Charts

Charts are the marine word for maps. You get them in paper or digital. You can download the digitals from a CD to any laptop computer or digital marine navigation unit. They can be combined with your GPS, radar and autopilot, to simplify your navigation (or require you become a computer hacker).

Because the risk of power shortage or electronic failure is major on an Atlantic trip however, paper charts should still be your major source for navigating. 

A labeled grid-system points out our position on the earth, and this is how it works: Look at a tabletop globe (the kind hiding the whisky will also work fine). Locate the equator encircling the middle. This is zero degrees.

Now move to the North Pole. This is 90 degrees north. Between every degree, there are 60 minutes - also called nautical miles (NM). One nautical mile is 1852 meter.

The straight distance between the Equator and the North Pole is 90 degrees x 60 NM x 1.852 km = 10000.8 km. The distance all the way around the globe will be 360 degrees or 40000 km.

On a chart, these nautical miles will be divided into tenths, each marked by parallel lines . These lines, encircling the globe, are called latitudes. Remember: One degree on the latitudes always marks 60 nautical miles.

Where you are - according to the latitude - will be stated by three sets of figures, and an indication whether you are north or south of the Equator.

Example: "Lat. N. 14 22.5 ", shows that you are 14 degrees and 22.5 NM north of the Equator. 

Well, now that we can find our way north and south (just as the Vikings could) let’s ad another dimension, east and west.

This time we start at London, or more accurate, Greenwich. Once again, look at the globe. Follow the line that goes all the way from the North Pole, through London and down to the South Pole. This is called a Longitude. The circle around the globe is 360° . Every degree is divided into 60 minutes. Every minute is then divided into tenths.

However, a minute when looking at longitudes is not related to distance. If you look at the globe and follow the longitudes north, you will see that the distance between them will narrow.

Remember: The purpose of longitude is to give position, not to give distance.

Just as the latitude will be connected to a north or south of the Equator, the longitude will be connected to an east or west of Greenwich, London.
 

 

Take the course from a chart

To make the globe readable on a chart, there is a standard procedure called Mercators projection. Lay down the chart on a flat surface. To the right you´ll find the latitude scale, at the bottom the longitude scale.

To check a distance or plot a course you need three things: a pair of dividers, a conveyor (triangle with degrees on) and a pencil.

Now, let’s make this very practical. Take a chart or a normal map where you can see Tenerife in the Canary Islands as well as St. Lucia in the Carribean.

Do a straight line between Tenerife and St. Lucia with the help of a ruler (use a pencil so you can erase the line later).

Before we check the exact course it’s good to make out in your head what it should be. If we go towards the Caribbean it should be pretty much going west and some south. So we could expect a course of around 230 degrees (see the chapter about the compass below).


At the base (the long side) of the conveyor you will see a straight line. Move the conveyor so that the line printed on it touches exactly the line you just draw.

At the middle of the conveyor base line there is a crossing line. Move the conveyor along the pencil line so that the cross is at a line on the chart that is going from down to up , a longitude.

Keep the conveyor at that position. Close to the top of the conveyor there is a scale that crosses the same longitude as the cross at the base line. You will now have two figures , one will be close to 230 degrees, which will be your course to St. Lucia, the other will be closer to 50 degrees, which will take you the opposite direction.

Make a note of the course on the map and a small arrow indicating which way you are going. "How dumb do they think I am?" you might think. Well, going without sleep and hot food for a couple of days, in the middle of the Ocean, a pitch-black night , that arrow might come very handy.

The best way from Tenerife to St. Lucia is however not the straight one. You will be best off by first going down to the Cape Verde Islands, and then do the crossing. Doing the course is simple though , just do two separate lines with your pencil instead of one.

 

Take the divider. Adjust them at the right side of the chart so that the distance between the needles is a suitable distance (1NM, 10 NM, 1 degree, depending on the scale of the chart you are using). Put one needle at Tenerife and "walk" along the course you marked until you arrive at St. Lucia. Count the "steps" you are taking and multiple with the chosen distance between the needles. This is easier done than described , have someone show you.

 

The wind might be more favorable at different places of the ocean. Orthe current might push your boat side ways , so you have to compensate for that.

By using a wind and current chart like the "Atlantic Pilot Atlas" you will not only get the current and wind information, but also how it changes for each month of the year!

A more complicated matter is the influence of the Magnetic Pole. All your charts will show the North Pole straight up and the South Pole straight down. However, the north on your compass will point towards the magnetic North Pole, which is way off the real Pole (120 NM down in Canada).

This is called the magnetic variation and it changes a little from year to year, and a lot depending on where you are. If you look at the chart, there will be a compass rose. That one will show you the way your magnetic compass will go. There will also be information about how it changes from year to year (don’t forget to check which year the chart was printed).

At the Canary Islands your compass will point you more to the left than the course you just plotted, which will end you down in South America , nice, but maybe not what you intended. Adjust the course you are going for by adding the variation to the course you got from the charts.

 

When under sail you have to constantly check your course and adjust it. This is how we do it.

Every 4-hour bring up the chart, the pencil, the conveyor and the dividers. Put on the GPS ,write down your position and time doing a small cross on the chart. Check the distance , write it down, and adjust the course if necessary.

Remember: Always write down your position , if your GPS fails you, this information is extremely important.

 

The principle is the same as out on open sea. However you have more to think about. There are islands, depth readings, lighthouses and other markings to regard.

Always use an up to date chart. Radar is great for identifying islands. A sonar will guide you into harbors and anchorages.

When getting close to islands and reefs , never trust the charts. There have been plenty of times when lighthouses are missing and the charts are smile wrong. An example is Isla la Tortugas outside Venezuela. Comparing a chart with a detailed guidebook , not even the shape of the island corresponded, even less the reefs.

The rules applied to buoys and markes will also differ widely from what you learned at your navigation course back home. Always read up on the local rules.

 

 You don’t need to learn how to cross the Atlantic. But if you got the time, learn how and you have a great back-up as well as something to look at during those lonely passage nights.

 

 There are two books you need. One is the "Atlantic Pilot Atlas" by James Clark (Adlard Coles Nautical), $ 70. The other is "The Atlantic Crossing Guide" by Anne Hammick and Gavin McLaren (Adlard Coles Nautical), $ 45. In the last one you will get information of all the charts you need for the passage.