A passenger ship was entering a canal and continued its passage after a passing a lock. Further ahead was a guest harbour for private boats. Beyond the harbour there was an old bridge abutment where the canal narrowed to about 14 metres. After leaving the lock the crew noticed that there was a sailing boat moored by the bridge abutment. Speed was reduced and the ship kept to starboard in the canal to clear the sailing boat.
By the bridge abutment the ship suddenly yawed to port due to the bank effect. The crew was not able to prevent the yaw and collided with the sailing boat, which was crushed and suffered hull damage in the form of cracks and holes, leading to extensive flooding. The roof above the wing of the bridge on the passenger ship was damaged. After the collision the passenger ship continued until it could be moored, when the crew went back to the place of the accident with the lock guard.
The rescue services were not notified until about an hour after the event. The investigation shows that the reasons and factors that lead to the accident were that the sailing boat was moored in such a way as to prevent safe passage. The crew on the passenger ship did not observe the sailing boat until they were out of the lock. At that point it was not possible to stop without risking running aground. The sailing boat was moored in a prohibited area. Despite this fact, the canal personnel had not reacted beyond putting a fine on the boat.
The investigation also states that the time taken to notify the rescue services was dangerously long and for this reason the rescue services did not arrive at the site before at least an hour after the accident. In this case it was not important since the sailing boat was unmanned. There were also deficiencies in the canal company’s procedures and instructions for handling accidents in the area. When a ship is sailing it is affected by various hydrodynamic forces that depend on its speed, the depth of water, width of passage and topography of the seabed. A mass of water is pressed up around the bow and then flows aftwards along the hull. There is a positive pressure at the bow and a negative pressure at the stern. This pressure differential has no lateral effect on the ship when sailing in open deep water since the flow is symmetrical along the hull and the forces cancel each other. In contrast, when a ship is moving in a canal and comes closer to one of the banks or a silt bank, the flow speed of water along that side of the hull will increase. A pressure difference arises that causes a tendency for the ship to yaw from the side with a higher velocity of flow. The negative pressure at the stern causes it to be sucked towards the canal bank. This is called the bank effect and can be described as lateral forces on the hull. The same effect occurs when moving alongside a stationary ship.
The depth of water also affects the manoeuvring characteristics of a ship. In general it can be said that if the water depth is less than 1.5 times the ship’s draught, the effectiveness of the rudder is decreased. In shallow water greater rudder angles are required for the desired effect. The bank effect can be reduced by keeping to the centreline of a canal and holding a lower speed. The helmsman must plan ahead and brace with the rudder. At lower speeds it is also possible to gain a better rudder effect by opening the throttle.
SFu journal no. 06.05.02 TSS 2010-1631