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Doug:

Any attempt to calculate and plot the position of a boat including
compensation for the effects of current or wind will have two components
(1) the boat's motion through the water, and (2) external effects
(current, wind) acting on the boat.

#1 is covered by what has traditionally been referred to as the boat's
DR position.  DR is calculated ONLY from speed, direction, and time.

Is the result the position of your boat?  Not necessarily.   If you know
your boat is being acted on by current or has experienced leeway, it's not.

Should a navigator correct the position of his/her boat by allowing for
leeway or current?   Absolutely, if he/she thinks these might have had a
significant effect.

But, again by tradition, this is called an Estimated Position (EP) and
dotted line is drawn between DR and EP with the cause for the EP being
different from the DR shown by a label on the line.  (Again, navigation
plots are usually meticulously annotated, both as a way of going back
and catching possible errors and to allow another navigator to read and
understand the plot).

In addition to showing EP vs DR for a position at any point in time, you
can show a "live" estimate of Course over Ground on your navigational
plot.  Simply extend two lines from your starting fix:  one for a DR
course and one for your estimated COG.  The former is labeled with
steered course (C) and knotmeter speed (S).  The latter is labeled with
COG and SOG.  And, again, there is usually some notation made on the
plot about how DR course and speed is being compensated to arrive at the
COG/SOG plot.

As far as set, drift, and leeway are concerned:   Leeway is sideways
motion of a boat due to the effect of wind on it.  It's an angle by
which the boat's actual course deviates from the course steered and it
naturally depends on the strength and direction of the wind.  Current
has a direction, called "set" and a velocity, called "drift."

In this discussion I don't think anyone is arguing that the effects of
wind and current shouldn't be taken into account by a good navigator.
It's just that there seems to be a misunderstanding of the terminology.
   But it's no different than why the latitude of the GP of a celestial
body is called its declination rather than its latitude.

So:  let's keep correcting for leeway and current.   But it would
probably be best to use the notation that's been used by navigators for
centuries rather than redefining the terms.

Lu Abel



Royer, Doug wrote:
>
>
>
> Bill wrote-
>
> "If I understand, DR is course and speed.  Once you factor in set,
> drift and
> leeway you are into the area of EP (estimated position)."
>
> I'm not sure what Bill implies. Is that really what the books, and the
> training courses, teach nowadays? Surely, if you're going to draw a
> line on your chart, representing your passage through the water, it
> makes sense to estimate a leeway correction to your course, there and
> then, before drawing that line. That's what I have always done, and I
> presumed that's what others did also. Perhaps I'm just out-of-touch.
> But if it's common practice to plot a course without allowing for
> leeway, then include it later with current set, can someone explain to
> me the rationale behind it?
>
> What, in this context, is "drift", as distinguished from "set", and
> "leeway"?
>
> George.
>
> Leeway is ONLY the action of the prevailing winds in a certain area that
> affects the desired course of a vessel underway.
> Set, drift and leeway must be compensated for to stay close to the
> intended track line.
> The larger the surface area of a vessel, and that vessel's surface area
> ASPECT to the prevailing winds, the greater effect the wind has on its
> ability to stay on its intended track line. Leeway is not a constant
> force but an averaged force. It is a local or regional force that may be
> in lesser or greater force at one point to the next in the same area.
> Yes, there are procedures to check or calculate leeway.
>
> Here's how it works on a commercial vessel. The OOD and helmsman are
> given the intended course line at the start of a watch. With today's
> computer navigational graphics and GPS it is easy and almost immediate
> to see the effects of set and drift and leeway.
> The master usually denotes the amount of distance off the intended track
> line is acceptable in routine situations. As an example he may denote
> 1/8 n. mile either way of the intended track line is acceptable but to
> correct anything outside of that limit to get the vessel back on track.
> But, in reality, the helmsman is constantly adjusting the wheel to keep
> the vessel on the track line. There is a monitor with the electronic
> chart of the area with the vessel's pos, the intended track line, the
> track actually followed and the vessel's position on the current heading
> at some set time in the future on the immediate heading.
> It's pretty easy now to compensate it all out as one is moving along.
>
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>

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