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Old 13-11-2006, 20:48   #1
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Hurricane Maximum CPA

Hi all,

I'm in the process of taking some tests at a USCG REC, and am having trouble figuring out how to solve a particular problem. It's common in Nav General

You are underway on course 050°T and your maximum speed is 10 knots. The eye of a hurricane bears 100°T, 90 miles from your position. The hurricane is moving towards 285°T at 19 knots. Which course should you steer at 10 knots to have the maximum CPA?
  • N a. 221°
    Y b. 226°
    N c. 233°
    N d. 238°
I know the answer is B. 226, but I do not know how to /get/ the answer. Anyone care to shine the light on this for me?

Thanks for your help,
Aaron N.


~~Update~~

I used my head and referenced Bowditch, which has a full explanation. I was trying to do it without plotting tools, thinking Nav General did not allow them. Bowditch says use a maneuvering board - makes life much easier!


Thanks again!
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Old 14-11-2006, 20:01   #2
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Look up Buys-Ballot's law then head away from the storm in the direction of the least wind.

TA
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Old 14-11-2006, 21:03   #3
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TA,

It's actually a bit more complicated than that. It suggests using a maneuvering board to plot the storms track and distance, and using the ratio of distance/time/speed, figure out the track that gives best. It can be found in the Plotting section of American Practical Navigator.
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Old 15-11-2006, 15:01   #4
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I'm sorry. I was reading posts in a hurry before going to work, and missed the whole question. My thought was that it was a storm avoidance question. But now I see that it is a maneuvering board question. Sadly, I looked on pages 514-516 of the 2002 edition of "Bowditch" where there is a similar problem worked out step by step, but without a proper MB sheet I'm not able to follow along. I'll keep looking at it and get back to you.

TA
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Old 15-11-2006, 15:05   #5
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"Maximum CPA "
YnGlitch translation please?
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Old 15-11-2006, 16:13   #6
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This is a lot easier with a plotting board, but you can do it on a chart or even blank paper if you have a protractor and a compass. Simple rel-vel speed triangle - plot storm's course and speed from the centre; around the centre draw a circle at your max speed (10 kts in this case); from the end of the storm's vector draw two lines which are tangent to your speed circle - this gives you two possible relative path lines, which you translate over to the storm's position (100 at 90nm). Use the line that gives the greater CPA - go back to the speed triangle and your course is from the centre to where that relative is tangential to your speed circle. Hope this helps.

Kevin
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Old 17-11-2006, 20:59   #7
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Isn't it interesting that sneuman was asking about outrunning a storm in another thread at the exact same time?

I remember this question from the Captain's License too and did the calculations. Its been a long long time and can't remember how we did it. I guess that's the problem with learning just enough to pass an exam.

Kind Regards, John L
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Old 18-11-2006, 06:40   #8
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IMO the best avoidance strategy is dependent on where you are - in relation to the storm and geographically. In the Atlantic, we know that hurricanes are 'born' southeast of the Caribbean; they grow up heading WNW through the caribbean; then tend to veer north to northeast around Florida. If you are south of Bermuda for instance, you could potentially find yourself in the dangerous 'right' semi-circle. The standard advice is to not "cross the T" (pass in front of the storm's path), so your avoidance course would be northeast. Then the storm hangs a right at Miami and you're smack dab in front of the storm, sailing into the dangerous semi. Of course you could head WSW across the path of the storm, predicting that it will alter its heading, and it does the unpredictable, and goes straight to shore - then you're in a hurricane, close to shore, getting peppered with hurricane-spawned tornadoes. Best strategy then is to be in a well-found vessel, well-practised in the art of heaving-to, or running with a series drogue or what have you.

Kevin
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Old 20-11-2006, 13:09   #9
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HelloSailor: CPA is "Closest Point of Approach". A maximum CPA is figuring out how to stay as far away from the storm as possible.
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Old 20-11-2006, 18:49   #10
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Thanks, Amgine. I understood the point of the exercise but was drawing a blank on the initials, it has been ages since I needed to calculate a "CPA" and think of it in those terms.<G>

Last time someone asked me "How far can you take us in?" I said "It's your keel, how much water do you want under it?" (Personal rule of thumb is to have masthead depth underneath me, so I can roll a boat without getting the mast stuck in the bottom.<G>) Other than that, I'll rockhop on the owner's orders.
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