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. Dyneema 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.

1. Make sure the yacht is stationary and cast the lead line at the same position as your echo sounder on one side of the yacht.
2. Let the line payout until it has some slack in it. Take the slack out, so the line is straight up and down.
3. 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.
4. Repeat the process on the other side of the yacht and average the two readings.

The measured depth should match the reading on your echo sounder depending on how your echo sounder has been calibrate - most people have the reading offset to show the depth of water beneath the keel or from the waterline to seabed.

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.