# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: DR thread from Nov-Dec '04
From: Bill B
Date: 2005 Jan 19, 16:36 -0500

```Doug wrote:

>> After reading my origonal post on this subject I noticed I wrote the set
>> discription incorrectly.I appologise for any confusion I caused and will
>> clear it up now.
>> If the wind is out of the north at 360 * T the set will be in the direction
>> of 180 * T.

George replied:

> Perhaps Doug is referring the "set" of the wind here, which is presumably
> the direction the wind is travelling toward. It's not a familiar
> expression, to me. Or is Doug referring to the set of the wind-driven
> current here?

Bill chimes in:

From my cheat sheet, distilled from several texts: "After 12 hours of steady
breeze a surface current may form.  It will be 15-45 degrees to the right of
the direction the wind is blowing towards in the northern hemisphere (to the
left in the southern hemisphere).  For example, an easterly wind (out of the
east) will create a north western current.  It will be approximately .01 of
the wind velocity."

Chapman Piloting states, "For a rough rule of thumb, the strength of a
wind-driven current can be taken as 2% of the wind's velocity.  The
direction of the current will not be the same as that of the wind, a result
of the earth's rotation.  In the Northern Hemisphere, the current will be
deflected to the right to a degree determined by latitude and the depth of
water.  The deflection may be as small as 15 degrees in shallow coastal
waters, or as great as 45 degrees on the high seas; it is greater in the
higher latitudes."

Perhaps the source of confusion here is "set" of the wind.  For an aircraft
pilot sailing in a sea of air, I imagine it would be fair to think of air
currents as having set (direction)and drift (velocity).  As I perceive it,
for the nautical pilot  the wind creates a water current with set and drift.
It is that current's set and drift which is of concern to the navigator.
Wind direction and velocity are not considered to have set and drift in this
instance, but rather are employed in determining the set and drift of the
surface current.

Other than bit of confusion, George does seem to have it right as presented
to date.

To muddy the waters a bit,  I have seen proportional drift figures from 1%
to 2% of the wind's velocity.  "The Annapolis Book of Seamanship" would
indicated the proportions are not a constant:

10 kt wind  "may" =  0.2 kt current
20 kt wind  "may" =  0.3 kt current
30 kt wind  "may" =  0.4 kt current
40 kt wind  "may" =  0.5 kt current

Some texts also toss in fetch (distance from shore to craft) and duration of
the wind, just to further confuse the issue.  Have never seen an equation--
let alone a look-up table--on the topic.

Bill

```
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