Questions About Navigation
Index
Trans-Ocean Navigation
Custom Log Book
Magnetic Compass
Time Signals for Sextant
Trans-Ocean Navigation (09/2009)
Question [edited for brevity]: 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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Custom Log Book (2005)
Question: I read your article, "Knowing Where You Are", in January's Cruising World magazine and was wondering if I could see a copy of your custom log book page.
Answer: Note that the blank column can be used for whatever you wish; we usually use it for recording water temperature, both when crossing the Gulf Stream and when in icy waters to monitor for the presence of pack ice.
To see a larger log book page, just click anywhere on the picture.
Magnetic Compass (2005)
Question: Do you use a magnetic compass to augment your navigation process?
Answer: Yes, we do. We have a large Ritchie compass mounted on top of our binnacle. Despite having two flux gate compasses (it’s a long story why two) we like to check the course with the magnetic compass, which we treat as the master compass. It’s also much easier to take a bearing sighting over it, particularly in rough weather, than using a hand bearing compass, since it has much better damping.
We have the Ritchie professionally adjusted for deviation any time we make any changes to the location of metals around it, or about every 3 or 4 years. I also check it occasionally via sun azimuth.
We would not consider relying totally on flux gates since their deviation drifts quite a bit as our latitude changes, particularly in the high latitudes.
An accurate compass is still important to us since we do not just blindly follow a GPS waypoint without checking that the course makes sense. By doing this, any error in entering a waypoint becomes apparent before it can do any harm.
Finally, the magnetic compass, together
with an old Walker log and sextant, is an important part of our equipment
to allow us to navigate in the event of a total power failure. Unlikely
I know, but Murphy is always lurking at sea.
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Time Signals for Sextant (2005)
Question: Do you still do sights with a sextant? If so, where do you get the time signal from when underway? I found several shortwave frequencies for time signals on the web but the reception is extremely poor. In fact, I can’t get any useful exact time at all with my SSB receiver (Lowe HF-150 from the UK, built 1995), which otherwise works properly. I am sailing in the Med and I am a beginner with the sextant. Maybe you have some proven frequencies you could share with me?
Answer: I have to confess that I have hardly used my sextant in recent years except for using it to measure the height of an ice berg. However, I always used to set my watch before each ocean crossing by listening to WWV, the time signal out of Colorado in the USA, which is broadcast on 2.5, 5, 10, 15 and 20mhz. These are huge transmitters and I can almost always pull one of them in on our Icom SSB. In fact, I use WWV to check that our receiver is working OK.
You may have to try different times of day to find a time and frequency combination that will give you a clear signal in the Med. If you have no luck getting WWV with any time and frequency combination I would suspect a problem with your radio or its installation. Radio problems can be difficult to analyze, as we found out when we had a problem with tuning on our old SSB that only showed up at frequencies above 10mhz. The radio worked, but receiver sensitivity was poor at the higher frequencies.
I say that “I used to use WWV” because now I tend to just set my watch
off the clock on our GPS, which is synchronized with the GPS system atomic
clock and is deadly accurate.
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