Navigation Explained Part 1

In this, the first of a three part post looking at navigation, We will look at what a chart is and what to do with it.

A young man, called Ernest is sat aboard a small sailing yacht with his uncle Ahab. Still secured to the dock but having had dinner they where discussing the plan for the following day.

Uncle Ahab realised that he would have to tell Ernest A lot about navigation. From the questions Ernest was asking, Ahab could see he was keen, just not very knowledgable.

Ahab Decided to talk to Ernest about:

  • What a chart is
  • How to use a chart
  • What a ‘course to steer’ is
  • How to create the ‘course to steer’
  • When to use the course to steer
  • What a ‘three point fix’ is
  • How to correctly make a three point fix

 

 

Introducing a navagational chart

So then young Ernest how do we go about finding our way at sea? Enquired uncle Ahab.

Look at the stars! Replied Ernest enthusiastically.

Well, that would be a more ‘traditional’ way of doing things, and it works. But for ease of use people have made ‘maps’ for sailing. These are known as ‘Charts’.

So why do we not just call them maps then? Asked Ernest.

Because they’re different in lots of ways. Replied Ahab. The information found on a chart is used specifically for navigating at sea.

For example there is very little detail for whats on land, where as the depth of the sea is included.

detail of chart showing spot depths

So that’s what all the numbers are for, exclaimed Ernest.

Exactly, but thats isn’t everything to be found a chart, said Ahab. We have so much more. There’s a compass rose so we know where north is.

The locations of markers or aids to navigation which can help our sailing

a picture depicting what lateral navigation marks look like when printed on admiralty charts
I stole this image of chart symbols from sailtrain.net

And there’s hazards to navigation, a good example of such a thing would be a rock.

 yacht aground on rocks

I see, said ernest, apprehensivly. So how can we use navigation to stay away from rocks?

The best way to do that would be to stay on course, said Ahab. Of course we will need a course for that.

Where might I find one of those then? Asked Ernest.

By taking it from the chart, I will have to explain how you can do that, said Ahab. But before that let me introduce you to the tools you will need to use a chart to it’s full potential, as well as create a ‘course to steer’.

To start with we have a ‘breton plotter’

bretton plotter

And you will also use ‘single handed dividers’ a lot when navigating, Ahab said to Ernest.

single handed dividers

There are other things we may need, like pens pencils and calculators, said Ahab. I think you may have used these before though.

Once or twice! Said Ernest, grinning slightly.

Well lets look at staying away from the rocks, navigating safely and creating a course to steer.

Course to steer

With the chart we have an over view of the area we are going to be sailing in, said Ahab. This means we can make decisions about where to sail, when we are planning our passage.

Thats a good idea, you could say it was the best course of action, said Ernest. Uncle Ahab rolled his eyes.

So if we take the breton plotter we can use its straight edge where we want to have our course, said Ahab.

Ernest did this and then said, Oh good, our course doesn’t seem to have any rocks on it.

Breton plotter on a chart

Quite right, replied Ahab. One thing though, we need to use the plotter the right way round.

How’s that? Asked Ernest.

What we need to make sure of, is that the arrow shape on the top is facing in the direction we want to go, said Ahab. Other wise we’ll end up with a course going in the opposite direction to which we want to go.

Lets not do that then, said Ernest.

No, agreed Ahab. Other wise we’ed never get there! What we need to do next is twist the compass rose on the breton plotter, so that it lines up with the lattitude and longitude lines on the chart.

What are lines of lattitude and longitude? Asked Ernest.

Good question, replied Ahab. For now I think we’ll just say the black grid on the chart. I’ll explain it another day.

Fair enough. Ernest responded. So twist this middle bit so the lines on the grids match up, like this?

detail of compass rose on bretton plotter

Just like that, said Ahab. Now if you look you can see a line down the middle of the plotter. Where this line goes through the compass rose is our course. More or less.

More or less? Asked Ernest.

We havn’t taken into account variation or deviation at this point. But on this boat in this part of the world, we don’t need to, replied Ahab.

So you’re saying that this deviation and variation can be very important but just not in this particular case? Asked Ernest.

Exactly, said Ahab with conviction.

Good, summarised Ernest. So I read the number form the compass rose by the line on the plotter. Then we can transfer that from the chart to sailing the boat when and using the compass to steer the boat.

Your not just a pretty face are you? Asked Ahab, This time Ernest rolled his eyes. Any way, we can look at our chart, find a safe route to our destination and then use the breton plotter to get our course. Are you happy with all this?

I am, said Ernest, but how can we know where we are, or how far we’ve come along our planned passage?

There are a few ways of doing this, replied Ahab. We can use dead reckoning, running fixes or a three point fix.

The three point fix

I think that the three point fix is the best one to start you off with, said Ahab.

So this is a technique we use when we’re sailing? Asked Ernest.

We can, replied Ahab. We can use land marks to take a bearing and then when we have three we can use those to triangulate our position.

So this is the same as techniques I might use orienteering, or say mountain climbing? Asked Ernest.

One and the same, Ahab answered. The execution is a little different but the principles are the same.

What changes? Ernest enquired curiously.

Well the boats moving, said Ahab flatly. But lets just run through the procedure, and then have a think about that. To start with we will need a way of getting our bearings, for this we use a hand bearing compass.

hand bearing compass

We can sight what we are using for our bearing, lets say a church.

church spire on a chart

By looking through the compass and reading the three digit number.

through a handbearing compass

This then gives us a bearing. We can then take that bearing and put it on the chart. The best way to think about this is its the same process as the course to steer, only reversed.

So we start from the bearing, twist the compass rose to the bearing we read from our compass, lined up with the boat on the top. Then place the straight edge of the breton plotter on the land mark, in this case the church spire

Then we draw a line down the side of the plotter for our first bearing line.

Then we just do this three times to get a triangle or ‘cocked hat’.

We are some where in that triangle.

cocked hat fix drawn on a chart

A light of recognition could be seen in Ernests eyes, You mentioned that some things in this process were specific to boats? Ernest asked.

Ah yes, Ahab said. Well its to do with accuracy. So how long do you think it would take you to take your bearings for a three point fix?

Oh, five mabey ten minuets, Ernest speculated.

About that yes, Ahab agreed. The thing to bear in mind is that if the boat is sailing along at six knots (nautical miles per hour) then you will cover a mile in that same ten minuets.

So if a three point fix took ten minuets, then it wouldn’t really be a fix, it would be a sort of idea of how you where travelling? Ernest asked .

Exactly, agreed Ahab. In an ideal world we would want to take our three bearings as quickly as possibly and then once we have three put them on the chart.

<link> slam a link into an outro kinda thing link to part two and robert is your fathers brother, pack up we’re done here</link>

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