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    Wind & Current Navigation
    From: Dan Allen
    Date: 2003 Apr 16, 18:08 -0700

    Today I went out on my boat, but leaving the slip became dicey.  This
    experience leads to a couple of questions about determining the
    cumulative effects of winds and currents.
    
    WHAT HAPPENED
    My slip faces due south.  The 15 knot wind was coming from the SE.  I
    released my lines and engaged reverse and pulled out of the slip, so
    far so good.  As I entered the fairway and the boat began turning so
    that the bow began to face East, I stayed relatively close to my slip,
    reasoning that the wind would push me out more to the middle of the
    fairway, and when I had turned enough, I would go forward and that
    would be that.  This plan has worked fine in no wind, or in light
    winds, or even winds this strong that I have encountered before.
    
    However, the incoming invisible tidal current was coming from the
    North.  It pushed me back towards the dock when I was starboard side to
    and I narrowly missed hitting docks and boats.
    
    The wind seemed to be controlling the situation.  The waves were all
    heading NW due to the wind from the SE.
    
    The current seemed negligible.  I knew that low-tide was just occurred
    about 30 minutes earlier; the flood was beginning again, but I did not
    think it had much strength compared to the wind, as I saw no sign of a
    current pushing me back into my slip.
    
    I was wrong!
    
    OBSERVATIONS
    The current was darn near invisible to me. The winds appeared to be all
    powerful, but the quiet, silent, invisible currents got me again.
    
    Let us say that we knew that the wind was 15 knots from SE and that the
    current from the incoming tide was 2 knots from the North; are they
    factored equally in how they combine to move the boat?  If they were
    weighted equally the result would be a 13.66 knot force coming from 129
    degrees, but this would not have blown me against the dock.  It still
    would have blown me away from the dock, but this is not what happened.
    
    A current from the North of 11 knots combined with a 15 knot wind from
    the SE would give a total force of 10.6 knots coming 2 degrees North of
    East.  This would have just barely begun to nudge me back toward the
    dock, but the tidal current certainly was nowhere near 11 knots!  It
    was more like 2 knots, perhaps 3 or 4 at the most.
    
    This leads me to believe that the current needs to be weighted much
    more than the wind, at least in my craft, but is this always the case?
    
    QUESTIONS
    How can you determine the relative strength of a current compared to
    the wind and determine the outcome of the two?  I know how to
    mathematically add two vectors, so it is not the actual math that I am
    asking about, but rather, how empirically does one determine the
    relative forces involved?
    
    How does one weight the effects of wind and weight the effects of
    current?
    
    How do the size, shape, and mass of the boat and the hull, and the
    superstructure (masts, flying bridges, etc.) effect these relative
    forces?
    
    HYPOTHESIS
    Let the vector "W" be the wind, and the vector "C" be the current.  Let
    "s" be a factor for the amount of superstructure on a boat, which the
    winds effect.  Let "h" be a variable representing the hull under the
    waterline, which the currents effect.  It seems to me that there should
    be some formula to determine the final speed and direction of the boat
    under these two forces of Wind and Current, the result being the vector
    B (for boat).  The formula would look something like:
    
           B = s * W + h * C
    
    s and h obviously are determined by a variety of measures and need to
    be further broken down.
    
    FINAL QUESTIONS
    Are there any rules of thumb that help one determine the sum of the
    wind and current accurately?  How do master mariners figure this out?
    What observations can be made to help predict the cumulative effects of
    wind and current accurately?  In other words, how could I prevent such
    near disasters in the future?
    
    Dan
    
    
    

       
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