# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Avoiding Wx Problem
From: Aubrey O´Callaghan
Date: 2000 Dec 16, 11:32 AM

```Carls answer is absolutely correct !
A very good source of relative motion problems is the Admiralty Navigation
Manual Vol. III. mine is a 1938 version.
The equivalent to this problem is stated somewhat differently in the manual
" Avoiding Action. A Ship of lesser speed escaping from a ship of greater
speed (In this case it was a merchant ship escaping a sloop at dusk ).
There are even torpedo firing problems... most useful for the recreational
sailor !!!!

Aubrey.

At 01:40 PM 15-12-00 -0500, you wrote:
>Sailing a a course perpendicular to the course of the storm will NOT
>achieve the
>maximum possible CPA unless the storm is traveling at the same speed as
>you.
>This type of storm avoidance question is a relative motion plot, easily
>accomplished on a maneuvering board or a plotting sheet. However, it can
>also be
>done on a chart.
>
>-- Plot your vessel (at the center if using a maneuvering board).
>-- Plot the position of the storm center.
>-- Plot the storm's speed vector (a line representing its direction and
>speed. In
>your second example, 265T for 22 miles). This line is drawn beginning
>from your
>current location.
>
>-- From the outer end of that line, draw a line tangent to the 12 knot
>speed circle
>(a circle with a radius of 12 nm with its center at your location).
>
>-- From your position, draw draw a perpendicular to this tangent line.
>This is the
>direction you need to travel to achieve maximum CPA.
>
>-- Extend that line in the opposite direction, until it crosses the
>course of the
>hurricane. That point represents the location of the hurricane (assuming
>course
>and speed remain the same) at the time of CPA. From there, calculate how
>long it
>will take the storm to reach that point, and you can establish the time
>'til
>CPA.
>
>--
>Carl Herzog
>Editor, Reed's Nautical Almanacs
>
>www.reedsalmanac.com
>carlzog{at}reedsalmanac.com
>-------------------------------------------
>120 Fulton Street
>Boston, MA 02109
>Ph# (617) 227-1300
>Fax# (617) 268-5905

=================================================================
Aubrey O'Callaghan
Wandrin' Star of Dart
Rival 38

Contact: Venezuela,
Puerto La Cruz
00-58-14 210 4625 (Cellular)
00-58-81 818 667
```
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