Frequently, the height change is slow and small enough so that those on watch can adjust the lines. On the mighty Thames, the tidal range is over seventeen feet. Tidal range is the difference between high and low tide. Instead of tending lines, the English build floating docks. Another item the cadets experienced was interaction.
Interaction is a phenomenon that happens when ships pass each other in shallow areas.
A ship acts as a wedge in the water. The bow cuts through the water and puts pressure on things forward of the vessel, pushing forward and away. When the ship passes, the water flows back, filling the space just occupied by the ship and creating suction that pulls the ship backward and inward. This back and forth movement is interaction. While docked at Tilbury, many of the cadets experienced this for the first time.