Sounding by lead and line

Acoustic depth sounders appeared in the twentieth century, earlier for ships than yachts. However, from at least biblical times depth was found by lead and line.

And sounded, and found it twenty fathoms: and when they had gone a little further, they sounded again, and found it fifteen fathoms.

ACTS: 27,28

The line is generally 25 fathoms in length, and is marked with strips of leather and different-coloured calico or string at the 2, 3, 5, 7,10, 13, 15, 17, and 20 fathom marks. These are known as “the marks,” and a sailor, in calling out the soundings where these marks apply, uses the expression “By the mark seven,” if the mark is close to the water; if more than the 7 fathoms, he calls, “and a quarter seven,” “and a half seven,” or “a quarter less eight,” these being respectively 7 1/4, 7 1/2, and 7 3/4 fathoms. For the fathoms where there are no marks, he calls, “By the deep four,” etc. The use of these terms is to prevent mistakes in calling out numbers which have similar sounds, such as 7 and 11, and to emphasize the number called; the word “deep” being derived from “dip,” the line having to be hauled up and dipped when ascertaining the unmarked fathoms. ¹Wheeler, W. H. Longman’s Civil Engineering Series – Tidal Rivers Their (1) Hydraulics (2) Improvement (3) Navigation. Read Books, 2009.

The practicality of the dissimilar materials is illustrated by this extract:

Q. How do you distinguish between 2 fathoms and 3 fathoms? A. The leather at 2 fathoms is put in the strand half-way, thus forming two tails; at 3 fathoms it is put in the same way, but one tail is slit, thus forming three tails.
Q. In a dark winter’s night, with your hand so be numbed with cold that you cannot tell the marks by your touch, what would you do? A. Put it to my lips. If the white be a bit of canvas, the red a bit of bunting, and the blue a bit of cloth, the lips will instantly tell the difference. ²Reed’s New Guide Book to the Local Marine Board Examinations of Masters and Mates for Certificates of Competency. By J.J. Stiles, 1867.

Particularly in survey work, but also to confirm location against a chart, tallow embedded in a recess at the base of the lead was, and still is, used to assess the material on the seabed. The marks are:

Notes and References

Depth sounding – Wikipedia

This is a distantly related but fascinating item on surveying invasion beaches with a lead line.

• 1
  Wheeler, W. H. Longman’s Civil Engineering Series – Tidal Rivers Their (1) Hydraulics (2) Improvement (3) Navigation. Read Books, 2009.
• 2
  Reed’s New Guide Book to the Local Marine Board Examinations of Masters and Mates for Certificates of Competency. By J.J. Stiles, 1867.