both for calculating GC routes, and also for doing sight reduction of celestial objects not covered by Pub. 249.
In my own case, I spent enough time with Pub. 229 to get comfortable with it, and to compare its accuracy to that which can be had in Pub. 249. Having done that, I said to myself, "I think I like packing Pub. 249 + a slide rule in my suitcase. That lets me do sight reduction of any celestial object I choose." Now, I have gone ahead and purchased Publ 249 Vol. 1...and it is quick and easy to use. At a certain point, I go to slide rule rather than Pub. 249, Vol 1, simply because it appeals to me more. I don't know why.
I think my take-away here is that another example for the user manual, where the GC is more distinct from the RL, would be better.
That said, I am working on producing an Ln scale, and will be making some explicit comments about when it is worthwhile to do GC calculations. In this connection, I am attaching a graphic that I am working with to illustrate GC/RL courses. What will go along with it is a chart of how much longer a RL course is than a GC, and when to choose. Obviously, you don't need to go to the trouble of GC calculations when you are sailing due north. If you are sailing from Hawaii to BC, there is some benefit to a GC course, but not much. If you are sailing Tokyo to BC, then following a GC course can save several days of sailing...assuming you don't mind sailing to far north.
The reality is that a small boat sailor is often going to take a longer route (see http://cornellsailing.com/publications/world-cruising-routes/) if that route gives him better winds, or if it avoids unpleasant weather. So GC calculations are helpful to make, but are only one factor to consider in route planning.