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    Re: Avoiding Wx Problem
    From: UNK
    Date: 2000 Dec 15, 7:50 AM

    Robert Owens [mailto:tugly{at}neosoft.com] asked:
    > I have been studying for my Masters license and have come upon this
    > You are underway on a course of 050T at a speed of 12 knots. The eye of a
    > hurricane bears 120T, 110 miles from your position. The hurricane is
    > toward 285T at 25 knots. If you maneuver at 12 knots to avoid the
    > what could be the maximum CPA.(Closest Point of Approach)1
    > The listed answer is 77 miles.
    > I don't have a clue how to figure this one out. My Bowditch must be hiding
    > the obvious but I can't find it.
    > My first inclination is to draw the hurricane vector, then at the west end
    > of it draw a 12 mile arc, the draw a line from the east end of the
    > vector to the arc. that gives me about a 313T. Then I don't get anyway
    > a 77 mile answer. More like 60 miles. Help.
    Construct a geographic plot with your initial position and the hurricane
    bearing 120dT at 110mi. Draw the hurricane's projected track of 285dT and
    extend it for 150mi (6 hours into the future). Since the test expects you
    to "maneuver at 12 knots to avoid the hurricane", construct a new course
    line from your initial position on a course of 015dT (perpendicular to
    the hurricane track) and extend it 72 miles (6 hours at 12kt).
    To find CPA distance:
     1. Lay your ruler along your new 015dT track and draw a line back along
        195dT until it crosses the hurricane track. This is the CPA.
     2. Measure the distance from the CPA back to the hurricane's initial
        position. Divide that distance (in miles) by 25 (hurricane's speed in
        knots) to get the time of CPA (hours in the future).
     3. Multiply that time by 12 (your speed in knots), giving the number of
        miles you will travel before CPA.
     4. Mark that distance on your 015dT track. The distance from there to
        the CPA point on the hurricane track is the CPA distance.
    I just tried this using the cheep protractor and compass at my desk and
    came pretty close to the book's answer of 77 miles.
     -- Peter

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