Tug Fast or Tug Pushing?

Make fast the tug 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 which arguments made to arrive at the best decision are.

  • 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;
  • To moor a vessel, the tugboat operating at a ship’s side can be used to push until the end;
  • To moor, a vessel, 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 to widen – complete 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 increase more and, despite the two tugs operating at a ship’s side pushing at full power, the vessel breaks some mooring lines. It is necessary to call another tugboat, but the situation became so precarious that it is needed to leave the port.

In this case, it is advisable to make fast the third tug forward. It is because, with all three tugboats to push, it would be at the wind’s mercy once the vessel let go 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, in order not to be crushed, would have to escape abandoning the assisted vessel.

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 would still have two fastened tugs that would 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.

With the two tugs to widen enough to get 5 meters away from the quay, let’s give a kick ahead to create a minimum of headway. 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 at a constant distance. 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 give us guarantees on the effectiveness at the 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.