Tug Fast or Tug Pushing?

23 Febbraio 2022
|By Maurizio Garipoli

Make the tug fast or make it ready by working on push?

It is one of those questions to which it is impossible to give a universal answer. Each situation has advantages and disadvantages.

However, we can make a series of examples to help understand the arguments to arrive at the best decision.

  • We know that the tug operating at a ship’s side can express, at ship stationery, 100% of its power;
  • The tug towing on a line must consider its rejected current (which impacts the ship’s hull), the Bollard Pull, the Bollard SWL, any line damage, the angle of work, etc.
  • The tug operating at a ship’s side can work in only one direction;
  • The  tug towing on a line can work sideways or change its action angle;
  • The tugboat operating at a ship’s side needs little manoeuvring water;
  • The  tug towing on a line needs space to work;
  • During mooring operation – the tugboat operating at a ship’s side can be used to push until the end;
  • During mooring operation – the tugboat towing on a line at a certain point will have to stop pulling because of no more space available, and – in case of wind widening – completing the mooring may be problematic.

Let’s now look at a couple of practical examples:

Let’s imagine a Container ship berthed. There is a widening beam wind, which increases more and more. At some point, it is necessary to call two tugs to keep the position because the ship begins to move away from the pier. The wind increases, and, despite the two tugs operating at a ship’s side pushing at full power, the vessel breaks some mooring lines. It was necessary to call another tugboat, but the situation became so precarious that it needed to leave the Port.

In this case, making the third tug forward is advisable. It is because, with all three tugboats to push, it would be at the wind’s mercy once the vessel let go of all lines. Furthermore, if the ship’s engine was not performing enough and she took time to pick up speed, it could happen that the tugs would lose much of their effectiveness using part of the propulsion to follow her.

The wind could win over the tug’s power, and the ship would go inexorably towards the dam’s concrete or the leeward quay. At some point, the tugboats would have to escape abandoning the assisted vessel not to be crushed.

However, we can decide on another strategy: to help keep the ship head to the wind and speed up more quickly, securing the tug towing on a line forward. In the meantime, the forward-pushing tugboat would go aft to be connected too, while the stern tugboat would continue to work pushing at the ship’s side. With this asset, you will have more chances to get out of the dangerous area, and if things go wrong, you will still have two fastened tugs that will work to the last to reduce the violence of the inevitable impact.

Another example

 

The tug towing on a line does not always work perfectly abeam. We always have a component that pulls forward for the fore tug and backward for the stern one. If it is true that sometimes this force is manageable, it is also true that it can lead us to severe difficulty in confined spaces with a contrary paddle-wheel effect.

Let’s take, for example, the departure of a large displacement cargo vessel 300 meters long,  berthed Port side alongside—two tugs towing on a line fore and aft.

Let’s give a kick ahead to create minimum headway with the two tugs widening enough to get 5 meters away from the quay. We stop the stern tug adjusting the position with the rudder to Port and the BT to Starboard. The idea is to turn around the pier’s edge constantly. The most important thing is to keep the speed over the ground to a minimum. Once the forward tug passes from the Starboard to the Port, it helps our swing.

If the forward component of the pulling tug causes the speed to increase too much, we can lead to a situation in which we cannot give the engine astern because of the propeller paddle-wheel contrary effect. The stern tug would not have enough room to help us swing. And the loaded/heavy ship would not guarantee effectiveness to stop. That’s a stalemate situation!

In that case, it would be better to keep the fore tug to push: it always gives maximum power without any pulling component force that would increase the ship’s speed.

We will describe other manoeuvre examples in later modules when we talk about ship handling in the presence of wind and current.