Electronic equipment at sea can sometimes let us down. To be a captain prepared for any situation, we must also know how to use manual marine instruments. The auxiliary equipment that allows us to measure the depth from the sea floor to the sea surface by hand is called a sounding line. A sounding line consists of a rope with a lead weight at the end and knots or color markings every meter.
When your depth gauge is malfunctioning, once your boat reaches the location where you plan to anchor, you lower the lead weight of the hand-held sounding line from the bow of the boat. By counting the knots on the line for each meter until the lead weight touches the seabed, you calculate the depth in meters. By subtracting the distance between the boat’s waterline and the deck from the calculated depth, you obtain the net depth. This allows us to easily calculate the amount of anchor chain we need to use at the location where we will anchor.
The hand lead also provides information about the seabed structure. After measuring the depth, we can determine the seabed structure based on the presence or absence of debris on the lead weight when we bring it back onto the boat deck.
The lead weight of the lead:

• If sand is stuck to it, it has a sandy sea floor,
• If there are pebbles, it has a small pebbly structure,
• If there is mud, it has a muddy sea floor,
• If there are no remains on it, it has a rocky bottom structure.