At a certain moment we experienced a rather curious effect in the 58 km long man made Manchester Shipping Canal which had us puzzled. We entered the Canal in Eastham near Liverpool and it ended in the center of Manchester. The Canal had transformed Manchester from a land locked city into a harbor. In those days the Canal was polluted up to the max because the industry discharged polluted water freely. The captain remarked to the pilot that they should cover over the Canal and that they would then have the biggest sewer in the world. The pilot was not amused…
Once while traveling through the canal, at a certain moment the pilot had to stop the ship because a bridge ahead had a malfunction. Totally unexpectedly when nearly stopped we could feel that the ship accelerated and at the same time strongly veered to starboard. No rudder or engine could prevent that we ended up with the bow in the mud. We could pry off ourselves with hard port-starboard rudder and engine full astern and a few times full ahead till the bow moved.
The pilot said that it was caused by some sort of stern wave. Later we learned that a vessel in very shallow water drags a volume of water astern, which can be as much as 40% of the displacement, When the vessel stops this entrained water continues moving and when it reaches the vessel’s stern it can produce a strong and unexpected turning moment, causing the vessel to begin to turn unexpectedly.
In the case of accompanying tugs they should tow on a long line or otherwise they may prove to be ineffective. The reason for this is that the proximity of the vessel’s hull and small underkeel clearance reduces or even cancels the tug’s thrust.