PART 3. DO-IT-YOURSELF-LO-TECH NAVIGATION

THE HISTORY OF NAVIGATION is the history of the pursuit of accuracy. At the risk of being accused of attempting to turn the clock back, I have just taken you through a range of technology-free techniques for determining your whereabouts at sea. Now I am going to introduce some technology to make things a little more accurate.

‘Why?’ you might ask. If I am going to introduce technology at all, why can’t I hand out GPS sets and be done with it? There are two reasons. Being able to
improvise navigation aids and knowing how to use them will serve to reinforce your understanding of the fundamental rules of navigation. It also makes the knowledge imparted in this book more potentially useful in survival situations.

DEAD RECKONING

DEAD RECKONING is a simple means of estimating a position by applying course steered (specifically assumed course made good) and distance sailed (speed x time) from a last-known position. The expression is derived from ‘deduced reckoning’, which is worth remembering because it does not sound nearly as accurate as ‘dead reckoning’! This caution is especially apt when barefoot navigating because the chances are that the ‘last-known position’ was derived from a ‘previously known position’ using dead reckoning. In other words, any new errors could be compounding old errors.

In Part 2 we looked at different means of determining and holding a heading. So you know the approximate course you have been making good, but how far are you along that course? To determine that, you need to know the speed your
boat has been making through the water, as well as the time travelled on that leg. In Part 3 we will look at methods for calculating your speed. Unless you have a watch, your time estimates are going to be in days or, at least, big fractions of
a day. Thus all we need to do this calculation is:

distance travelled = speed x time travelled

In most barefoot navigation – especially the survival kind – you will be working to fulfil a long-term strategy: ‘I’m going to head due south to 13º and turn west, keeping slightly north of the latitude for ten days until the jets taking off and landing at Grantley Adams International Airport tell me I’ve reached Barbados.’ Or more starkly, ‘I want to get to the nearest coastline and get off this leaky tub!’ Subtle course changes will be rare, and probably irrelevant, because they presume
a confidence in your position that is not justified by the navigation techniques you are employing.

This straightforward approach might be imposed on you by the fact that you are having to hold the course in your head; you have no chart! The ‘global position emergency locator’ in Appendix 10 is designed to help you get a feeling for where you might be and what your options are for reaching a safe haven. This is best used in conjunction with the charts showing world ocean currents (Appendix 12) and prevailing wind systems (Part 2). Hopefully you copied these, or printed
them from the website, and laminated them before tucking them away in your survival ‘grab-bag’. The other possibility is to have the Admiralty pilot chart for the regions in which you are sailing; these show the prevailing wind directions and
speeds and are the right kind of scale for survival situations.

There is another practical problem. A major cause of error in dead reckoning is the navigator’s failure to make correct allowance for the extent to which surface currents, tide streams and the wind are conspiring to push the boat off the desired
course. If you have been formally trained you will work out a course to steer by drawing the relevant vectors49 on the chart. If you don’t have a chart it is possible to make these calculations on a plain piece of paper as long as you have a straightedge (preferably a ruler), a pencil, a protractor and, of course, a sheet of paper. If you lost all those, or you are trying to work without them as the ancient navigators did, then you are going to have to make the adjustments in your head.

What follows are rudimentary methods of calculating a position by dead reckoning; later we will consider how you might be able to make slightly more accurate estimates of latitude and longitude.

IMPROVISING A COMPASS

No compass? This is unlikely to happen on board the boat itself, but it could occur in a survival predicament on a liferaft, deserted island or isolated coastline. These are the steps to follow if you need to improvise your own compass:

• Locate a short length of wire, a sewing needle or, at a pinch, a pin. A needle is best, but whatever you choose needs to have high ferrous (iron) content.

• Now magnetise the needle. There are a number of ways of doing this. If you have a magnet, use it to stroke the needle repeatedly, always in the same direction, until it is able to attract/repel other metal objects. (If you are still
  on a boat, you might be able to find a magnet inside unusable electrical gear. Start with one of the speakers from the hi-fi system. You will also find magnets in the engine starter motor and alternators.) The second technique involves the use of a battery or batteries with a potential in excess of 3 volts. Using insulated wire, fashion a coil around the needle (the more coils the better) and connect the ends to the positive and negative terminals. Leave for 5 to 10 minutes. Finally, although I’ve never tried this, some magnetism can be induced into the needle by stroking it with a piece of silk cloth. (You may need a volunteer with silk underwear for this.)

• Now you have to suspend the needle in some way. A piece of untwisted thread tied around the centre will serve. If you have a well-magnetised needle, one that snaps reasonably sharply to north, it is better to use a glass jar or plastic beaker partially filled with water (see Fig 3.1). The needle can be suspended
  on a sliver of balsa wood, a slip of paper or, if you are ashore, a piece of leaf or seaweed.

• Calibration comes next. Make sure you know which end of the needle is pointing north. The sun or stars will help you do this. If you have the needle floating in a container, you might be able to mark it up (using a Chinagraph pencil or pieces of adhesive tape) with a crude compass card. If you can do
  better than eight divisions (north, north-east, east, etc), then you are doing very well. A light line or wire stretched tightly between north and south on your improvised card might well prove helpful to the helmsman should you be able to get under way.

I understand that it is possible to magnetise a razor blade by stropping it against the palm of the hand then suspending it vertically. (Don’t try this in rough seas and without the supervision of a trauma surgeon!) Whatever device you use, remember to top up the magnetism every few days, at least. Also remember that the needle will be pointing towards magnetic north. The variation from true north will depend on where you are in the world and may well be the least of your worries. Some metal objects seem to hold a little magnetism, maybe enough to provide a rough indication of north. Take the
pull-ring or lid from a soft-drink can or the lid from a can of baked beans, clean it up, flatten it, and float it in a tumbler
of water. Now watch it for a while. Does it settle in a particular direction? Move it and see if it happens again and it rotates to the same place? Mark the lid in some way. You shouldn’t have too much trouble in determining whether it is pointing north or south and it should be pointing to both.

• See The Barefoot Navigator in our catalog.
  
  
• Back to main Excerpts page.