NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Avoiding Wx Problem
From: Aubrey O´Callaghan
Date: 2000 Dec 16, 11:32 AM
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@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
