A lead line is a very simple device used for measuring the depth of water / soundings. With the use of tallow, beeswax or grease, it can also tell you the nature of the sea bed.
A lead line consists of a straight lead weight, much like a plumb bob, with a thin line attached. Dyanema is a very good choice of line as it has minimal stretch and thus gives very accurate readings. The line can be knotted every meter or marked with colour coded whippings at equal intervals.
- Stand at the widest point of the yacht, make sure the yacht is almost stationary and cast the lead line slightly forward of the yacht.
- Let the line payout until it has some slack in it. Take the slack out, so the line is straight up and down.
- Count the knots or marks as the line is paid out and when the line is straight up and down you will have your depth sounding. Make sure the yachts engine is in neutral, just in case you run over the line and it gets caught in the propeller.
If you have put tallow, beeswax or grease on the bottom of the weight, the nature of the seabed will be revealed by what is stuck on the bottom of the weight. This method of depth sounding takes a bit of practise, however it is extremely accurate, does not require any power and takes up very little space to store. They can also be used in an emergency if you have no power on your boat.
Lead lines are ready available to buy, however they are also very easy to make. A shackle could be used, however it will not sink as fast as a tapper shaped lead on a length of dyneema, allowing it be moved with tide or drift of the yacht.