Skipper’s Tips #17 – Magnetic or True

plotter-picture

I’m sure most boaters or walkers or anyone who navigates their way around using charts or maps will have come across the situation where their compass is pointing one way and their maps or charts are pointing in a slightly different direction.

The reason for this, in simple terms, is that the molten core of the earth, which is constantly moving, is what compasses point at.  However, when you look at the globe from a human perspective, it has been drawn with the North Pole at the top and the South Pole at the bottom, a slightly skewed axis and to add insult to injury, the beautifully drawn land masses are not quite in the position that our trusty new GPS gadgetry now correctly indicates.   Here’s an interesting observation to chew on; if you went out into space and looked at the world, how would you know which way is up? Is the North Pole actually at the top?

I digress…

Obviously, if we are to sail around without bumping into things we need to work out how to work on the same page as the compasses we are utilising.  What we do, is allow for the difference in opinion (Variation) and once applied to the (True) or unadjusted heading, we end up with a (Magnetic) heading figure.  The story doesn’t end there though.  Stick a compass on a boat and all the electronic and magnetic forces on the boat will have a further effect on the compass, confusing the ‘poor dear’ even more, this we call (Deviation).  Making an allowance for the Deviation inaccuracy of the compass gives us the most accurate heading and is called a (Compass) heading.

Do not despair. When you buy a new boat, a compass adjuster will get on board and eradicate as much of the Deviation as possible.  The bit that’s left will be listed on a compass Deviation card so you can allow for it when navigating.  Beware though.  If you change any electronic equipment, or bring items onto your boat that have any electronic or magnetic properties, they will have an effect on the compass and it will have to be ‘swung’ again!

By now you must be starting to wonder how you’ve managed to navigate safely between ports without bumping into rocks and chunks of unexpected land. The answer, of course, is that we all readily use the chart-plotter/GPS equipment that’s become almost as intrinsic as the hull!  The chance of this equipment failing is remote and I have touched on this in another tip but if it were to fail or cease to operate correctly we will have to deal with Variation and Deviation before we can start to steer a course by the boat’s compass.

So, how do we work with Variation and Deviation?  Importantly, you must remember to make these ‘allowances’ in a specific order.  Allow for Variation first and then Deviation. You will also need to decide how you apply the correction.  When dealing with Variation this correction is essentially adding or subtracting the appropriate amount of degrees difference between what the compass is indicating and what the local Variation is shown to be on your chart as either degrees (West) or degrees (East)

Doc 3 Sep 2017, 12-37This is an example of a compass rose on a chart.  The Magnetic Variation shown is 2 degrees, 45 minutes West, correct in 2006.  Each year, this ‘error’ is decreasing by 8 minutes.  There are 60 minutes in a degree, so for 2017 – 11 years on – that’s a reduction of 1 degree and 28 minutes.  Therefore the Magnetic Variation in 2017 is 1 degree and 17 minutes West. Yes, there will come a point when the error disappears completely and then starts to become an East Magnetic Variation.  When I first started teaching, the Variation in parts of the Med was just under 2 degrees West and the last I saw it was almost zero – Point & Go!

N.B. Please remember, Magnetic Variation is nothing to do with whether you are going in a Westerly or Easterly direction, it is actually about your position on the globe relative to that molten core of the Earth which we spoke about earlier.

The question now is, how do we know whether to add or subtract these degrees of Variation or Deviation?  I don’t doubt that there are many different ‘aide memoirs’ but the one I favour, naturally, is the one I made up myself.  If you are calculating from True (shown on the chart) to Compass (the heading you are going to steer to by the ship’s compass) then any Variation or Deviation you allow for that is West must be added.  In other words True To Compass Add West – TTCAW.  It therefore follows, that any Variation or Deviation that you encounter when going from True to Compass that is East must be subtracted; the opposite, see?

Furthermore, when you are calculating from Compass to True, as you might when taking a Magnetic bearing with a hand-bearing compass, you would add East and subtract West. Clear as mud isn’t it?  Have a look at the table below, which has some examples.

Just to reiterate, don’t fall into the trap of thinking the direction you are pointing your boat in determines whether the Variation or Deviation is West or East.  The Variation figure comes from the compass rose on your chart and the Deviation figure from your boat’s deviation card.

True    Variation        Magnetic       Deviation      Compass

231        6 W                  237                  2 W                  239                             add West

079        3 E                   076                    0                     076                             subtract East

147       4 W                  151                   1 E                   150

013       2 E                    011                   2 W                 013

348       2 W                   350                  1 E                   349

To remember the order try,  ‘True Virgins Make Dull Companions’.  Alternatively, if that’s too racy for you, remember the reverse order with,  ‘Cadbury’s Dairy Milk Very Tasty’.

This has been another Captain Corbett’s Adventure.  If I’m not on Jersey teaching a private tuition Day Skipper theory or Yachtmaster theory course, then I’m either spending time with someone on their boat, giving them the confidence to take their boat out with their family and friends on board or I’m off somewhere exotic delivering a boat. Either way, I’ll write it up and put it on the Blog for you all to see, so keep popping back to see my most recent adventures.

 

 

Skipper’s Tips #7 – Chart Plotter Rules, OK?

Hello all!

Please excuse my prolonged absence, I’ve been rather preoccupied with work lately and everything else has had to take a back seat.  However, I have finally managed to find the time to sit down and post a new Skipper’s Tip.

I do hope you enjoy this tip and also that you find it informative.  If you have any questions or are at all confused by what I write, then please contact me and I will try to explain.  

This tip is all about paper charts and the tool we use to extract navigational heading information from them. Electronic chart plotters are amazing pieces of technology these days and they hardly ever go wrong.  Nevertheless, we must not forget that as good as these pieces of equipment have become, they are still no substitute for knowing how to navigate ‘manually’.  You should always plan on a paper chart and always keep a paper chart with you when you are on your passage and you should make a regular note on the paper chart of your position.

The single best tip I can give you regarding navigation, is to know exactly where you are, at all times!

The Chart Plotter

When you first see a paper chart and start looking carefully at the detail, it can be a little awe-inspiring. Imagine all the work that went into collating that information.  Especially impressive, is the fact that charts have changed very little since the world was first mapped.  I have a mental image of a couple of men in naval uniform in a small boat with a long piece of string and a heavy weight, a big pad of paper and infinite patience.  Today, we take for granted the effort that must have gone into making the world’s seas and coastlines appear in 2D, so that we can safely navigate in our boats. However, if it weren’t for the efforts of these determined explorers we wouldn’t stray very far from home would we.

To use a chart effectively you need to understand a few basic rules.

  • Always measure distances from the side of the chart nearest to where you are planning to sail
  • Look at the compass rose shown on your chart to determine what the allowance for variation must be
  • All the numbers peppered across the chart are ‘charted depths’ or ‘drying heights’ and as such make no allowance for tidal height
  • These depths or drying heights could be in feet, fathoms or metres and you need to look at the key for the chart to know what you are dealing with
  • Make sure the chart is designed to be used with GPS, if that is how you intend to navigate
  • The squiggles, lights, lines, abbreviations, etc. are all explained in the Admiralty publication ’Chart 5011’
  • The tide stream data will be referred to one specific port and will be listed as ‘true’
  • In fact, everything shown on the chart is ‘true’ not ‘magnetic’
  • Pay particular attention to the scale of the chart

In order to work out which way to point the boat or indeed, to describe a line on the chart to show where we have been, we will need to employ an accurate process.

Enter the ‘chart plotter’.

There are many incarnations of the modern chart plotter, a great improvement I have to say on the ‘parallel rules’ of the past. Essentially, the one I am going to describe using in this example looks like a very large and wide rule with, in the centre, a dial that you can swivel.

There are two key things to remember with ’chart plotters’. Firstly that whenever you place the plotter on your chart, the centre dial, which spins, must always have ’N’ (North) pointing to and aligned accurately with, true North on your chart.  The second rule is that the big arrow at one end of the plotter will always indicate the direction or heading of the bearing.

If you are trying to draw a line in an easterly direction, i.e. 090 degrees and yet the arrow on the plotter is pointing west (270 degrees), then you have it upside down…  It will be really helpful if you try to think of these numbers as directions, rather than just random numbers.  000/360 is North, 090 is East, 180 is South and 270 is West.

So, the way it works, is that you lie the plotter on the chart, parallel to a line which you want to know the bearing of, spin the dial until the ‘N’ at the top aligns with North on your chart and then read the bearing figure from the dial, which lines up with the large arrow at the end of the plotter.  This will tell you the ‘true’ bearing of that line or indeed, the bearing between two points on your chart.  The next thing you will need to do, of course, is to convert this ‘true’ bearing to a ‘magnetic’ or even ‘compass’ bearing, so that you can use it with the ship’s fixed compass to navigate your course.

If you are using the chart plotter to show your course travelled on the chart, in the way you might if you were showing your passage as part of your log, then you will need to do all this is reverse. Firstly, assuming you know the direction you have been travelling in, convert that direction from compass or magnetic back to true. Then align this number with the chart plotter dial and the large direction arrow.  Lastly, place the chart plotter on the chart, being careful not to turn the dial and align the whole plotter with true North (use the N on the dial for alignment; DON’T TURN THE DIAL, TURN THE WHOLE PLOTTER) and then draw a line down the length of the plotter to show your track.

I have to say, trying to describe this process in words is not easy.  It is much easier to actual show someone this but if you sit down with my explanation, your chart plotter and a chart you will soon get the idea I’m sure.

I think my next Skipper’s Tip had better be about True, Magnetic and Compass bearings!

This has been another Captain Corbett’s Adventure.  If I’m not on Jersey teaching a private tuition Day Skipper theory or Yachtmaster theory course, then I’m either spending time with someone on their boat, giving them the confidence to take their boat out with their family and friends on board or I’m off somewhere exotic delivering a boat. Either way, I’ll write it up and put it on the Blog for you all to see, so keep popping back to see my most recent adventures.