Swinging the lead

[As] old sailing ships neared land, it was important to know the depth of water beneath the keel. Most sailors had no formal education, therefore a very simple foolproof system was invented. A lead weighted rope was set to have a knot tied every fathom length. A fathom being 6 foot

A sailor had the job of throwing the weight forward and feeding out the rope until he felt the weight hit bottom. By now the rope should be vertical in the water. The sailor retrieves the rope counting the knots that were submerged and calls out very loudly 30 fathoms.. (or whatever the count was)

...

It was very hard work throwing and retrieving this heavy lead weighted rope. So the lazy sailor would pretend to do it when he thought that no one was supervising him. He would stand there "swinging the lead" so that as soon as an officer appeared he could let it go forward and appear that he had been correctly doing his job all along