NavList:

Lu Abel sent a message on July 8 addressed to Doug, but quoteing what
I had written to Doug, and Doug's response, so I suspect he was
arguing with what I wrote, rather than Doug.

I will interpolate some comments-

| 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.

Not such a simple division as Lu makes out. Wind, acting on the boat,
affects the boat's motion through the water. For a sailing craft, it
CAUSES the boat's motion through the water.
|
| #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.

What I have questioned is simply WHY it should be done that way. If
you know that your course through the water is different from your
heading from the compass, why not enter your best guess as to what
your true course really is, before drawing in your vector with respect
to the water?

| 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.


We agree.

| 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).

It's that "tradition" that I am questioning. For a sailing craft, it
seems to me far more sensible to estimate, as precisely as possible,
the path of the vessel through the water, to include any leeway, as
courses and wind directions change. Then add in the vector for set and
drift, for the motion of the water, independent of what the craft
does. How does the "traditional" method that Lu advocates deal with
the situation of a tack being made, so part of the day's work is on
one tack and part on another? How does he allow for the effects of
leeway then? Or is Lu's method intended to apply to powered vessels
rather than sailing craft?

| 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."

Yes, we've sorted that one out; It's one of those cases of two nations
divided by a common language.

| 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

The difference between our attitudes is not whether one should or
shouldn't correct for leeway and current. Nor is there any real
difference about the terminology. It's simply that there are two
vectors to be added together, and Lu thinks that "traditionally",
leeway should be included with the motion of the water, whereas I
think it should be included in determining the boat's motion through
the water, because it depends so closely on course and wind direction.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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