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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 08-14-2011
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Correction for Leeway - Estimated Position

Hello All,

Refer to the following log book entries:

Time Log Course Weather Comments
----- ---- ------ ---------- ----------

1300 0 004M WNW 18K Depart point A, close-hauled,
leeway 5 degrees

1400 5.5 264M WNW 18K Tacked, Leeway 5 degrees

My question is:

When I'm drawing my DR on a chart, do I plot 004M at 1300 and 264M at
1400 to come up with my Estimated Position? Or do I account for leeway
and plot 009M (004M + 5 degrees of leeway) and 259M (264M - 5 degrees of leeway)?

Set and Drift ARE NOT accounted.

My answer would be to plot 004M and 264M, because the log indicates "COURSE."

Thanks for your comments.
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Old 08-19-2011
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what course are you taking?

From this and your other thread it sounds like you're in a sink-or-swim course where you're not getting much instructor help.
You might be better off to find your local Squadron of the United States Power Squadrons. The national site is http://usps.org Although the name says "power squadrons" it is a sail and power educational organization and you should be able to rely on your instructor to help you sort out issues like these, and get for sure the right answers and reasoning rather than a bunch of educated, or maybe not, guesses.
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Last edited by junkrig; 08-19-2011 at 02:21 PM.
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Old 08-19-2011
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I think you are saying,

"Course" is the direction you actually made good, not the direction you steered, which is referred to as "heading". Therefore the log already accounted for leeway.

I would agree.
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Old 08-20-2011
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Thanks for responding to my question and the Power Squadron recommendation. I'll look them up. I'm actually taking a home study seminar as I travel a lot for work. I've been able to understand 90 percent of the material except for those few questions. This forum has been very helpful, and most of the responses have been educational. I hope that by me asking questions, it will be educational for the rest.
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Old 08-20-2011
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AdamLein, so I would plot on the chart 004M and 264M, right?
Thanks.
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Old 08-21-2011
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kwindancer,
You seem to understand the problem from your OP, and are simply trying to decide what to plot.

Seems to me the problem unfolds this way: If the logbook entries are 004M and 264M, they were taken from the compass. Doesn't have to be that way, but generally, the compass is your most reliable heading instrument. Could have the GPS display in magnetic, or the chart plotter, but you can get fix right off the display if you have good signal, and that makes no sense for the stated problem. If you are manually plotting, as the post sez, and leeway is known (how? as a known characteristic of the vessel when it is hard on the wind in 18 knots, which is the only way I can think of unless you are comparing compass heading against course on the GPS), then DR would be plotted along the mag course 004, and EP would be plotted at the end of a vector from your starting point along mag direction 009, at a distance equal to your estimate of distance traveled from the starting point.

When you tack to 264M, plot your DR along the 264M course from your 1400 DR position.

When you get a fix, reset your position to that fix, evaluating its quality, and start a new DR from that fix.

Dead reckoning is a simple integrated plot of course using the instrument you steer by and the speed you read on your log. Estimated positions include leeway, current effects not measured but historically observed, and other factors.
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Old 08-22-2011
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Quote:
When you tack to 264M, plot your DR along the 264M course from your 1400 DR position.
I'm a bit rusty on this stuff, but, why wouldn't you plot the second segment from the EP position (with leeway accounted for), rather than the DR position? ie: Magnetic heading indicated you should be in a certain spot by DR, but, knowing your boat you have 5 degrees of leeway, so a better estimate is that you are in a position that is 5 degrees leeward of your DR position.
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Old 08-22-2011
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DR is maintained along the steered course from fix to fix, but DR can be "reset" to an EP if you have a good one. DR is a plot of the track you have steered, are steering, and will steer at the speed you observe on your log, and distance observed on your knot log. EP is estimated position, a "best guess" based on all navigational factors you have at hand, including historical info. In general, DR error can be estimated as a percentage of your speed, a few degrees of course error due to compass error or helmsman capabilities, historical or observed set and drift, leeway, or calibration errors. By knowing these errors, and applying them along your track, you can build a "wedge" (expanding with errors as time since last fix increases) within which you are quite certain your current actual position is. Then if, for example, you have a sounding that corresponds to a depth you can correlate within the wedge you will estimate that to be a much more accurate position than the DR location. If you want to reset your DR to that ESTIMATED position, that's the navigator's or captain's call. When you get a fix, you always reset your DR to that FIXED position. You can then evaluate your error between DR and the fix and determine whether the factors you use in determining your EPs (or building your wedge) were good.

In this age of near continuous fixes, DR and EP are being used less and less. If GPS is not available for whatever reason, and you're away from visual navigation aids, making log entries allowing for accurate DRing and good EP estimates from noon sight to noon sight, or other fix source and interval, will be invaluable.
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Old 08-23-2011
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fryewe: everything you said seems correct, but if the log book said "Course = whatever", then that's not a compass reading. You said yourself that the compass gives an accurate indication of heading. So either the logbook was filled in incorrectly, or for some reason the navigator filled in his magnetic course instead of his true course.
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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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