In coastal navigation, the most important tools are the compass, chart, spyglass (telescope to check for landmarks) and “hand lead-line” to measure depth, the 30 second sandglass to measure time and the “log line” to measure distance through the water. These two measurements allow the ship’s speed to be determined at a given moment. As the speed of the ship varies regular use of the log line is necessary to ensure that total distance travelled can be estimated with some accuracy. The log line consists of a lead-weighted wooden board (the chip log) attached by a bridle, to hold the board vertically under the surface of the water, to a long line, on a drum or reel, with knots tied at regular intervals. The interval calculated is the distance travelled in 30 seconds at one nautical mile per hour. The drum or reel is on a stand. A seaman  drops the chip log into the water and turns the 30 second sand glass. Another seaman holds the log line and, as the line passes through his fingers, counts the number of knots in 30 seconds. That number is recorded, the line rewound and the procedure repeated. The speeds recorded in one hour are averaged and that number added to the Traverse Board.

The Compass

The compass consists of a lodestone (magnetic) needle, floating in a cup of heavy oil, which always points to Magnetic North. The oil prevents the needle from moving erratically. Magnetic North lies a few degrees away from True North. That difference is called the magnetic variation. On top of and fixed to the needle is a card. It is divided and marked into 32 points and is called the compass rose. As the needle follows the direction of magnetic north, the compass rose turns with it.

The major points – north, east, south and west at 90 degrees to each other in a circle. Between each point and at 45 degrees, four further points called – north east, southeast, south west and northwest.

Those eight points further equally divided into eight further points, each point now 22½ degrees apart- nor’ nor’ east (NNE), east nor’ east (ENE) , east south east (ESE), south south east SSE) continuing around the Compass.

Then finally, those sixteen points further equally divided, making 32 points, in all 11¼ degrees apart – north by east, NNE by east, NE by east, ENE by east, ESE by east, SE by east, SSE by east, south by east. To the west, south by west, SSW by west and on to north by west.

The compass with its rose is housed in the binnacle. A round metal casing attached to the deck with the compass at the top and covered in glass. The binnacle has an etched line showing the fore and aft line of the ship’s axis. The direction the ship is heading toward (called the Heading) is read off the compass rose by the point under the forward end of that etched line.

The Traverse Board

The Traverse Board is used to keep a record of the heading and the speed of the ship and, as a result, the direction and distance travelled during a watch. The Board has an image of the compass rose on it with 8 concentric rings of holes, each ring with a hole on each point of the compass. There are 8 pegs attached to the board by twine. At the start of the watch a peg is inserted in the hole in the first ring, nearest the centre, on the point of the compass that the ship is heading. If the ship is heading NE, the peg is inserted in the first hole on the NE point of the compass. Every 30 minutes, another peg is placed in the appropriate hole in the second and subsequent rings, each signifying the heading of the ship at that moment. By the end of the 4 hour watch the 8 pegs will have been inserted into a hole in each of the 8 rings.

At the bottom of the board there is a set of 4 rows of 10 holes, and 4 other pegs attached to the board with twine. The average speed in the first hour of the 4 hour watch is represented by a peg placed in first row from 1 – 10 knots. There is another set of 4 rows of 3 holes to the right, with another 4 pegs. These signify ¼, ½, ¾ knot.. The speed is calculated from the average speed measured by the log line through the hour. For a calculated speed of 4 ½ knots in the first hour of the watch, pegs would be placed in the top row in the 4th column of the first set and the 2nd column of the second set. This would be done for each hour of the watch.

At the end of the watch, the officer of the watch records the distance and heading into the ship’s log.

Once out of sight of land, latitude is measured by the backstaff, or cross staff which, used with a set of written “Tables”, determines the ship’s latitude from the position of the sun or certain stars.

It is necessary to hold the cross-staff in both hands. Look through a hole in a vane at near end of a wooden staff to a horizontal slit in a vertical plate attached to the other end of the staff. Next step, line up the horizon through that slit. The vane is made to slide up and down a curved, tabulated scale. Move the vane to align with the horizon and a corresponding number could be read off the scale.

Then by adjusting another smaller vane, at the far end of the staff, have the shadow of the sun, caused by the vane, to fall on the slit the observer is looking through at the horizon. This smaller vane is on its own curved, tabulated scale. With the horizon and the sun’s shadow aligned read off the corresponding numbers from both scales. Add them together to obtain the sun’s altitude in degrees.

 

Latitude consists of imaginary lines parallel to the equator around the world, where the latitude at the equator is given the angular value of 0 degrees, while the latitude at the North Pole has the value of 90 degrees north.

Knowing the latitude of the ship’s destination and the ship’s current position enables the course to the ship’s destination to be determined.