Tuesday, September 30, 2008

Lesson 17: DR Plots

Deduced Reckoning aka Dead Reckoning

De(a)d Reckoning (DR) is nothing more than projecting a vessel’s intended course or track line from a fixed or known position by using true direction, speed, time, and distance to be traveled. “I reckon I’ll be there at 1020.” DR is one of the simplest and oldest methods of navigating in which a vessel’s position is deduced or computed mathematically in relation to a known point of departure. Dead reckoning is always used every time a vessel is underway. Do DR’s at least once hour, on the hour. The underlying reason for using DR is that a navigator can reasonably determine of his vessel’s most probable position without taking a fix. Dead Reckoning is the process of determining a vessel’s present location or future position along a known or projected course line from a previously known position by using the vessel’s actual or anticipated course, speed, and run time. The effects of current or wind are not considered in determining a DR position.

Some things to consider when Dead Reckoning:

1.Be neat, accurate, and complete. You will be less likely to make mistakes if you properly notate your work on the chart. Plus you'll have the information available if you need to refer to it at a later time.

2.The DR course (or track) is always plotted in degrees true. That means your compass course will have to be adjusted for variation and deviation.

3.The DR track always starts from a known location (your Fixed position).

4.The speed you use for DR is always your boat's known speed through the water. Don't use it's speed over ground. What's the difference? Your boat's speed through the water is just that. It has nothing to do with the actual speed that you're making over the sea bottom. The progress of a boat is influenced by many factors. You should measure your vessel's speed through calm water on a windless day with the use of a log (a boat speedometer) or some other method, and correlate it with engine RPM. This way you'll know your speed by looking at the tachometer. If you know that your boat goes 10 knots at 2000 RPM in a calm sea, and you're going into a 2 knot current, your speed over ground (SOG) may be 8 knots, but for your DR use the 10 knots. You'll apply corrections later.

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