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Subject: Re: Avoiding Wx Problem
From: Smith, Peter (Smith_Peter@XXX.XXX)
Date: Fri Dec 15 2000 - 09:50:29 EST
Robert Owens [mailto:tugly@XXX.XXX] asked:
> I have been studying for my Masters license and have come upon this
problem:
>
> 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
moving
> toward 285T at 25 knots. If you maneuver at 12 knots to avoid the
hurricane,
> 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
hurricane
> vector to the arc. that gives me about a 313T. Then I don't get anyway
near
> 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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