Lot (shipping)

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Strip of the seabed mapped with a fan-echo sounder. The recorded area becomes narrower as the water depth decreases.

The plumb bob in shipping is a device for measuring the water depth. The classic hand plumb line consists of a plumb line and a weight at its end. The modern echo sounder measures the depth electronically using sound waves. The plumb bob is used to measure water depths, for ship safety and to detect schools of fish.


Surveying the depths of the sea

The depth of the sea floor is measured with the plumb bob ( bathymetry ). The results are shown on nautical charts as individual depth indications or as depth lines.

Ship safety

Sufficient water depth ("a hand's breadth of water under the keel") is important for safety in shipping. The current water depth can be measured with the plumb line. The course of the subsoil results from several measurements. This can reduce the risk of grounding and a leak . A single shoal can only be detected with predictive echo sounders. If the contour of contour lines is known from the map, the echo sounder can also be used as a makeshift for navigation.


With modern echo sounders , schools of fish can be detected and their position, depth and size displayed. Even the size of individual fish can be distinguished. The direction of movement of the swarm can also be derived from several measurements.

Hand plummet

Manual depth measurement, illustration from 1844
British sailors practice manual plumbing, the plumb bob is open at the bottom (1939–1945)

The hand plumb bob is a plumb line weighing 5 kg on an approx. 30 m or 100 foot long rope (plumb line). At first the plumb bob was lowered until it touched the bottom of the water, then it was pulled in again and the length of the cord was determined in arm spans. One arm span corresponded to a water depth of one thread (1 thread = 6 feet = 1.828 m). Later the plumb line was provided with colored marks every 2 m and leather strips with the corresponding number of holes every 10 m, so the plumb line did not have to be drawn in every time to measure the depth. A hand plummet can only be used when the ship is slow, the current is low and the depths are shallow.

The plumb bob was often provided with a cavity on the lower surface, with which a soil sample from the sea ​​floor could be obtained. This cavity was smeared with the solder dish. It consisted of beef tallow , into which soil particles were pressed and stuck when the solder reached the bottom. In addition to the depth of the sea, the nature of the seabed was often noted on the nautical charts, so that this method also made it possible to determine the location. Knowledge of the nature of the sea bed also helps to find a suitable anchorage. Not every anchor holds up equally well on every ground.

Deep down

The deep plumb bob, also known as Thomson's plumb bob, is a metal body that weighs up to 30 kg and is attached to a wire and lowered with a winch. In the solder body there is a thin glass tube on the underside into which water penetrates depending on the depth and the pressure present there. After pulling up, you can read the depth on it.

Bar solder

The plumb bob is a rod or a tube made of wood, metal or plastic with which you can measure the water depth in shallow waters. The plumb bob is dipped down to the tangible bottom and the depth is read off the scaled rod at the level of the water level.

echo sounder

The echo sounder measures the water depth with an electro-acoustic sound signal. The time that elapses between the emission of a sound pulse (water-borne sound) and the arrival of the sound waves reflected from the bottom of the water (echo) is measured. Every larger recreational yacht has an echo sounder. Echo sounders are standard equipment on commercial ships and fishing boats.

Fan echo sounder

The multibeam echo sounder is used to measure the sea floor. It sends several hundred measuring beams. The fan captures strips of the sea floor along the ship's course. A digital terrain model can be calculated from this and a topographic map can be created. The multibeam echo sounder is also increasingly used on yachts and by anglers and fishermen ("fish finders"). It shows a two-dimensional profile of the sea floor on a screen. Modern devices show a three-dimensional image of the area under water by recording the individual measurements one after the other and thus conveying a three-dimensional impression.

Sediment echo sounder

The sediment echo sounder penetrates the sea floor up to 100 m with a special frequency and provides geologists and geophysicists with information about the hardness and nature of the upper soil layers ( sediments ).

Compressed air solder

The compressed air solder measures the water pressure as a measure of the water depth. The dive computer and the nansen bottle also measure the depth using water pressure.

Sound solder

The sound plummet or free pilot is a kind of small bomb that sinks at a certain speed and detonates on the ground. The depth can be determined with a time / speed measurement. It is accurate to about 40 m.

Literature and usage of language

The writer Mark Twain (actually Samuel Langhorne Clemens) derived his stage name from this device. “Mark Twain” was a pilot language expression and meant “two threads deep”. With this understatement, he humorously mocked his own "depth".

In the legend of the Blautopf , a mermaid repeatedly steals the solder in order to keep the true depth of the Blautopf a secret.

The word "exploring" is often used in a figurative sense when one goes into "unknown territory". It indicates that caution is required.

See also