At a time when green, remotely controlled and autonomous tugs attract attention, let’s draw breath and reflect on how tug design itself has developed in recent decades.
Progress with improving efficiency and environmental performance rightly continues to grab the headlines and fill column inches. It is easy therefore to overlook how shiphandling tugs themselves have evolved over a relatively short period of the industry’s history which dates back over 200 years.
That history be it ocean towing or shiphandling dates to realisation that the power of steam could be harnessed as a means of mechanical propulsion and a commercial saviour for wind-powered vessels which after racing around the world under sail with their precious cargoes were at the mercy of the wind direction for that final tantalisingly short journey to the berth. Steam-powered seeking tugs would meet clippers at ever increasing distances from their destination ensuring they reached traders ahead of the opposition.
Early paddle-wheelers gave way to screw-driven tugs and other than developments within the engine room itself, the basic design remained that of a deep hull powered by one or two propellers. The towing hook or winch was typically located amidships, an arrangement suited for long distance ocean tows and at the time adopted for shiphandling towage. This design remained largely unchanged for many years.
The development of shiphandling tugs to today’s azimuth stern drive (ASD), tractor and what seem to have gained the name ‘novel’ designs was prompted by social, technical and economic pressures to improve safety and enhance manoeuvrability and efficiency when handling increasingly larger ships in ever tighter spaces.
Kort nozzles increased available bollard pull but it was the introduction of azimuthing thrusters and the ASD tug towing over the bow that transformed the shiphandling tug scene and has remained the dominant choice for tug operators since. Tractor tugs with the propulsion units located forward and towing point positioned towards the stern was another variation from the traditional. The thrusters could be either azimuth or Voith Schneider cycloidal type typified by the Voith Water Tractor but let’s not forget that Voith propulsors date back to the 1930s or even earlier according to some sources.
As these different configurations emerged designers and builders wrestled with what to call their latest offerings. The labels reverse stern drive or reverse tractor appeared to apply to an ASD tug going backwards while connected to the ship bow to bow, while the RSD of Damen Shipyard’s latest innovative design the ‘always bow first’ RSD 2513 stands for reversed stern drive (note the extra ‘d’). And the configuration of Robert Allan Ltd’s popular Z-Tech design appears to be something different again ... or is it?
At a recent tug industry conference an unscheduled debate started about the meaning of these definitions, there was little consensus. It did however illustrate the options tug operators and owners have and the hunger for innovation in fine tuning designs aimed at providing the client with the best possible solution to satisfy their needs which at times can be very detailed for specific operations and locations.
ESCORT TOWING
Similarly, when examining the subject of escort towing, the origins of this form of towage assistance has been debated. Escort towing, also known as indirect towing generally refers to a tug connected to the stern of an often large vessel while it maintains a relatively higher speed than normal (when connected to a tug) transiting open water and still steering from its own rudder. This could be in environmentally sensitive locations where the ‘insurance’ of readily available intervention in the event of propulsion or steering failure is deemed necessary. If required to intervene, the tug influences the ship’s heading and speed using hydrodynamic lift from the tug hull’s underwater profile which usually incorporates a large skeg, rather than just the direct force of the towline.
Professionals with long personal memories point out that this ‘paravane’ or ‘otter board’ principle (often at lower speed) was adopted in their conventional single screw tugs. Indeed, it has been pointed out that artist JMW Turner’s 1839 painting The Fighting Temeraire depicting the warship under tow by the paddle tug Monarch to breakers on the River Thames doesn’t clearly show the tug connected aft, perhaps the first escort tug application?
ASD tugs remain popular for replacing older vessels or with new towage contracts that include requirements for newbuild vessels. Mass-produced and well established they are often available from stock with short delivery times, they also have advantages for the transfer of skills.
SOMETHING NOVEL
Tug designers and shipyards are always keen to improve their products and some interesting designs have emerged. Different to the popular configurations, they have been described elsewhere as ‘novel’ designs so let’s look at some that have made it beyond the drawing board, not including remote-controlled or autonomous tugs here.
Rotortug was established in1996 as the shipbuilding and trading arm of Kotug International, the Rotortug design characterised by three azimuth thrusters, two forward and one aft, the latter described as an active skeg. Full bollard pull is provided in virtually any direction along with enhanced manoeuvrability from the aft thruster offering easier berthing of ships, able to push via the towline rather than physically pushing the ship itself. Rotortugs are popular at locations requiring high-spec towing provision, typified by Kotug Australia’s operation at Port Hedland, Western Australia where nine powerful Rotortugs are deployed.
Another alternative design is the Giano Tug which features a double-ended symmetrical and tunneled hull and a single CP inline thruster at each end. It has substantial escort winches forward and aft and is available in 55tbp and 70tbp versions in a unified design.
The Eddy Tug has a similar balanced double-ended hull with inline thrusters. The accommodation is biased towards one end with a single towing winch close to the vessel’s mid-length. It is offered in 24m, 30m and 34m variants with bollard pulls between 30t and 100t. Telstar is currently the one example of the 24m version, originally delivered in 2016 to Iskes Towage & Salvage of IJmuiden and now part of Svitzer Netherlands BV’s fleet.
Another diversion from the traditional sees the towing connection, be it a hook or winch free-moving on a ring encircling the accommodation. A well-known danger for a tug and its crew is the risk of girting, this is where for example with deepsea towing perhaps in bad weather the tow takes a shear and sets off on its own adventure, potentially pulling the tug over if the towline runs abeam of it. Enhanced manoeuvrability and better understanding has reduced the risk but this addresses the problem in a revolutionary way, literally.
In 1999 the Dutch Muller family showed an interest in an idea presented by a naval architect where the towing connection rotates freely around a circular ring thereby moving the towing point away from the tug’s centreline as the towing angle changes. Given the name Carrousel system, Multratug’s Multratug 12 was retrofitted in 2002 to prove the principle in operation and for the next ten years those involved invested in R&D under the banner Novatug. This led to the building of two tugs and arguably one of the most revolutionary developments in shiphandling towage for many years.
Novatug was not alone in the ideas department however and in 2008 Peterhead Port Authority’s, Macduff UK-built Ugie Runner became the first tug to employ Mampaey’s Dynamic Oval Towing System with a free-running towing hook mounted on an oval ring encircling the deckhouse.
In 2013, Novatug and Multraship teamed up with Canadian naval architect Robert Allan Ltd to design a tug incorporating the Carrousel system. A high-spec model from the RAL catalogue was selected, the RAVE design (Robert Allan Voith Escort) developed in conjunction with Voith Turbo Marine featuring the longitudinal alignment of two Voith propulsion units on a basically symmetrical hull. In 2015 construction started on what were to become Multratug 32 and Multratug 33, the hulls built by Theodor Buschmann of Hamburg and completed by Damen Maaskant at Stellendam, the Netherlands in 2018. The following year at World Harbour Days in Rotterdam the 77tbp pair were christened and demonstrated their agility connected to each other, each rotating through 360’ presenting an impression of being remarkably stable with the towline running abeam. It is notable that three of these four novel designs include generally symmetrical hulls with inline, centreline thrusters. While currently each may be limited in numbers it will be interesting to see if the principle is taken further.
The examples above send a clear message that the industry is hungry for innovation and to repeat, no mention of the subject of remote-control and autonomous tugs. Being a pioneer may well involve extra investment and standard designs will always be popular but much of the technology with off-the-shelf models is proven and will suit the more innovative designs. Privately-owned companies have traditionally led the charge with new concepts but such is the demand nowadays for emissions reductions and more efficient operations it will be interesting to see how major operators navigate emerging challenges.
By Peter Barker
