December 7th, 2016

Tea Break Talk – Navigation in a Nutshell

Posted in Article
theory on table 1400x875


Tea Break Talk – Navigation in a Nutshell

There are many things to think about before you head out on the water in command of your own boat for the very first time. We have previously discussed theory and practical training courses, but one course you will need to do before you take to the high seas is your VHF Radio Licence.

In the same way as when you are finding your way on land, using a map and compass, at sea you can use your compass direction and the things you can see around you to work out where you are on the chart (a map for the sea). And in the same way that you can use the contours on an OS map to help guide you the contours of the sea bottom on the chart can also be of use. So really, navigation at sea is much the same as navigation on land, it just moves about a bit more!

With the advent of GPS and other modern technology (I just recently have been checking the motorcycle GPS reviews and ratings, for my personal use this time), which allows us to pin point our position using satellites, to within a few metres, it is now much easier to work out where you are on the sea or land. However, whilst this is a massive help, it is never good to rely on technology, as occasionally it does fail and usually, when you really need it the most… So, in the same way that you would have a back up plan whilst walking on Dartmoor, it is essential to know how to navigate the old fashioned way so that you always know where you are.

What you will need…

Chart – A map of the sea.

Plotter – This has various names from Portland to Breton, but all are essentially the same. A piece of plastic with a wheel at one end, it does everything from measuring distance to plotting your course on the chart.

Dividers – Like a compass (for drawing circles), but with points on both ends, this tool helps to measure distance.

Hand Bearing Compass – Usually a round compass which can be held up to your eye to take a compass bearing from a landmark or buoy.

Almanac – An annual book publishing the tide times and information for your part of the world. It also has useful information on local weather forecasts, Coast Guard stations and harbours.

The main difference between navigating on land and navigating at sea is that you have the motion of the tidal flow to contend with. So, whereas on land you can stop entirely, take stock and get your bearings, at sea you are continually being pulled in one direction or another, so it is important to take these flows into account.

If you are coastal sailing or in an area like The Solent, the easiest way to navigate is by line of sight. This means that you will take bearings from at least 3 landmarks or buoys that you can see, in a triangle, and plot them on the chart to give your position. You will end up with a small triangle and you are somewhere in that triangle. This is called a Fix.

If you are sailing further out to sea, or the visibility is poor and you can’t see any landmarks, you will need to do a few more calculations to work out where you are. In this case, you will need to know what the tide is doing, your boat speed and the direction you are travelling in. By drawing a line on the chart with the course of your boat and working out how far down that line you are using your boat speed, you can work out a basic position. This is called Dead Reckoning and doesn’t tell the full story, although gives a good idea of the vicinity that you are in.

An Estimated Position takes a bit more information to give a clearer idea of where you are. By adding the direction and speed of the tide to your course and boat speed, for each hour that you are sailing and, if the information is known, how much you are being pushed sideways (if at all) by the wind (this is called Leeway), you will be able to plot a position on the chart which should be very close to where you actually are. When verified against a GPS position, you will find that, if your information has been plotted correctly (and is correct!) then you will be only a few metres out!

Basic navigation is not tricky at all and is something that everyone can learn, even if it looks like a black art. Why not sign up to an online sailing theory course. You will soon see how easy it is!