Dredge on the Anchor Copy

The anchor is the most suitable “tool” available.

Let’s see what the conditions could be in which we should plan to use the anchor.

The anchor is used for dredging when a certain amount of chain is dropped at sea to slow down the ship’s headway without stopping and increase control over the steering.

How does the ship slow down?

To hang cable. Simply holding (tightening on the brake) a quantity of chain that, thanks to the engine Ahead, allows the anchor to drag on the bottom.

And how does control over the ship’s steering increase?

The ship’s pivot point will move very close to the bow, helping change direction using the engine Ahead with small rudder angles. It will make it easy to get the bow exactly where we want it.

In practice, when is it convenient to dredge anchor?

The classic situation sees us handling a small-sized ship, light draft, without or with insufficient manoeuvring thrusters power, which must moor unfavourable to the Paddle-wheel effect, whether with the fix or variable pitch. We will explain this manoeuvre in detail below.

Another situation where dredging the anchor is particularly effective is moving backwards without a bow thruster’s help.

Imagine, for example, a lightship without a BT (bow thruster), which must proceed backwards in the presence of a fresh headwind. The propeller effect, whatever it is, will cause the bow to yaw on one side or the other, and the wind will increase this tendency.

Walking back the chain corresponding to one and a half the depth will solve the problem: the bow will be held in the wind allowing us to move backwards while maintaining the direction.

There are places where it is not allowed to leave the anchor at sea once moored, oil ports, for example, where we must manage an emergency exit quickly, canal ports, or shallow water situations with the transit of ships near the quay. When it can be helpful to use the anchor to facilitate mooring, it is usual to dredge it to be in the condition of heaving up a modest quantity of chain once moored.

Let’s now look at the first case mentioned:

• General cargo vessel – 100 meters long.
• With variable pitch right-handed propeller.
• Without bow thruster.
• Fully loaded.
• Must moor with the starboard side alongside the quay.

Let’s start with some considerations:

The controllable-pitch propeller has a much lower efficiency than the fixed-pitch when astern and has a tremendous Paddle-wheel effect. Considering that the ship is fully loaded, we must expect difficulties in stopping and a marked tendency for the bow to come to starboard. Unfortunately, the lack of the bow thruster does not allow us to counteract the right-handed effect.

It is easy to foresee the need to use the engine astern to stop the ship in the desired position in the final phase. The Paddle-wheel effect will affect the bow that will inevitably tend towards the quay and the stern that will widen from it. We will lose control of all the elements and risk hitting the dock. It may seem exaggerated, but the ratio of stopping effectiveness and turning effect of a heavy vessel having a controllable pitch propeller (even if it is small and proceeds slowly) is very disadvantageous. In reality, some tricks allow to manage the situation without using the anchor, but they are delicate operations that do not guarantee the result.

• The first advice is to forget the rush: it is imperative to go slow and privilege precision at the expense of power.
• The second tip concerns the instructions to the Master: for the manoeuvre to be successful, it must follow the steps fluidly.

Therefore, the first thing to do is make sure that the anchor is ‘paid out to the water level ready on the brake.

Based on the depth, type of the seabed and anchor characteristics, the amount of chain to walk back will be decided so that the ship is braked but not stopped. In practice, we will have to dredge the anchor. If in doubt, it is better to walk back less cable and slack a little more later.

We’ll drop one shackle in the water considering 12 meters depth and a well-proportioned anchor. The Master must adjust the engine near the mooring as previously told: the ship will not have to stop!

After dropping the anchor, it may be necessary to increase the engine while maintaining a low speed. If the ship’s speed were so low that the anchor could hold and stop the vessel, it would become challenging to resume motion ahead. The crew must send the headline and the bow spring.

Let’s go into detail by analysing the final part of the manoeuvre.

We will drop the port anchor when we arrive one and a half or two hull lengths from the mooring, and we will do it with the engine stopped and a residual speed of about two knots. The bow will look just beyond the centre of the intended position at approximately 30 degrees. When the time has come to hold on to the anchor, we will pay particular attention to the headway, increasing the engine to prevent the anchor from grabbing the seabed and stopping the vessel.

The ship’s pivot point, which will move very close to the bow, will help us vary the direction using the engine Ahead and small rudder angles. In this way, it will be easy to bring the bow exactly where we want it.

Once we have reached the position, we have already passed the spring and the headline ashore. We can reduce the engine while maintaining a minimum movement ahead until the residual speed is wholly lost. Then, finally, the rudder hard over to port will allow us to bring the stern toward the quay to send the remaining lines ashore.

INSERIRE VIDEO

‘This manoeuvre is not to be underestimated!’

Particular sensitivity is needed to control the ship. It is necessary to understand when and how much to increase the engine. Headway should not stop. It is essential to instruct the crew not to pay out too much chain and get close enough to the dock to send the messenger ashore.

Wanting to provide some references, even if empirical, we can say that the cable length to be paid out is the one that permits zero speed with the Dead Slow Ahead while allowing the ship to maintain a light headway with the engine Slow Ahead.

Generally, we can obtain this condition by walking back a chain equal to 1.5 times the Depth. This parameter can vary according to the nature of the seabed, the type of anchor, and the Dead Slow Ahead real power. Another necessary precaution is not to exceed the speed. Otherwise, the anchor will begin to jump on the bottom, losing its holding capacity.

The second case proposed is an unloaded ship that has to move backwards.

• We always consider a ship of 100 meters, this time unloaded.
• Light drafted bow – significantly influenced by the wind direction.
• No bow thruster.
• Variable-pitch with the right-hand effect.
• Presence of a fresh headwind.

The scenario includes obstacles that make it impossible for the vessel to turn on the spot. The only ‘way to go’ is to move back to a large swinging basin.

As already mentioned above, the right-handed variable pitch propeller, once in reverse, would struggle to make the ship increase its speed while, at the same time, it would immediately cause the bow to swing to starboard. If we add the wind force acting on the bow of an unloaded ship and the absence of the bow thruster, it becomes easy to imagine the challenge of managing the direction as we would like.

The solution is to drop the port anchor and increase the engine to move backwards. The bow will immediately keep into the wind—the quantity of chain to be paid out depends on the depth. We cannot generalise. However, we can use this rule of thumb: Hold the brake once the anchor’s crown digs and the chain starts to look forward!

It may also be helpful to use the rudder hard-over to one side – in our case, starboard – to contribute to going straight back. The presence of generously sized rudders as the speed increases helps counteract the propeller’s Paddle-wheel effect.

When the ship has finally reached the turning area, we must first stop the engine, slow down the speed, then with the engine forward and the rudder hard over, turn on the anchor, weighing and proceed to the exit.

INSERIRE VIDEO

In narrow places where small yawing on the bow may be dangerous, to provide a straight sternway, the assistance of one tug aft would be handy to pull the vessel safely.