speed measurement was called the "chip log." chip as in chip of wood, and log as in to record in a log. the chip was a wedge of wood about 18" in size; it was tied to one end of a rope on a large spool. the rope had knots tied into it about every 47'3" (more about how that was calibrated below).
the wooden chip was thrown overboard at the ship's stern (back end). because of its wedge shape, it would "grab" the water and start pulling out rope as the ship moved forward at some yet unknown speed. one man would hold the spool of rope as it played 