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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---.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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