At this point in our navigation education, we're able to find our latitude and longitude using the position of the sun, we know how to deal with magnetic declination when setting a course, and we're able to calculate the speed of a ship using a chip log. Now we're ready to put all of these skills to use!

You might be thinking, "But why do we need to know how to use these skills when we can rely on GPS and other electronic equipment?" The problem with electronic equipment is that it doesn't always work, and when you lose your instrumentation, you're forced to use more primitive tools. And while this doesn't happen often, it certainly does happen. Here is one account of an emergency that a friend of mine experienced on a trip to the Falkland Islands and South Georgia. And another friend shared her account of a trip where a wave knocked out a window on the bridge, destroying the instrumentation and injuring several crew members. While I have been lucky enough to never experience an emergency, I have been through storms where the ship has rolled at least 30 degrees, which can be scary.

So how do you navigate without GPS, a speedometer, and a computer-corrected compass? You use dead reckoning. Dead reckoning involves using a previously-determined position, the ship's bearing, and estimated speeds to determine current and future positions. This information can be gathered as follows:

• Previously-determined position: Determined by GPS, a known location (the departure point, a port, or another visible landmark), or the movement of the sun.
• Ship's bearing: Determined by a magnetic compass and the magnetic declination information from a navigation chart.
• Estimated speed: Determined by previous calculated speeds or by using a chip log.

When you know this information, you can use it to estimate your position as you travel. This is done by the following process:

1. Plot the previously-determined position on a navigation chart.
2. Use the ship's bearing to plot a course on the chart from the previously-determined position.
3. Use the estimated speed, along with the amount of time that has passed, to calculate the distance traveled from the previously-determined position.
4. Plot the new position on the chart by moving the calculated distance along the course.
5. Use the chart to determine if a change of bearing is necessary to head in the direction you wish to travel. If so, use the magnetic declination to change the course.
6. If possible, use visual landmarks to correct for possible errors (which can accumulate given the nature of using so many estimations).
7. Plot a new position every hour, as well as any time the ship changes course, the ship changes speed, or an error is corrected.

Most ships that navigate in open seas keep a written logbook where they detail their position, bearing, speed, and weather information every 30 or 60 minutes. While this is kept for many reasons, it is particularly important in case the ship loses instrumentation and the crew needs to use dead reckoning to navigate.

Sample Problems

Suppose that you were navigating a sailboat on a trip to the Antarctic Peninsula. You had just left Elephant Island in the morning and were due north of it, with coordinates 55°W, 61°S. From there, you traveled at a bearing of 0° (due north) at a speed of 12 knots for five hours. However, a huge wave crashed over your boat, knocking out the electricity and all of your instrumentation. You need to head back to South America. You only have a chip log, a magnetic compass, and this navigation chart at your disposal.

1. Use your previously known location, your bearing, your speed, and the amount of time traveled to determine your current location.

2. You need to navigate to 65.5°W, 55.4°S, near Isla Nueva. At this point you can easily navigate into the Beagle Channel and head toward the port of Ushuaia. Use your current location, your desired location, and your navigation chart to determine the bearing you need.

3. Use the true bearing you need and the magnetic declination at your current position to determine the magnetic bearing you need to use. (Recall that to convert a true bearing to a magnetic bearing, we add the magnetic declination, with the convention that east is positive and west is negative.)

4. Approximate how long it would take to travel to Isla Nueva based on an average speed of 12 knots. What about an average speed of 9 knots? What about an average speed of 6 knots?