Nav Tip: Dead Reckoning

There aren’t many sinking sensations to compare with the one you get when your GPS decides to take an unscheduled break, especially if you’ve been relying on an electronic chartplotter. One minute you know exactly where you are. The next you’re surrounded by a trackless sea, feeling distinctly insecure.
Author:
Updated:
Original:
deadreckoning2

There aren’t many sinking sensations to compare with the one you get when your GPS decides to take an unscheduled break, especially if you’ve been relying on an electronic chartplotter. One minute you know exactly where you are. The next you’re surrounded by a trackless sea, feeling distinctly insecure. It’s no good saying, “This will never happen in 2012,” because there are a number of reasons why it very well might. Your first priority is to find out where you are, a fairly straightforward job if you’ve taken a few basic precautions. Here’s what you need to do.

• Always carry a paper chart. You don’t have to have it open if you’re using a good plotter (although it’s a must for passage planning), but when the plotter goes off-watch, that chart is your lifeline. 

• Maintain an up-to-date ship’s logbook on paper. This doesn’t have to be fancy. All it needs is some columns for time, position, course and the distance-log reading. You may choose to enter more data, and all credit to you if you do, but those basics will get you home.

• Make a log entry once an hour, or whenever you pass a significant mark, such as an offshore buoy.

Armed with this data, you can look an electronic meltdown in the eye without missing a heartbeat. 

deadreckoning

Dead Reckoning

Because you’ve logged your GPS positions, you know exactly where you were an hour ago. With luck it’ll be a good bit less time than that. Plot this on the chart as your “last known position.” 

Starting from there, using parallel rulers or a chart protractor, plot a line in the direction you’ve been steering. Unless there’s been serious current, you’re somewhere on this course line, or pretty near it. Label it with, “C 065M,” for example, if you’ve been steering 065 on the compass— the “M” is a good idea because it denotes a magnetic heading to make sure you don’t confuse it with a true one. If you’re motoring, you might note the speed as well–“S 5.5” perhaps.

Now read the distance log. The difference between now and what you noted at the last known position is how far you’ve come down the course line. Mark this off on the latitude scale using dividers, then transfer it to the course line by pricking one leg onto the last known position. The position at which the second leg lands on the course line is your “dead reckoning,” or DR, position. Mark it with a dot and a tiny semi-circle, label it with the time, and then log it with distance run. 

Note the soundings on the chart for this position and see if it stacks up with the reading on your depthsounder. If it does, the chances are that you’re not far away from the DR. To be sure you’re safe, the next little task consitutes the bedrock of all good navigation—double-checking. Along the coast, the best confirmation is to take a fix. 

Fixing your position

First have a good look around. You may well find that some easily recognizable object, such as a buoy the DR suggested was a mile away, is really only 100 yards off. If the depth corroborates this, you won’t find a much better fix than that. Log your position and set a course for home. If you aren’t close enough to anything immediately recognizable, you’ll have to deduce where you are by using lines of position, or LOPs.

An LOP is a line drawn on the chart in an observed direction from a known object. There are many ways of producing these, but the universal favorite is a magnetic bearing taken with a handbearing compass or the steering compass if it’s convenient. Plot the line onto the chart, noting the time on its upper edge. You know you are somewhere along it; the question is where. To nail this down, find another LOP and plot that as well. You’re now on two lines, so your only possible location is where they meet. 

If you can only produce two LOPs, reference the intersection with a time label and log it as a fix with distance run and the rest. You now have a “last known position” from which to start your next DR plot. All that’s left is to get going. If you can see a third object offering another useful LOP, grab that as well. The compass on a rocking, rolling boat is an imperfect instrument. The extra line will either confirm the accuracy of the first two, or cast some honest doubt on the proceedings so you can exercise seamanlike caution.

Related

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com The back door Satisfied with your headsails? So was I, until one day I took a long, hard look up the luff of my genoa, making sure I inspected the leeward side as well. The sail had plenty of life ...read more

02-Lydia12-01

Losing Sight of Shore

I arrived on the docks of Beaufort, North Carolina, in late April with two backpacks filled with new gear—everything I’d need for my first offshore passage. Though I’d been sailing for 16 years, graduating from dinghies to keelboats to a J/122, I’d spent my time racing and, in ...read more

Squall

The Face of a Squall

They are the worst of times, they are the best of times There’s a fabulous line from an old Paul Simon song that I often sing to myself while sailing: I can gather all the news I need from the weather report. It is part of the magic of sailing, this ancient process by which we ...read more

ntcktshtrstk

Cruising Southern New England Waters

One of the most wonderful childhood vacations I can remember was back in 1971 when my best friend invited me to his family’s summer home on Nantucket Island. For a 10-year-old kid, this was a thrilling trip for many reasons, not the least of which was the fact it was also my ...read more

IMG_8287GR16Mykonos

Cultural Charters: Mykonos

In last month’s column, I covered the amazing mix of cultures that have called the Dalmatian Coast home over the centuries. Croatia cruising is like a smorgasbord of intertwined centuries, and the islands are a movie set. A little farther south, though, you’ve also got Greece, ...read more

cookinglead

Cruising: No Oven? No Worries

Many cruising boats, especially smaller ones, don’t have a conventional oven. But that doesn’t mean that you can’t have all the baked foods you want, from bread to brownies to breakfast rolls to casseroles and even a roast chicken. All it takes is the right bit of gear and a ...read more

ZK-Seaboot-900

Gear: Zhik’s Seaboot 900

A Better Sea Boot Following up on its successful ZK Seaboot 800, Zhik’s Seaboot 900 was created in partnership with team AkzoNobel and Dongfeng Race Team, the latter the overall winner of the 2017-18 Volvo Ocean Race. Designed for serious, long-distance offshore racers and ...read more

01-LEAD-FP-Astrea-42-Gilles-martin-rajet---Navigation

Switching to Solar Offshore

No sensible bluewater sailor would consider setting off on a long cruise these days without some means of generating power other than by burning fossil fuels. The good news is that solar energy is becoming less expensive by the day, making it an obvious answer for providing the ...read more