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Efficient selection and deployment of terminal tugs

Edition 60 - VTMIS & Aids to Navigation (ATON), VTS, navigation, mooring and berthing

 

Introduction

When sea-going vessels slow down to enter port, their rudder effectiveness and manoeuvrability become restricted by the loss of water flow or water speed across their rudder. The paradox of slowing down to enable safe navigation of highrisk areas, while losing effective steering control at the same time is what drives the demand for tugs.

At the zero speed condition a ship’s control ability can be boosted with bow and stern thrusters, but the effectiveness of these drops dramatically if a ship gains a little speed and a pilot is not always sure how effective a ship’s transverse thrusters will be.

At medium speeds tugs apply steering and braking forces supplementary to a ship’s own steering system and at higher speeds we refer to this type of assist as ‘escorting’. There is however a sliding scale between escorting and shiphandling and modern tugs are generally required to provide assistance over the full 10-0 knots speed range, creating an overlap in the existing definitions.

This is why modern day terminal tugs are required to feature both suitable escort capabilities and superior manoeuvrability during ship-handling operations. In effect, one could state that terminal tugs cover a somewhat intermediate position between escort tugs and ship-handling tugs as far as operating speed (not maximum sailing speed) is concerned. This issue focuses on the working principles of modern terminal tugs and how tugs generate maximum towline forces during escort duties and rapid vector response when ship-handling in confined spaces.

Escorting duties

Escorting and associated technologies are like the extreme sports of towage industries. High towline forces are generated by manoeuvring the tug under an oblique angle to the sailing direction. The hydrodynamic hull forces (lift and drag) generated in this manner create the high towline and steering forces on the assisted ship (see Figure 2). Because of the indirect power generation, we generally refer to this manoeuvre as operating indirect or in an indirect mode.

The same working principle applies at lower speeds too and at about 6-4 knots tugs generally shift between the so-called indirect mode and direct mode wherein they pull using direct thruster power only.

• To maximise the indirect steering forces, naval architects increase the lateral surface area of escort tugs by adding skegs

• The application of skegs in tug designs limits manoeuvrability when providing ship-handling services

• Generated steering forces deteriorate more than 40 percent when the assisted vessel reduces speed (because the hydrodynamic lift and drag forces deteriorate in turn by a power of two.)

The tugs’ supplementary steering forces deteriorate as the ship slows down, while the ship’s own steering system becomes less effective too. Where tugs should mitigate the navigational risks, they actually become less effective due to their working principle. This paradox in ship design required a new tug concept providing steering and braking forces over the full range of speeds of ships with hydrodynamic forces being part of, but not the only power generating force.

The triple Z-drive or Rotortug® displayed in figures 3 and 4 uses a combination of the hydrodynamic and direct thruster forces to generate effective steering and braking forces at the same time. The combination of indirect and direct thruster forces enables enhanced ship control for the pilot over the full 10-0 knots speed range. The Rotortug® generates the forces without increasing lateral surface area of the hull and is therefore easier to manoeuvre during the ship-handling phase.

• The combination arrest mode (see Figure 4) offers enhanced safety for a tugboat’s crew because the towline is lined up with the tug’s hull creating only limited heeling angles (5-7 degrees heeling).

• Switching between the indirect and direct or combi mode is a simple and safe manoeuvre. The Rotortug’s doubl e - ended cont rol abi l i t y increases vector response to assisted ships.

• Smart application of a ship’s own steering systems and pre-deployed tugs can further increase a ship’s manoeuvrability in restricted waters.

• The ability to deploy your tugs in multiple configurations extends the pilot’s options to adapt to all circumstances.

So what level of supplementary steering forces can we expect from these Rotortugs when deployed on the stern tug position in addition to a ship’s own steering systems? The bottom line answer to this is displayed in figure 5. Figure 5 displays the generated steering forces over the entire speed range for both the triple Z-drive Rotortug® and conventional escort tugs relying on hydrodynamic forces to generate steering forces. Another interesting feature is this ‘rotoring’ or indirect
towing at low speeds. This enables full control ability while operating within the beam of a ship while transiting bridges or entering docks.

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