In reading the post from last year on the subject of the use of knots and nautical miles it was never mentioned how 47.33ft became the standard measure of one knot. Was there something common on tall ships that was this length or is it a simple division of a different unit of measure that has not stood the test of time translated into feet?
So, to clear up any confusion, a nautical mile was defined as being equivalent roughly to one minute of arc along the globe. The distance you're talking about is the number of actual knots (or other markers) in a rope which would be thrown overboard and the number of actual knots counted out, the number of knots being equal to the nautical miles per hour (speed over water, not over ground, of course). To measure their speed, sailors would toss a line overboard, attached to a log, and measure the number of knots that played out
There could actually be two intervals for measuring knots: the 47'3" interval would be used with a 28-second timing glass, and a 50'9" interval with a 30-second glass.
The intervals are related to the actual portion of a nautical mile they're measuring. In other words, to do some math:
Which is close enough for government work!
Source: A Sea of Words: A Lexicon and Companion to the Complete Seafaring Tales of Patrick O'Brian, by Dean King