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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Allowing for current. was: Re: Noon sun fix example
From: George Huxtable
Date: 2009 Sep 30, 22:39 +0100
From: George Huxtable
Date: 2009 Sep 30, 22:39 +0100
This was triggered by Jeremy's reference to the effect of current, but it's about a rather different situation, of tidal currents. It's arisen here before, a long time ago. I can illustrate it best by a little problem that faces many small-boat navigators crossing the English Channel, between Anvil Point, a headland South of Poole, and Cherbourg, to its South by about 50 miles. I will simplify the details, but for many small craft that passage will take about 12 hours, or just two tides' worth. Say the tide flows at about 2.5 knots, a total displacement of about 11 miles East, over the first 6-hour period; then 11 miles West, over the next 6 hours.. Many navigators will set the destination of Cherbourg as an intended waypoint, then religiously adjust their heading to keep their ground-track along that intended line, angling against the current to keep it so. They are, of course, wasting time and energy. Because, over 12 hours, the net tidal displacement, East then West, will sum up to be close to zero, there will be no overall effect of tidal current. In which case, the correct and easy procedure is to head due South, allowing your craft to be swept up-channel first, then back again later. By ignoring the instantaneous tide, and the resulting cross-track error, the vessel is heading exactly Southwards all the time; the best that can be done. Indeed, set out like that, it seems pretty obvious, but it's hard to convince many navigators that they can, and should, ignore those warnings of cross-track error. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---