Q&A: Trans-Ocean Navigation

Question: We are crossing the Atlantic from the Canary Islands to St. Lucia. I found a large scale chart of the Atlantic (Imray Passage Chart 100). This single chart shows the entire Atlantic Ocean taking the earth’s curvature into account and therefore does NOT have a compass rose to help steer by. I am having trouble finding a series of smaller charts that DO have a compass rose that might be a bit easier to navigate by. Do you know of a source where I might purchase charts of a smaller scale?

Answer: The issue here is much deeper than that. No such charts exist. At sea, navigation was, before chart plotters, done on plotting sheets that are basically blank charts for each area of latitude. These sheets showed a true compass rose only. Each day the navigator looked up the variation on a passage chart and then set the compass course after factoring in the great circle course, if appropriate. To do this manually you need to understand lines of equal variation and the difference between great circle and Mercator (rhumb line) routing. These days many, probably all, chart plotters and GPS units will do all this for you automatically, although you still need to make overall routing decisions and understand the difference between a great circle and a rhumb line course.

The chart shows various great circle (dotted orange) and rhumb line (solid gray lines). Counter intuitively, the curved courses are actually shorter. The difference between the two tracks is almost nonexistent on the Caribbean to US east coast passage; larger on the westbound trans-Atlantic passage; and very large on the eastbound trans-Atlantic passage, to the point that the great circle course takes you over land and the iceberg infested Grand Banks—a good reason to understand which type of course your GPS is using!

In my opinion it is vital that before you set off on an ocean crossing, someone on your boat have a good understanding of these basic navigation issues. I would suggest a good book on navigation, or better still a navigation course. Otherwise, in case of an electrical failure, or the death of your plotter, you will be, very literally, all at sea.

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David Head

I hope my post will re-open this subject. I am gradually weaning myself off all the gizmos and gadgets. Failure can only be a blown fuse or a scorched PCB away, so I have adopted the older methods. While I watch my AIS I miss the under 300 ton fishing boat, with his set off that strays into my path. I feel much more confident, but moreover, my methods are in my remit of control rather than a slavish acceptance of what I see on a screen must be therefore true. I have learned the formulae for my scientific calculator and have a printed set of sine / cosine tables as a backup. With an Almanac; an AP, and a sunsight I can get a pretty good fix within minutes. My eyes see the lumpy bits before I hit them and my compass and log give me a good idea for the next AP. GC distance can be easily calculated as can initial departure course, re-calculated daily. We ignore these skills at our peril! John. What do you say?

Oliver Rocher

Hi David,

I’m planning a multi-year cruise with my family, and for sure, I don’t want want to fully rely on electronics and computer. So, I am looking for sources of information to learn the basics of cross-ocean navigation. However, so far, I have not been very successful in finding books or writings on the subject. Any suggestion or any material to share ?

Thanks in advance
Olivier

RDE

AIS vs EYES

Perfect arguments for having the display mounted in the cockpit. AIS tells you if its big and moving. EYES tell you if its something you don’t want to meet. Dito for eyes glued to the radar down at the chart table.

And that is why a dodger should be a shelter rather than a house— you should be able to look over it when standing so you can actually see rather than looking through an opaque piece of plastic.

Michael Lambert

Thanks John,

So, on TZ I just pressed go to a point off ushant from here in Maine, and it looks like it shows a rhumb line course. Wouldn’t you want it showing the great circle route?

Edit, this was just a test.

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