Safe tug: From a tug master’s point of view: Bas van Hoorn

September 1st, 2014 by JCB
Posted in Articles, Features, Technical and Training | No Comments »

This article is reproduced with the kind permission of the publishers of the International Tug & OSV magazine. It was first published in their July/August 2013 edition. www.tugandosv.com. The Pilot Magazine would also like to pass on their thanks to Capt. Bas van Hoorn.

My name is Bas van Hoorn and I am senior captain/instructor on all types of tugboats ranging from conventional single screw, stern drives, Voith Schneiders, tractors and Rotor®tugs. I’ve been involved as an operational tug master in simulators all over the world to run tests for new port infrastructure and to train tug masters. In the past year I have also been driving tugs during model tank tests. First of all I would like to thank Henk Hensen for initiating the safe tug discussion in International Tug & OSV, and Markus van der Laan, who reacted to that article from his professional point of view as a naval architect. I myself would like to take this opportunity to give my reaction to both articles from a tug master’s/instructor’s point of view.

Making fast on the bow of an incoming vessel: the ideal speed

During this procedure, making fast at the bow of an incoming sea-going vessel, speed is a key factor. When a tug master is matching the speed of the vessel and manoeuvres his tug in front of the sea-going vessel, he can estimate and calculate the speed of operation very precisely. In this way he will know how much there is left to get away in an emergency situation. A tug master should always contact the pilot if he deems the speed too high for his comfort zone.

This ‘making fast’ speed will differ largely between the different type of tugs – even within the tug type categories – and it will vary largely per individual tug. The just-clutched-in speed will be the standard. Tugs with a just-clutched-in speed of 6 knots will not be happy to make fast at a speed of 4 knots. When this is required, the tug master will have to ‘kill’ the thrusters, or clutch in and out, thus making the manoeuvre more complex. A number of tugs I have sailed with have a clutched in speed of 8 knots. It would be ideal to make fast just above this speed.

As a tug master you can handle the tug at its most safe and responsive range, and enough power is left to escape in an emergency situation. This is the main reason not to prescribe the ideal speed to connect. In a port such as Rotterdam, with a large number and variety of tugs, many different tug master opinions will be heard. The communication between pilot and tug master is therefore critical. Both will need to have consensus about the operations. With the high dead-slow speeds of the modern vessels, this consensus is getting increasingly more important.

 Making fast on the bow of an incoming vessel: the various type of tugs

The conventional tug and ASD over the stern. 

 

This is the manoeuvre with the highest risk level. In order to get away from the bow or bulb of the sea-going vessel, the tug has to manoeuvre its stern towards the danger first, with the risk of colliding and girting. The unfortunate accident with Fairplay 22 is an example of this manoeuvre going wrong. The choice of a tug master to make fast in this conventional way can be influenced by the design of the tug and the present circumstances, such as sea conditions. The master can decide to assist in this conventional way due to a low freeboard of the aft deck of the tug when going stern first and/or when there are high seas. The fact that a conventional connected tug requires less space to manoeuvre may be of influence when bridges or locks need to be passed. With the present length of the ASD tugs this can be an important consideration.

The ASD over the bow 

 

Interaction between tug and ship are of major concern. As a tug master it is of importance to know whether the tug is pulled or pushed away by the vessel to assist. This is especially important when making fast with an ASD in the bow-to-bow operations, but also with a tractor tug. The effects are biggest at the bow of the vessel due to the relatively big skegs of the ASD and tractor tugs. Manoeuvring with the thrusters at high speed and the influence of the surge of the bow of the vessel can reinforce turning moments. By pushing the wash of the thrusters against the vessel’s own skeg, other turning moments than those you might expect could occur. This is especially critical with ASD tugs – which have a long skeg – sailing backwards, as this influences the speed of making fast.

I must also mention the taking on of water on the aft deck of an ASD sailing backwards. Whilst operating in high seas and at high speed, this can cause a dangerous situation: the speed of the tug drops enormously when corrective steering needs to take place, and the danger of girting is eminent. The main reason for the execution of the push-pull system all over the world is this eminent danger of sailing backwards with an ASD in combination with making fast at the bow. The disadvantage of push-pull is less control of the vessel you assist and a lot more time is needed to turn or berth a vessel.

 

The tractor tug over the stern

As mentioned by Marcus and Henk previously, this type of tug is a lot safer during a making-fast operation at the bow of the vessel. Sailing bow first was one of the primary contributors to the Voith-Schneider and other tractor tugs’ success when introduced at the time. But also as mentioned, the tug master must be fully aware of the playing field of forces on the large skeg, directly influenced by the wave of the bulbous bow of the vessel. When a tractor tug is making fast at the stern of the vessel, speed, unexpected wash of the screw, repositioning while crossing this wash, and indirect towing, can cause dangerous situations due to the relatively low aft deck of the tug sailing backwards.

The Rotortug over the bow or stern

Adding it all up, one can come to the conclusion that a tug is at its safest during operation when its bow and stern can be controlled at all times, and the tug is assisting with the bow in the direction of sailing. The tug must therefore have a towing point at two sides: where the main (two thrusters) propulsion is in front of the towing point when operating over the stern, and where the main propulsion is located aft of the towing point when operating over the bow.

 

The Rotortug combines the best of both worlds. With this tug, the master is always in control of both the bow and stern of the tug. He is free to choose the direction of sailing, depending on the given circumstances, without compromising the performance of the tug. Making fast at the bow of the vessel at speed can be executed with precision. The master is also controlling the part of the tug where the towline connection is to be made. With all other types of tugboat, the towline connection is relatively far away from the thrusters. The bulb wave is not influencing the tug because it has no skeg.

 

In case of an emergency, the Rotortug at the bow can always get away with the forward thrusters, and this movement is reinforced by the aft thruster, to create a sideways movement. Making fast at the stern of a vessel, the tug master can manoeuvre with the two forward thrusters, at the side of the tug where the towline connection is made. This is the most direct way of steering, ensuring a safe connection procedure. The screw water does not affect the tug, because the Rotortug has no skeg. The connecting procedure is done more indirectly with all other types of tugs, on the bow or stern of the vessel to assist, causing time loss, which can be crucial in an emergency situation.

It’s incomprehensible that the triple Z-drive or Rotortug has been ignored in all Safetug (I and II) investigations over the last 14 years since their introduction, but this is maybe because people are not fully aware of the phenomenon. These Rotortugs have been operating effectively in port areas with extensive lock systems, bridges and significant currents. It is the most forgiving (there is always room for human error) and flexible tugboat type I have operated by far.

Conclusion

1. There is no such thing as a general ideal speed for making fast a tug at the bow of an incoming vessel. This fully depends on tug (type), vessel and skill level.

2. Before making fast a tug, the master and the pilot must have consensus about the procedure.

3.Making fast with an ASD tug at the bow of an incoming vessel can prove to be a hazardous operation.

4.Making fast with an ASD tug at the stern of an incoming vessel is a safe operation.

5. Making fast with a tractor tug at the bow of an incoming vessel is a relatively safe operation, depending on the speed and bulb wave influence on the skeg.

6.Making fast with a tractor tug at the stern of an incoming vessel can be a hazardous situation depending on speed, but the danger caused by the vessel is in sailing away from the tug.

7. Making fast with a Rotortug on the bow and stern of an incoming vessel is always a controlled and safe operation. No external pickup devices are needed. The safe zone of this tug is only limited by the hull of the vessel assisted.

8. The bow-connected tugboat working over the stern is much safer and more efficient compared to push-pull with an ASD tug over push-pull, and gives the tug master more control of the vessel being assisted, at all speeds below 10 knots. Turning ships in turning basins or wherever is done a lot faster.

With this article, I hope I am able to contribute to the overall safety of  our industry.