Current and its effect
A feature of any river berth is the current. It is common for a river berth to lie in the same direction as the prevailing current so that the current can assist with berthing.
In this case, a berth can be approached bow into the current in order to give the advantage of relatively high speed through the water with a reduced speed over the ground. Consequently, steerage at low ground speed is improved by the good water flow over the rudder. The ship will be easier to stop.
Another advantage of berthing into a current is that it can be used to push a ship alongside. Position the ship off the intended berth but at a slight angle towards it. Then allow the current to produce a sideways movement of the ship towards the berth.
Masters should note that currents are usually complex, with varying rates and directions that can change hourly. For safe navigation, local knowledge is essential. A ship making headway into a current, but stopped over the ground, will have a forward pivot point.
Berthing in a current
Berthing with a following current is difficult, since the ship must develop sternway through the water in order to be stopped over the ground. In these circumstances, control of a single screw ship will not be easy. Use a tug to hold the stern against the current.
Care is needed when berthing into a current, because too large an angle between the berth and the direction of the current will cause the ship to move rapidly sideways. Unless corrected, contact with the berth may be unavoidable.
If during berthing the bow’s angle to the berth is over-corrected then the ship could move away from the berth as the wedge of water between ship and berth becomes established. This may cause the ship’s stern to strike the berth.
Once alongside, care must be taken to prevent the ship dropping astern before back springs and head lines are set.
Points to remember:
• In many places a counter current flows in opposition to the main current close to the bank. Only local knowledge will provide this information.
• Current can vary with depth of water and large deep draught ships can experience different current effects at differing parts of the hull. Caution is needed.
• When close to the berth in a head current, there is a danger that flow inshore of the ship becomes restricted and the ship is subject to interactive forces. These forces can cause the ship to either be sucked towards or pushed away from the berth. Local knowledge will help anticipate this phenomenon.
• As speed is reduced, take care that the increased proportion of the ship’s vector which is attributable to current does not set the ship close to obstructions.
• Always make a generous allowance for current. Its effect on the ship increases as the ship’s speed reduces. A mistake made during berthing is often impossible to correct. Remember that current predictions are just predictions and meteorological conditions may result in a greater or lesser rate than forecast. Local VTS information will normally advise of any significant anomalies.BLOG COMMENTS POWERED BY DISQUS