Why Nautical Charts are Essential.
by: Clive Peterson

Navigation & Chart Plotting.

Many new boating people who are out in the sea do not feel important to carry nautical charts onboard. I must say that it is a risky habit. A nautical chart gives you the relevant information required to make a safe passage plan using the charted fixed marks such as buoys and landmarks, so that you can take bearings and maintain a correct course.

Photo: Sea Lover

Pilotage Information: With the help of the nautical chart, you can gain pilotage information regarding the position and nature that is favourable to the navigator. The nautical chart holds crucial information such as seabed information, landmarks, and seamarks. There are symbols that describe the seabed, such as the depth of the ocean, possible hazards, and tidal information. The charts also have symbols for lights, lighthouses, buoys and land structures and features, which is very essential for position fixing.

Many charts also use colours to distinguish between manmade features. One has to be clear with the charts to understand the indication in the nautical charts. The chart may indicate a seabed that lies permanently below the water level or the seabed that becomes dry at low tide. There are various symbols to indicate water levels, but contour lines are shown to identify changing depths.

To understand the depth of the water: Numbers marked in the chart indicates mostly depths that have been measured. Most charts use meters to indicate the depths. However, the charts that are published by the US Government use feet. Depth curve lines indicate about the relief underwater. The differing colour portions in the sea mean that there is obstruction underwater or indicates shallow water. Such points are only understood when one is aware about importance of nautical charts and are familiar with the symbols and signs.

Photo: Suomis

To understand about tidal information: Nautical charts provide information on tidal movement. The strong water currents and tidal races have special chart symbols. The nautical chart will carry tidal diamonds, which indicate the bearing and the speed of the tidal flow, every hour of the tidal cycle.

Important points to keep in mind when you are reading a nautical chart:

• Make a thorough reading of the chart.
• Carefully monitor your position, which you will follow and pursue the course you will take.
• The notes and other indication such as obstructions, water depths indicate something so you need to be clear with them.
• Make a note of whatever you observe on the nautical chart.
• A careful monitoring of the chart will show you visual objects and you must be able to identify with your position.

The chart has a superimposed compass rose on it that indicates true North and the magnetic correction required to your course to steer.

Photo: Hrvoje Komljenovic

To make a fix on the chart i.e. mark where your boats actual position is on the chart, this can be carried out as follows;

With the aid of a hand bearing compass, from the boats deck, take three bearings of fixed objects on the land or in the sea, as far apart within the 360 degree compass circle as possible. At the same time make a note of the boats ‘log’ (distance travelled through the water) the boats speed, the depth of water under the boat and the time.

Photo: Mhairi A

Now on the chart table, identify the three fixed objects you have used to take your bearings from on the chart. Now draw a line in pencil on the chart of the actual hand bearing compass readings from each fixed object. The three lines you have drawn will converge forming a small triangle, your boats position is within that triangle. Mark next to that triangle the boats log reading taken and the time for reference, you can then check the depth of water from the chart is the same as the depth taken when making the fix, this helps to confirm your position. If the depth or your three lines do not converge errors have been made and you need to repeat the process again.

Your new course can now be plotted from that new datum fix on the chart.

Source: learntosailnow.com