Navigation and Seamanship

3.1.2. Construction of lead line

Hand lead line

The hand lead line consists of a tapered bar of lead weighing from 7 to 20 pounds, which is attached to a graduated rope. The long rope is marked with strips of cloth and leather to indicate the various depths, and the rope is wound on small wooden reel. The cavity available in the bottom of the tapered bar of lead is filled with a hard waxy substance, so that when the lead touches the sea-bed, some of the soil gets stick in the waxy material in the cavity. This will be useful to identify the nature of the sea-bottom.

The cable-laid hemp rope or nylon rope is used with the length of 25 fathoms (150 feet) and 10 mm in diameter. One end of the rope is attached with lead weight by means of eye splice. Before marking a new lead line, it should be stretched, and thereafter mark the line before it get dried. Mark the line every fathom, (1.83 metres) with any marking colour and number them 0, 1,2,3,4 etc. Now place the end of the eye splice on the first marking ( zero) and mark the lead-line at 2,3,5,7,10,13,15,17,20 .Fathoms which correspond with the depths marked are called marks. There are no markings at 1, 4, 6,8,9,11,12,14,16,18 and 19 fathoms. These are called “Deeps”.. There are altogether nine marks and eleven deeps. Different types of materials are used for marking, such as leather, linen, bunting, serge and cord, so that a navigator can identify the markings even if there is no light, by touch and feel the material with hand or lips. Lead lines should be measured frequently which wet and the correctness of the marking verified. During operation, a lead line is subjected to jerks when the lead is being swung. These jerks may be absorbed by the elastic nature of the rope used for a lead line. The cable-laid rope of cotton or manila has the maximum elasticity amongst all types of ropes. Hence a cable-laid rope is used for lead line.

In addition to the depth measurement, the lead line may be used to check the readings of echo sounder for accuracy, when the ship is stopped or when at anchor. Navigator should be familiar with the markings on the lead line.