The log line is an old instrument for measuring the speed of the ship. It consisted of a flat piece of wood (the log), which was weighted at the bottom edge to enable it to float upright in the water. To the log was attached a long rope (log line).

The log line was wound onto a spool (log reel) so that it could be reeled out after the log was thrown into the water at the aft (rear) of the ship.

The friction of the water held the log in place as the ship moved away from it.

The log line was 150 fathoms (900 feet) in length and was marked with a piece of white bunting (duck) to start the measuring and after that by “knots” to represent the speed.

The line was then let to run out for a given interval of time - the number of “knots” counted as the as the line reeled out during that time would show the speed of the ship.

The log line, which is knotted every 47 feet 3 inches, was used to measure the ship's speed. Three seamen were needed for this task: one to hold the reel, one to heave the log line over the ships stern, the third to watch the twenty eight second time glass.

As the line ran over the stern the knots were counted against the time glass.

The number of knots counted = the ships speed in knots.
1 knot =1 nautical mile per hour.
1 nautical mile = 6,080 feet or 2,026.6 yards

The distance of 47 feet 3 inches as a fraction of 1 nautical mile (i.e. 6,080 feet) is directly relative to 28 seconds as a fraction of 1 hour in seconds (3,600 seconds).