NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: DR plotting techniques
From: George Huxtable
Date: 2003 Oct 17, 17:42 +0100
From: George Huxtable
Date: 2003 Oct 17, 17:42 +0100
from George Huxtable. This is a note about my own navigation, covering rather more than DR plotting techniques. I'm a "coastal fringe" sailor who doesn't expect to be out of sight of land for more than 24 hours or so, though my passages may be somewhat longer than that. Mine is a small craft (8 metres) without gadgets: no plotter, no radar, but in recent years a handheld GPS, which uses external power and an external antenna. Just recently my cruising radius has become somewhat curtailed, but I will describe my navigation practices over the past several years. I've had the same boat for 30 years now, and sail with my wife Joan as crew. We've become rather familiar, over the years, with both sides of the English channel, and the French coast as far as South Brittany. My navigation is always on paper Admiralty charts, and I have about 80 of these on board. Very few of these are the current edition (or even the one before that!). Joan keeps them corrected from weekly Notices to Mariners, though whenever a new edition appears some changes occur that are irretrievably lost to us. You may find my attitude somewhat casual. As far as possible, we sail rather than use the engine, which is mainly for getting to and from the berth. Where we can, we find anchorage rather than marinas. Because the weather is so unpredictable and dominates our sailing, conventional passage-planning is rather unrewarding. We don't bother much about that. Instead, we just accept where we have got to, take another day perhaps, and if necessary alter our plans, for example by changing our destination. However, in our waters the tide dominates, and I will always note beforehand the times of its reversals, and the total distance it will sweep in the 6 hours or so between those reversals. That is about the extent of my pre-planning. Until GPS appeared, we would always stream a Walker log, except when trailing a mackerel line (because the two would always tangle). This is how GPS is used on my boat. I don't bother with waypoints, or with tracks between them, because in a cross tideway such straight ground-tracks are often highly inefficient. Steering is always done from the ship's compass, not heeding any off-track error indication. At irregular intervals I will read a GPS lat and long and plot it on the chart, writing down the coordinates alongside the plotting point. That's all. I don't keep up any other log, just those points on the chart. I will also have plotted a line showing my intended track with some tide vectors added, to give some sort of DR position. When sailing over the edge of a chart, and on to the next, I transfer what I can that's relevant to the new chart. It gets a bit less casual as we approach a destination, or pass close to hazards, of course. In pre-GPS days, radiobeacons would be used as a final check, but now these are mostly silent. I must comment that knowing exactly where you are from GPS has taken a lot of the fun out of navigation. It's no longer a challenge. My most satisfying navigational experiences have been to enter an unfamiliar harbour at night, then look around after dawn to see what I had got into. Now that's easy. Whenever the chance occurs I take a round of visual magnetic bearings, using a Morin "Minicompas", which I know and trust; an "Opticompas" is effectively the same thing. I've got rather skilled at this over the years and can take a round of bearings in well under a minute, shouting out the first two to Joan, to remember, keeping the final one or two in my own head. Then below to plot, using a Breton plotter, set to allow for variation. The plotting to get a cocked hat takes only another minute or so. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================