Conventional Tugs

Conventional Tugs

Conventional tugs can be single or twin-screw, with or without bow thruster (BT) and usually only one rudder.

The conventional tugs towing hook is generally at a distance of about 0.45 x LOA (length overall) from the stern, therefore close to the hull’s centre. In addition, the propulsion aft and towing point near midships compromise its efficiency making this tug exposed to particular risks. For example, it may happen – while towing a ship under-speed – that the cable work abeam of the tugboat, trapping it in a stall position (girting) until it causes it to overturn (capsizing)—a towing winch with a quick-release system lower this risk.

A modern variant involves using a retractable azimuth BT —the Combi-tug. The 360 degrees steerable azimuth bow thruster permits this tug type to step sideways, turn on the spot, sail straight ahead and astern, and increase the Bollard Pull. They reduce the related risk of girting tanks to this increase in manoeuvrability.

The emergency quick-release mechanism either trip the hook or release the brake on towing winches to take the load off the towline allowing the tug to regain control from a potentially dangerous situation. This system may be remote-controlled by bridge or manually at the winch. We will talk about it later, and we will deepen the topic.

To move the working point from the centre of the hull towards the stern, the “gog” or “gob” – wire or rope – is generally used. It can be rigged in many ways. It practically serves to move the tow point away from the pivot point, minimizing the risk of girting; however, it considerably reduces the tug’s manoeuvrability. Therefore, the Modern version or Combi-tug are equipped with special openable fairleads to substitute the gob rope at the aft end.

When it is secured forward, a conventional tug danger derives from its sensitivity to the interaction forces. When it has to give the tug’s line close to the ship’s bulb, the tugboat in motion suffers the effects of the wake’s turbulence of its propeller that collides with the water displaced from the bulbous bow, making it challenging to maintain directionality.

Furthermore, its hull shape and propulsion type make the conventional tug not very agile. Unlike other types, it cannot pull or push simply by reversing a lever; direction changes are also laborious and time-consuming. When ‘pushing at the ship’s side’, they have difficulty keeping a right angle.

Precisely because of these manoeuvrability limitations, the conventional tug can suffer more than others the risk of capsizing due to the ship’s speed related to the working angle of the towing cable:

• forward, when towing on a line, it is overtaken by the vessel;
• aft, during transitions in pulling working positions.

Among the advantages, we can mention a lower construction and maintenance cost, a yield – between horsepower and pulling capacity – above average, which determines excellent performance in pure towing, intended as towing from point A to point B.

This last aspect is often further improved by using the Kort Nozzle – which we talked about in “The Propellers” course – a particular duct accelerates the propeller’s water flow, increasing its thrust.