A Simplified Guide to Small Marine Craft Navigation.

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Chapter 15 (v.1) - Measuring Water Depth by Poles, Sounding Leads and Lines.

Submitted: March 17, 2017

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Submitted: March 17, 2017

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Measuring Water Depth by Poles, Sounding Leads and Lines.

 

 

It is obviously extremely useful for a navigator to know the depth of water in which his craft is floating. A measurement of the depth of water is termed a sounding, and measured in meters, fathoms, or feet.

A fathom is a nautical measure equal to six feet. In conformity with the introduction of metrication into commerce and industry, current chart soundings are given in meters and decimeters, tenths of a meter. However, many charts are still in use with their soundings in the imperial system. Therefore, it is important to understand both measurements as used for soundings.

The simplest method of taking a sounding in shallow water is with a pole, and for this reason barge-poles, bearing-off spars and boat-hooks should be marked off in decimeters, feet, or both. In deeper water, soundings are taken by a lead and line, or by the use of echo sounders.

For small marine craft purposes, a Hand Lead Line suitable for measuring depths up to 20 meters, 66 ft. or 11 fathoms, consists of a 25 meter, 82 feet, length of special 9mm. diameter cable-laid, left-handed, rope with a long eye-splice at one end and a back-splice at the other end. The Lead consists of a tapered bar of lead weighing about 3 kilos, 7 lbs, with a hide strop becketed round an eye in its upper end. The base of the lead is hollowed out into a score which can be filled with tallow or soap if a sample of the sea bed is required.

This process is known as arming the lead so that the sand, shells, gravel or whatever the seabed is composed of can adhere to the tallow and be recognized when the lead is recovered. This compared with the seabed characteristics shown on the chart and can be extremely useful when trying to check your position in fog, or in locating a suitable place in which to anchor.

To attach the Lead Line to the Lead the large eye of the line is rove through the strep of the lead, the Lead itself then being passed through the eye. The standard lead line markings are inserted through the lay of the line, and are as follows:-

 

1 and 11 meters. ->  One strip of leather.

2 and 12 meters. -> Two strips of leather.

3 and 13 meters. ->  Blue bunting.

4 and 14 meters. -> Green and white bunting.

5 and 15 meters. -> White bunting.

6 and 16 meters. ->  Green bunting.

7 and 17 meters. ->  Red bunting.

8 and 18 meters. ->  Blue and white bunting.

9 and 19 meters. ->  Red and white bunting.

10 meters.--------->  Leather with a hole in it.

20 meters.--------->  Leather with a hole in it and 2 strips of leather.

All 0.2 meter markings. ->A piece of mackerel line.

 

The first meter measured does not include the length of the lead, thus in any soundings obtained there is a depth of water that much extra. This is termed as the benefit of the lead. When a new line is required it should be soaked and stretched before inserting the marks, and as it will still be liable to stretch or shrink, should be re-measured from time to time to ensure the marks are correct.

The proper way to take a sounding with a lead and line, termed making a cast of the lead, is to take the line coiled small, about the size of a dinner plate, and divide the coil in two. Then holding the Lead in one hand with half the coil allow the Lead to hang down about half a meter and give it a good sweeping underarm swing, releasing it as it comes parallel with the waterline. It is not necessary, and certainly not advisable, to swing the Lead around the head when making a cast as it may connect with your own or someone else’s head with fatal results.

 Stand on the lee-side and well forward and aim to land the lead well ahead of the bows so that as the craft sails forward, the Lead, still sinking, reaches the bottom before the line is vertical. The feeling of slack shows when it has bottomed, and if a third of a meter or so of line is quickly taken back and released when the line is vertical the up-and-down depth can be noted.  


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