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:
Publish date:
Updated on
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 Check the waypoint  Most errors with GPS and paper chart navigation are caused by the operator punching in the wrong numbers or plotting the lat/long incorrectly. The surest way to double-check a ...read more

Furlex-Electric

Gear: Seldén’s Furlex Electric

Furl Power Seldén’s Furlex Electric offers an easy path into the world of sweat-free headsail furling. The compact unit can be retrofitted to an existing manual Furlex unit or installed as a replacement for whatever you’ve got now. Its DC-DC converter accepts your boat’s 12V or ...read more

11_DSC8423Tom-Zydler

Cruising: Nova Scotia

There’s a unique cruising ground that combines access to urban locations with easy escapes to wilderness and nature. Its native people may be the friendliest on the east coast of North America. Its coastline runs 250 nautical miles in a straight line, but that should be ...read more

01-LEAD-shutterstock_727849660

Boat Monitoring System

Boat Oversight In a world where you can track your friends’ locations in real time and stream yourself live on the internet, it should come as no surprise that you can also keep a close eye on your boat from the comfort of home. In fact, not only is there a plethora of options ...read more

pilot_saloon_42-_en_navigation_11

Boat Review: Wauquiez Pilot Saloon 42

Old salts grouse about modern aesthetics. It’s just what they do, and the hard lines and spartan interiors of today’s production boats give them many reasons to complain. French builder Wauquiez, however, seems to consistently be able to marry contemporary elements with ...read more

JuneWaterlines

Sights and Stories Cruising the Caribbean

Though I hate to think of myself as a “disaster tourist,” I can’t deny one of the things I was most curious about as I sailed south last fall to visit St. Martin, the Virgin Islands and Puerto Rico was how much hurricane damage I would see. I’m sure no one needs reminding that ...read more