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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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