Using experience to assess required tug power



Captain Henk Hensen, Fellow of the Nautical Institute, Marine Consultant, Maasland, The Netherlands



The optimum use of tugs can have different interpretations depending on the economic priorities of the parties involved. The shipowner for example may want the fastest operations which in turn may lead to stronger tugs, the port on the other hand may not have the repeated use for very large tugs so that the owners cannot recover their investment. So the answer is a balance which is appropriate to the operating environment in the port concerned.

The experience of the shiphandler is, however, always a crucial factor when deciding on the appropriate use of tugs. This experience factor is essential to answer the questions of how many tugs are needed and what tug power is required? The importance of the experience factor is not always taken into consideration, and this article emphasises this important factor.

Why we should take heed to the experience factor?

The basic reason is the trend in the use of ever stronger tugs and the related important aspect of the difficulty in assessing how much tug power is really needed to handle ships safely in a port. The latter is also important for port authorities to be aware of when assigning ships what tug assistance should be used. A proper assessment of the tug power needed is important for large ships and particularly for ships with a large windage, such as container vessels, car carriers and LNG carriers, whose number and wind area increases continuously.

The trend towards the use of more powerful tugs and the difficulty in assessing the tug power really needed will also have its effect on ports under construction and ports with a small financial budget. While a professional assessment of the situation, taking into account the local available experience, may result in a more realistic and economical tug use and tug fleet, it should be well noted that the larger the shiphandler’s experience the better the results will be. This also means that for a port, training of shiphandlers, pilots and captains, by capable instructors with the right practical experience is a major factor of importance as well.

The present situation and its consequences

As mentioned above, in many ports a tendency can be observed towards ever stronger and often ever more complicated tugs. Tug power increases continuously and tugs with a bollard pull of 70 tons can be found already in a number of ports or are under construction, while harbour tugs with a bollard pull of 100-120 tons are already considered. No port is the same and tug requirements differ from port to port. However, in general more powerful tugs mean larger investments and higher costs.

The dimensions of tugs change. Ever stronger engines can be installed in ever smaller tug hulls. An extreme example is the so called compact tug: Tugs, often with azimuth thrusters, with a length over all of about 24m and a bollard pull of up to approximately 70 tons. The small, very manoeuvrable and powerful tugs are a welcome development as space in ports is often limited and high power is sometimes needed.

Replacing a number of smaller tugs by a more powerful tug could produce savings for a tug fleet owner due to the smaller tug fleet and savings on tug crews. However, the availability of stronger tugs does not automatically mean that ships will use less tugs. Ships normally using one or two tugs will, when using more powerful tugs, still use one or two tugs. Even ships using, for instance, four tugs on arrival generally use the same number of tugs when even more powerful tugs become available. An example is given below. In such cases the towing company made a large investment, but ships are more or less using the same number of tugs and may pay the same tug dues.

Reducing the number of tugs because of the introduction of more powerful units may have consequences for a port. A reduced availability of tugs may cause problems in case of severe weather conditions or in case of peaks in shipping traffic. The introduction of more powerful tugs may in certain cases indeed result in a reduction of the number of tugs used per ship, for instance from four to two units. This has the consequence that an additional risk is introduced. The reason is as follows: In case the ship is assisted by four tugs and one tug has a break down, there are still three tugs left to assist, which is, in most cases, sufficient to handle the ship safely. In cases where the ship is assisted by two powerful tugs, and one tug has a breakdown, safe handling of the ship becomes problematic with just one tug.

Tugs should, furthermore, be suitable for the size of ship. This means that small ships should preferably be handled by small tugs. Smooth and gentle handling of small ships with powerful tugs is more difficult and bollards and fairleads of small ships are not strong enough for the forces that can be delivered by the powerful tug. The latter is already a problem for large ships when handled by the present powerful tugs of 60 – 70 tons bollard pull. Several complaints regarding damaged bollards and fairleads have already been reported.

It is clear that although the development of safe and capable tugs should continue, following blindly the trend towards stronger tugs is not the way it should be done, although the fact that shiphandlers gradually get used to the larger tug power is a factor not to be underestimated.

Nevertheless, the type of tugs, the required number of tugs and the required bollard pull should be determined in a professional way, based on the local situation, including the local available shiphandler’s experience. A number of ports do carry out careful studies to determine what the required type and bollard pull of new tugs should be, taking into account pilots’ experience.


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