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post #10 of Old 08-23-2011
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Course or heading can be either magnetic or true. Consistency is important in keeping the log. Every log book should have instructions for keeping it. If the log keeper is directed to observe the compass and enter his reading of the magnetic compass at the logging interval, or GPS in true heading, that is okay, but it's not the instantaneous heading that is important for DR. The helmsman, or the autopilot or wind vane, is directed to steer a course about which there is some wander. That course should be the DR course, and in my view the log should reflect whatever the helmsman is using to steer and monitor the autopilot and/or wind vane performance by, be it magnetic or true. Even if the ordered course is not along the desired track or route because you are avoiding something, or because you can't lay a desired course due to wind direction and have to fall off, at the point and time that happens, the navigator or log keeper should note the time, and mark the DR track at the time of the turn. From that point, he lays out the new DR in the new ORDERED direction. The most important reasons for keeping a DR plot are predictive...that is, to look ahead at where you are TRYING to go to prevent foul water, weather, traffic, and for efficiency in making way toward your destination.
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