Janet Tatum, age 14, of Casper, Wyo., for her question:
WHAT IS A CHIP LOG?
A log is an instrument that measures the speed of a ship. The six most important kinds are the chip, or common log; the taffrail log; the harpoon log; the ground log; the Forties log; and the pitometer log.
The chip log is a piece of board about a half inch thick and shaped like a quarter of a circle, about six inches long on the two straight sides. the curved edge is weighted with lead so that the log floats upright with the curved edge down.
A sailor throws the log into the water behind the ship and a line attached to it unreels as the ship moves. By noting the amount of line that runs out in a given time, a sailor can determine the ship's speed.
The taffrail log works on the principle of the automobile speedometer. It consists of a rotator with spiral fins that cause it to turn as a ship pulls it through the water. The rotator is connected by means of the towline to a recording device that looks like a clock. The recorder, attached to the stern of a ship, shows only the distance traveled. Because of this, a sailor must make two readings, with a known interval of time between them, to obtain the speed of the ship.
The harpoon log resembles the taffrail log, but its register is towed in the water with the rotator and must be hauled in to be read.
The ground log is used in shallow water when a ship is moving slowly. It has a lead weight on the end of the lead line. A sailor throws the lead overboard and it sinks to the bottom. A known amount of line is paid out and the time noted, as in the case of the chip log.
The Forties log consists of a small rotator in a tube that projects through the ship s bottom. The speed of rotation varies with the ship's speed. The pitometer log, like the Forties log, records both speed and distance but it operates by the action of water pressure.
The basic part of a pito meter log is a Pitot tube, which forms part of a rod meter that extends from 24 to 30 inches below the bottom of the ship. The Pitot tube has an opening on the side facing the bow of the vessel. Another tube surrounding the Pitot tube has an opening that faces toward one side of the ship.
When the ship is at rest, the water pressure at the opening of the Pitot tube and the surrounding tube is the same. This is known as static pressure. As the ship moves, the speed of the vessel increases the water pressure on the opening of the Pitot tube. This is known as dynamic pressure.
The total pressure on the Pitot tube equals the sum of the static and dynamic pressures. But the opening in the outer tube receives only static pressure, whether the ship is at rest or in motion. Various devices inside the ship measure the difference in pressure between the two tubes and translate this into speed and distance.
Large, modern steamships measure speed by counting the revolutions of the ship's propeller. The accuracy of this method is affected by the weather, the shape of the ship and the ship's bottom.