Tugs get smaller but pack a bigger punch

Just as there are horses for courses, so are there tugs for ports. Local conditions and specific situations require different solutions, which are reflected in the development of a range of tug types. Factors determining the choice of tug type include the kind of harbour and approaches, i.e. conventional ports, ports with terminals or ports with piers and jetties; the type of ships calling at the port; and the services required in and around the port.

Over recent years there has been a rapid growth in new types of harbour tugs designed with high manoeuvrability and increased pulling power while modern steering systems, new towing appliances and new materials for towlines have also become available. Paradoxically, the improved manoeuvring capabilities of modern ships together with the enhanced towing performance of modern tugs has led to a decrease in the number of tugs in port areas. This, allied to financial pressures on shipowners, has led captains and pilots to use the minimum tug assistance.

Tug types

Tug types

are commonly named after their type of propulsion. This can be confusing since, for example, when talking about a Z-peller tug it could mean a tug with azimuth propellers located either forward or aft. Although the difference may not appear to be significant, it does affect the performance of the tug. Viewed in this manner, there are only two fundamental tug types. Those with their propulsion aft and towing point near amidships, i.e. conventional types, or those with their towing point aft and propulsion forward of amidships, i.e. tractor tugs.

Within these two broad categories, there are several sub-types depending on the propulsion systems fitted. Conventional tugs can be single-screw tugs, twin-screw tugs, ASD tugs (azimuth stern drive) or Combi type. Tractor tugs comprise Voith Schneider and azimuth propeller tugs.

To confuse matters, there are intermediate types of tugs that can be classified under both the main types. Reverse-tractor or pusher tugs as used in Japan, Hong Kong and Taiwan, are conventional tugs but built to operate mainly over the tug?s bow and therefore act as a tractor tug. Similarly, the azimuth stern drive (ASD) tugs can be built to tow from the bow and operate in the tractor mode.

Design requirements

Operational requirements for a harbour tug are optimum manoeuvrability, a short response time in terms of moving quickly from push to pull and vice versa, fast and easy handling of towing equipment, bollard pull and tug?s stability. In addition, a tug?s wheelhouse with an all-round view as close as possible to 360 deg and small windows located in the deck head that give an upward view (which is important for making fast to vessels with a high freeboard) are important. Another feature is that the tug?s superstructure should be located well inboard of the deck edge since tugs regularly have to work near a ship?s bow where the flair and overhang are often pronounced.

Good fendering is necessary to protect both the tug and the towed vessel from damage.

Propulsion systems

The vast majority of tugs entering service these days employ one of the many types of fully steerable thruster, the well-established Voith Schneider cycloidal propeller, or high efficiency conventional screw propeller and nozzle. Development on all of these systems continues unabated with the aim of improving bollard pull in relation to horsepower and, increasingly, the cost per tonne of bollard pull, both in terms of capital expenditure and running costs. Prime movers are predominantly medium- or high-speed diesel engines and the diesel mechanical drive is still the preferred propulsion method. Diesel electric propulsion systems are used in harbour tugs, but these are fairly uncommon because capital outlay and running costs are much higher.

Apart from the normal fixed pitch propeller or controllable pitch propeller, many tugs have nozzles fitted around their propellers, which increase thrust by 15 to 25 per cent in towing and pushing conditions. The effect is most noticeable with high propeller loads at low speeds, which is the way harbour tugs have to perform. Various types of nozzles have been developed although research is still ongoing and are very cost effective giving a good astern thrust of up to 70 per cent of the ahead value. Nozzles can also be steerable.

Fixed nozzles can also be used in conjunction with the Towmaster rudder system, which is a shutter rudder type consisting of several rudders (usually three) mounted behind each nozzle. Providing a rudder angle up to 60 deg, the Towmaster system gives good thrust and steering characteristics when going ahead or astern at the expense of increased complexity. Another solution is to fit a retractable or fixed azimuthing bow thruster to aid manoeuvrability and bollard pull of a conventional single screw tug.

Future trends

With the requirement for more bollard pull and manoeuvrability, there is a distinct move towards tugs becoming more and more compact with a higher concentration of power. This means that the hull length of a traditional multi-purpose ASD tug is becoming less than 30m whereas the beam has increased up to 12m. This gives a length/beam ratio between approximately 2.6 and 2.0 and the smallest of these tugs may have the largest bollard pull and the smallest length/beam ratio. Interest in compact tugs has also grown because of lower costs and the possibility of handling the vessel with two or three crewmembers.

These compact tugs can be classed as harbour tugs with an overall length of between 20 to 24m and a forward bollard pull of up to 70t although there is a strong demand for tug boats having a minimum thrust of 50t in all directions. These are generally of the ASD/reverse tractor type. For this purpose Schottel has developed a compact rudder-propeller type SRP 1215, which can take powers of up to 2,070kW with a propeller diameter of 2,300 to 2,400mm. This is claimed to be a very cost-effective tool to provide tugs with a maximum bollard pull range of 55 to 60t.

The newest designs to use these units are the Z-tech tractor tugs designed by Robert Allen in Canada. Two of these were recently built by Keppel Singmarine in Singapore and delivered to Keppel Smit Towage for the Port of Singapore Authorities. They are powered by two 2,070kW SRP 1215?s. Robert Allen has also introduced the new ?Ramparts? series of azimuthing stern drive tug designs to the world market. These designs are based on a modular approach to design and construction with the intention of bringing the best performing tug designs to a broader market at a more competitive price than one-off custom-built vessels.

The first Ramparts 2400 class tug went into service with Lyttelton Port Company of New Zealand bearing the name Blackadder. Constructed by Titan Marine Engineering of Auckland, the vessel is 23.85m in length overall with a beam of 11m and maximum draught of 4.6m. A pair of Caterpillar 3516B diesels producing 3,675kW power Rolls-Royce US 205 fully azimuthing propulsion units to give the tug a bollard pull of 60t and free running speed of 12.5 knots. Other examples of this design are two of the 2500 series with 50t bollard pull destined for owners in the US and six larger Ramparts 3000 type with a 55t bollard pull are on order, one in the US and four in Brazil.

Damen Shipyard Group in the Netherlands has also produced a compact design in the shape of its ASD tug 2411 which has a loa of 24.55m, a beam of 11.50m and a draught of approximately 4.70m. Another example of modern compact tug is the 2001 built Cape Pasley which has a 68t bollard pull on a 2.7 by 10.7m hull with a draught of 4.60m.

ASD tugs are also being used more and more with improved ice breaking capabilities and the first tug boat orders have been placed with class designation ice class 1 A to ice class 1 A super. An important business is also the modernisation of existing conventional tugboats such as in the case of US company McAllister Towing. Here an existing conventional tug boat with single propeller and a narrow beam of only 8.80m was converted into an ASD tug with a power of 2 x 1,231kW by using Schottel thrusters type SRP 1010 with reduced propeller diameters of 1,900mm.

The general tendency to build smaller but more powerful tug boats has given the opportunity to simplify shaft lines between thrusters and motors. Increasingly, hollow shafts or composite shafts are being used to reduce the number of intermediate bearings and to reduce the risk of failures due to misalignment of shafts.

In designing tugs, there is a requirement to reduce the lateral resistance as low as possible which means that they have no skegs, or just an open docking skeg thereby moving the centre of lateral pressure forward of amidships. For the reverse-tractor tug operating mode, this results in high pushing, pulling and towing capabilities and short response times. For a number of reasons compact tugs may have a skeg to improve course keeping stability, or to increase performance at higher speeds as a stern tug in the indirect mode, or as an ASD when towing astern.