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I originally wrote:

> I was taught that DR is determined only by course (heading) and speed.
> Current, windage, etc., get factored into EP, but not into the DR.

and Dave W. responded:

> I've heard your definition before but it seems to make a
> distinction that makes no sense to me.
>
> Do you plot your compass heading and ignore known deviation and variation?
> If you know your speed log registers a 1/2 knot slow, do you leave this out
> of your DR plot?
..
> So... My boat sags off 5 degrees when sailing close to the wind.
> I know this from  experience.  Why would I not include this in my DR.
>
>  On a passage from Jacksonville, Fl headed for Bermuda, I know that
> I'll not be remiss if I include 2.5 knots set to the north from when
> I hit 080 west longitude till 079.20 west.

And now I respond:

Deviation, variation, and errors in the knotmeter are considered to be known in
advance and do indeed go into the DR plot.  Current and leeway are considered
unknown and do not go into the DR plot. From the first paragraph of Chapter 7 of
Bowditch (1995 edition):

"The DR position is only an approximate position because it does not allow for
the effect of leeway, current, helmsman error, or gyro error."

I can think of two possible reasons for this:

1. Course steered and speed through the water (corrected for variation,
deviation, and known errors in the knotmeter) are both known factors within the
control of the vessel's operator and can be kept [relatively] constant.
External factors such as wind and current are not known with certainty in
advance, are not within your control, and can change unpredictably. Better to
plot what you know for sure in your DR plot.

2. A DR plot, like a log book, is a legal record of the vessel's actions and can
be treated as such in a court of law in the event of a collision or other
mishap.

To use your example, you would indeed be foolish to ignore the effects of the
Gulf Stream in planning a passage from Jacksonville, Florida (USA) to Bermuda.
That same chapter in Bowditch shows how take this into consideration (in Figure
708b. "Finding the course to steer at a given speed to make good a given course
through a current.")

Bowditch also talks about something called "the estimated track made good",
which it defines as the direction from the last fix to the EP.

If it were me, I would do the following:

1. Plot my Intended Track in advance (labelled as such and perhaps plotted in
blue pencil so as not to confuse matters any further).
2. Compute in advance the course to be steered in order to make good that
intended track, taking all known factors into account. Adjust this as new
information comes to bear.
3. Plot my actual course steered and speed through the water as my DR.
4. Plot a dashed line showing my estimated track, considering wind and current.
5. Navigate in such a manner that both my DR and my intended track stay out of
harm's way, allowing plenty of margin for error.

A copy of Bowditch (1995) can be found on line at

http://pollux.nss.nima.mil/pubs/pubs_j_apn_sections.html?rid=100

Regards,

Chuck Taylor
Everett, WA, USA

   

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