The Sounding Lead

Since at least the sixth century B.C. the sounding lead or sounding weight was in use in the Mediterranean area for maritime navigation. It is the oldest known marine navigational instrument and it remained a primary navigational aid in coastal waters and rivers well into the twentieth century. A sounding weight is a roughly bell-shaped mass, usually made of lead, averaging about five kilograms in weight, with a sturdy attachment lug at its apex and a tallow cup in its spreading base. A line was attached to the sounding lead (lead line) with graduated markings at fixed length intervals. The sounding lead was led out in the water hanging on the lead line and the line was veered until the lead reached the sea bed. From the marks on the lead line the depth of water could be read.

Ancient mariners used sounding-weights, not only to determine the depth of water, but also to bring up samples of the bottom (stuck in the tallow on the base of the lead), comparing the result with their knowledge of coastal geography and river sediments. With this knowledge a method of navigating from one depth to another based upon the condition of the bottom developed. Herodotus wrote in the fourth century B.C.: "..When you get 11 fathoms and ooze on the lead, you are a day's journey out from Alexandria,..". And a sailing directions from the 14th Century reads "Ye shall go north until ye sound in 72 fathoms in fair grey sand. Then north until ye come into soundings of ooze, and then go your course east-north-east". 72 fathoms is about 130 meters - that's a long line! From later reports we know that "heaving the lead" was a standard procedure as a ship sailed into harbour. A sailor (the "leadsman") would throw the sounding lead as far forward as he could, and when the ship came up to where the lead had landed he would count the knots or markings on the lead line and call out the depth of the water to the pilot.

In 1895 Joshua Slocum - on his sailing journey around the world - passed Sable Island (New Scotland - Canada) in a foggy night. In his book "Sailing alone around the World" he writes: "By the lead, which I cast often, I found that a little after midnight I was passing the east point of the Island, and should soon be clear of dangers of land ans shoals".

The Hand Lead Line

A hand lead line is a light sounding weight (7 to 14 pounds), usually having al line of not more than 50 meters. It is used in coastal waters and anchorage places to determine the depth of water and to bring up samples of the bottom.

Since modern yachts are equipped with electronic sounding systems, the issue of analysing the anchorage soil is the main application of lead lines today.

Nevertheless, the hand lead line will be used in regular intervals to check and calibrate the installed sounding system. This will be done especially for the critical shallow depths.

Acoustic Echo Sounding Systems

Modern electronic sounding systems are based on ultrasonic echo transceivers. A short burst of an ultrasonic wave is transmitted by an underwater transducer. A part of this acoustic wave will be reflected by the sea bottom and this echo is received by the transducer.

The time elapsed between transmission and reception of the acoustic burst is proportional to the distance travelled by this burst from the transducer to the sea bottom and back. By measuring the time, the acoustic wave needs to come back, the depth of the water below the transducer can be measured.

In deep waters, secondary echoes may occur on cold water layers. Since these echoes may be stronger than the main echo coming from the bottom of the sea, a correct interpretation is difficult. Only more expensive sounding systems will be able to give correct results. Since this is only a problem for water depths well above 20 meters, this is not an issue, for pleasure yachts.

The advantage of an electronic sounding system is that it enables a continuous and instantaneous monitoring of the water depth. Especially when navigating shallow waters such as anchorages or harbour entrances, this system is much more comfortable and safer than working with the hand lead line. Standard features of electronic sounding systems include alarm functions, which will activate an acoustic signal, when the measured depth is below a selected critical depth. For use as anchorage alarm two critical depths, defining an allowed depth range, can be selected. The alarm is activated if the measured depth is not within the allowed range.

"Look-Ahead" Sounding Systems

Normal echo sounding systems determine the depth of the water immediately below the underwater transducer. The transducers are mounted such that they are aligned approximately along a plumb line. With this constellation it is not possible to detect shallow waters in advance - before the vessel has actually reached the shallow.

By tilting the transducer, such that it transmits in a forward direction it is possible to measure the water depth of a point which lies before the location of the vessel. This however requires additional logical circuitry to calculate the depth from the time interval between burst transmission and echo reception. Moreover this method requires more sensitive transducers, since the received echo will be weaker. This will generally result is a system, which is more prone to incorrect measurements. The increased sensibility to environmental conditions has to be considered in the system monitoring software, which must be able to recognize and ignore unreliable transducer signals.

Recently effective "look-ahead" depth scanners have been developed for maritime applications. They continously scan the water depth in a range up to 100 meters ahead of the vessel producing a depth profile, which can be displayed graphically on an LCD display.