In today’s world of GPS enabled devices some will argue that the art and science of real navigation has been lost and in many cases, they’d have a point. With many electronic devices now GPS-enabled, including one’s phone, watch and, believe it or not, even the odd automated lawnmower, the need to actually ‘navigate’ is much diminished.
How then, to navigate without ready access to a GPS? Well, there are purists out there that will say there’s no need for a GPS when you have a sextant, a reliable time piece and some sight reduction tables. Others will say that, inshore at least, a well kept chart, a compass, depth sounder and skilled eye is all that is required. And both would be right.
But what if, halfway across the North Atlantic, you lost your compass, GPS and sextant? It might seem a worrying development. But whilst it might focus the mind, there is no need to panic. There is still a straightforward way to get yourself safely to landfall and impress your crew to such an extent that you’ll never need to buy a dark ‘n stormy ever again!
How then, do we proceed?
First of all, we must assume that we have religiously kept a detailed log and have been plotting our progress through accurate GPS or sextant-derived fixes. Now we’ve lost our navigation equipment, dead reckoning and calculating our EP (estimated position) becomes even more important. So far so good. After all,as an RYA Day Skipper or above, you can plot an EP in their sleep.
Second, we must check our estimated daily progress against approximate position fixes. After all, we don’t want to miss landfall. How do we do that without GPS or sextant? Well, we navigate.
We know that when the sun is at its highest it is noon local time. At that time, if we’re in the Northern hemisphere, the Sun is probably due South of us (unless in extremely low latitudes) and, if we know the exact time when the sun is at its highest point (local noon) and we take an accurate time of that event, we can establish our approximate longitude. We do this by comparing the time of our local noon with noon at Greenwich (0 degrees Longitude). Of course, when checking the sun’s height against the horizon, don’t look directly at the Sun! Instead, use a stick and a bit of paper and mark the end of the shadow every 20 seconds as local noon approaches. Alternatively, make a sun shadow board.
Once we have a time of local noon, we compare it with noon at Greenwich. We know that for every hour difference in noon there is a distance of 15 degrees of longitude from Greenwich. So if our local noon is 1500hrs Greenwich Mean Time, we must be 3 hours West of Greenwich. In other words 45 degrees West of Greenwich. If the local time of noon was 0900 UT then we’d be 45 degrees East of Greenwich, in other words, the sun has reached our noon 3 hours before Greenwich. The sun rises in the East and so we must be East of Greenwich by 3 hours or 45 degrees.
Of course at noon you can also calculate an approximate latitude by measuring the Sun’s zenith distance and allowing for declination, but that’s for another time. We have an approximate longitude then, but what about latitude? Well, we know we can calculate latitude by reference to its altitude, but how else?
Every evening, there is one star in the Northern hemisphere that sits pretty much directly above the North Pole. If we are in the northern hemisphere, we can see it, as long as the sky is clear. For mathematical reasons I shall not go into now, the angle of inclination from the polar star to our horizon is equal to our latitude. But how do we measure that angle?
There are ways and a sextant is our best option, but you will recall that ‘nipper’ dropped that over the side together with the GPS earlier today, so what to do?
The simplest way is to find the right latitude to find us land, then sail across that line of latitude. In other words keep our latitude constant. To do that, all we need to do is grab a stick. hold it vertically in front of your eye with a straight arm and line the top of the stick up with Polaris. Then carefully ‘nick’ the stick at the point where the horizon intersects it. Every evening, check the stick against Polaris. If the ‘nick’ is still on the horizon, you are at the same latitude. Simple. Of course, for this to be effective, you must mark your stick as soon as possible after your last reliable fix, use the same stick every time and the same person should do it. When you don’t know what latitude you’re starting from, you might need to estimate this. This is where the help of Kaho’s apprentice comes in (we’ll let you google what that is). Don’t worry, there’s always one to hand..
Of course, these approximate fixes are not accurate, but coupled with an experienced helm steering a constant course and accurate chart keeping, that first rum punch can’t be far away. With a magnetic compass a constant course to steer should be achievable, but if you are having a really bad day and have also lost your compass, consider these facts which will help you steer a constant heading.
First of all, in the trade winds wind direction is likely to be pretty constant. Secondly, for the same reason, wave direction should also be pretty constant. Therefore, maintaining your heading as a constant angle to these forces should help you maintain a course. After that, we know that the sun rises from the East and sets in the West, although this exact bearing will vary dependent on the time of year and the latitude you are at. We also know Polaris is due North of us when we can see it and at local noon the Sun is due South. On top of this, if we have an accurate watch with hands, we can use this to establish the points of the compass in the northern or southern hemisphere. However, the watch must be set to local time and the lower the latitude, the less accurate it becomes. In the Northern hemisphere, simply lay the watch face flat and line up the hour hand with the sun. A line bisecting the hour hand and 12 o’clock on the watch is North. If you know North, you can establish East, West and South.
Finally, as you estimate landfall should be near, from your dead reckoning, EPs and position fixes, start looking for birds and cumulus cloud formations on the horizon. The smaller the bird, the closer land is likely to be, perhaps 30 - 50 miles distant. Larger birds might venture further afield and, of course, a single bird could always be lost! Clouds gather over land as the land heats up and convection occurs, causing sea breezes. Always a good sign. Finally, listen for VHF radio traffic and watch for other vessels. You can always ask directions for the last bit!
This is a very basic summary of some of the ‘approximate’ techniques of navigation more fully explained in the wonderful book, The Barefoot Navigator by Jack Lagan. In it you’ll learn how to rig a dutchman’s log, set your heading using the star, kochab, and how to make a ‘quadrant’ and ‘sun shadow board’. It’s well worth a read for those of you interested in pure navigation techniques without the aid of GPS. Of course, all these forms of navigation are approximate and should form part of your survival strategy in extreme circumstances!