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## 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: Doug Royer
Date: 2005 Jan 19, 10:16 -0800

```Hello George.I hope your new year is going great.

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?

The set of the wind driven current not including the Coriolis Effect.I meant
to project that the current derived from a wind blowing from 360 * will move
the vessel towards 180 * without the Ekman Effect.

>If the wind is on the starboard beam the resulting current will
>push the vessel toward the south.

Well, that would be true if the direction of the current set was indeed
180*T, and that would be true whatever part of the ship the wind was
blowing on. But Doug will deduce, in the next sentence, that the
wind-driven current set will be to the South-West, in the direction 220*T,
not to the South, so that's the direction in which it will push the vessel.

Correct.Stated better than I could do.

>That issue now being corrected the set of
>the wind driven current will be in the direction of 220 * T,not 320 * T.
>Again, 360 *T - 180 * T = 180 * T. 180* T + 40 *(to the right of the wind
>direction)= 220 * T.

Agreed.

But there's another matter that's confusing. His original posting contained
the passage-

>The drift will be 2% of the true wind speed.
>The set will be 40 * to the right of the true wind direction(Northern Hem.)
>and reversed when used in the S.H.
>
>Ex:
>The true wind speed is 20 knots sustained with the wind coming dead into
the
>starboard beam.
>The ship's head is 270 * T .We'll make the true wind direction 360 *  T to
>meet the above condition.
>The wind generated drift will be 0.2 knot.

But surely if the wind speed was 20 knots then a drift of 2% of the wind
speed would be 0.4 knots, not 0.2 knots.

Correct again.Doing arithmatic in my head is not my strong suit.

Have I got it right?  As Doug says, at sea a small offset current is an
important matter (more important still to us small-boat sailors, who only
aim to travel through the water at 4 knots or so). Perhaps Doug might
scrub-and-replace his original posting, with its later amendment, with
another that's entirely self-consistent, to avoid losing the meaning of his
message.

Thanks for your inputs and helping me clarify what I wanted to get across.

I remember taking a short course on oceanography, 50 years ago, and one of
the topics that has stuck in my mind ever since is the Ekman theory of wind
drift, developed just 100 yearss ago, and using some ferocious (to me then)
maths. Ekman deduced that in the Northern hemisphere the current at the
surface was 45 degrees to the right of the wind, and deeper down it was
further still around, to the right. The mechanism is similar to that which
makes the wind spin in circles in storms and depressions, and is due to the
rotation of the Earth. There is a return current, which allows the water
that's shifted by the surface current, to flow back the other way at a
deeper level, deeper even than Doug's ships. It's satisfying to think that
Ekman's academic work is now saving fuel on the World's oceans.

George

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

```
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