When two ships in the same edition of this magazine carry the name ‘innovation’, or its translation in Dutch, it shows what the Dutch shipbuilding industry is up to. This Innovation is the first of Damen Shipyards’ new Reversed Stern Drive tugs, and was nominated for the Ship of the Year award 2018.
Damen's tug 2513
is a two-in-one solution
The tug industry is one of continuous developments and many new concepts have been launched in the past few years. While each of these innovative concepts excels in one particular domain, it’s Damen Shipyards’ goal to design and build tugs which provide a cost-effective answer to the everyday challenges of 95 per cent of the customers. The new Reversed Stern Drive (RSD) Tug is a clever combination of two previous Damen designs: the Azimuth Stern Drive (ASD) Tug and the Azimuth Tractor Drive (ATD) Tug.
The ASD tug is the most popular in Damen’s range. It features in general a high foredeck and low aft deck, with a relatively long and round bilge hull, with a single keel. When it is used at the bow of the ship, this tug always sails stern-first, as the thrusters are then further away from the ship’s bow. This avoids the risk of contact between thrusters and bulbous bow and ensures that the prop wash pushes less against the towed vessel. It also provides a very good and safe lever arm between thrusters and towing point for steering corrections. The disadvantage is that it is sailing with the low freeboard of the aft deck at the front in this case. The tractor tug, or ATD in short, was designed specifically to operate at the bow. It has its thrusters in the foreship and a single keel or twin-fin keel in the aft. In the ideal case, a ship would be escorted by an ATD in the front and an ASD at the stern. Both vessels then operate bow-first.
The wheelhouse is exceptionally quiet
Builder: Damen Shipyards
Operator: Kotug Smit
Length: 24.73 m
Beam: 13.13 m
Depth at sides: 4.95 m
Draft: 5.50 m
Displacement: 525 t
Main engines: 2 x MTU 16V4000 M63L
Thrusters: 2 x Rolls Royce US 255
Generators: 2 x Caterpillar C4.4 TA, 81 kVA
Max speed ahead: 13.0 kn
Max speed astern: 12.8 kn
Bollard pull ahead: 75.3 t
Bollard pull astern: 71.2 t
Fuel tanks: 77.4 m3
Fresh water tanks: 10.2 m3
Bilge water tanks: 6.0 m3
Sewage tanks: 6.0 m3
Luboil tank: 2.5 m3
Dirty oil tank: 2.5 m3
Because many tug operators prefer to have a standard fleet of tugs of a single type, Damen started the development of the Reversed Stern Drive tug, which - in a simplified explanation – has the bow of the ASD on one side and the bow of the ATD on the other side. It can operate either way and has a distinctive sheer line with a relatively high freeboard both forward and aft. As this also means that the towing point is located higher up, the vessel is beamier to ensure the same stability. The RSD tug is a therefore a relatively short and wide vessel, which is excellent for maneuverability.
Course-keeping is also very important however, particularly when towing at an oblique angle. To achieve the adequate course stability, a typical centerline skeg is not sufficient. The RSD tug has two vertical fins at the bow, a patented concept called ‘Twin Fin’, which was also used on the ATD. While the profiles on the ATD were wing-shaped, they are elliptical on the RSD, to ensure good performance in both directions. The skegs’ shape and positions were optimized in collaboration with MARIN, using both computer simulations and model testing.
In comparison with a single keel on centerline, the twin fins have a center of pressure which is further away from the thrusters, leading to less interaction between the thruster flow and skeg(s), and also a longer lever arm to work with. A second difference is that the center of pressure is located higher than a center keel below the bottom. This means that for a given side force (during drift), the vessel will heel less. Therefore, the ‘Twin Fin’ fit perfectly in the quest for maximum stability in relation to the wetted area. This is an important ratio, as most of the sailing of a harbor tug is done at five to six knots, a speed at which the frictional resistance (and thus wetted surface area) is dominant. For tugs operating a lot at speeds above nine knots, for example on long harbor approaches by river, a longer and slenderer tug will be more efficient. The RSD tug is more geared towards ship-handling such as berthing and unberthing.
The Rolls Royce thrusters
provide excellent manoeuvrability
MTU engines provide propulsion power, Caterpillars the electrical power
When the tug is pushing, the skegs are in front of the thrusters, but the water is much more blocked by the object being pushed than it is by the fins. When the tug is pulling, the thrusters receive an undisturbed flow and the fins are in the wake of the thrusters, like any rudder on any ship.
The stability of Innovation complies with the new stability rules, which Bureau Veritas introduced in 2018 and which IMO will adopt in January 2020. These rules make a differentiation in tugs depending of their use, be it harbor tugs, escort tugs or salvage tugs. Her higher freeboard gives her more reserve stability, without penalizing on wetted surface.
Propulsion is with two azimuthing thrusters, both with a nozzle, from Rolls Royce, of type US 255. The propellers have a large diameter of 2,700 mm. Each thruster is mechanically driven by an MTU 16V4000 main engine, rated at 2240 kW at 1,800 rpm. Each RSD Tug 2513 built after Innovation will be IMO Tier III-ready, which means that space is reserved in the exhaust silencers for retrofitting of an SCR catalytic converter and in the engine room for a freestanding Urea tank and pump. The second vessel has also been completed and is equipped to IMO Tier III standard. To ensure the comfort of the crew, Innovation is built to BV class notations COMF-NOISE 3 and COMF-VIB 1, which specify the limits for accommodation noise and vibrations. On main deck level, there are three cabins, each with their own wet cell, a mess room and a galley.
The towing winch has 175 tons of brake power
Sister vessel Bis Viridis is built to
IMO Tier III emission requirements
The deckhouse is mounted on a vibration-insulating strip and has a sound-absorbing ceiling, and this paid off. The noise level in the wheelhouse is only 58 dB(A), compared to 67 dB(A) for a similarly sized ATD tug, in spite of slightly higher numbers for engine power and bollard pull. The wheelhouse has excellent sight lines in all directions, due to the very compact window mullions and the overhead windows. The glass in the wheelhouse is shatterproof all around, ensuring the safety of the crew in the event of a towing line breakage. To ensure safe operations on deck, both the foredeck and aft deck were designed to be as clutter-free as possible, eliminating tripping or line-snagging hazards. The towing winch is located on the foredeck, has a 175 tons brake and can pull cable up to 38 metres per minute.
The RSD tug Innovation is equipped with the Damen Gateway, a system which collects all data onboard and sends it to the shore. When there is temporarily no connection, the data is stored and sent later. The ship owner can access this data through the PC- based Damen Platform, a dashboard where the operational manager can see the ship’s position, engine speeds, fuel consumption and a lot more. A visualization of the data from suppliers (Rolls Royce, Caterpillar) can be included in this dashboard, leading to a proper remotely visible Condition-based Maintenance System.
While there are tugs that can pull from either the foredeck or the aft deck, or even from all around the wheelhouse, the RSD tug has a different philosophy. The towing point is always on the foredeck, but the sailing direction can be adapted to whether it is working at the bow of a ship or the stern. As usual, Damen Shipyard builds these tugs for stock, ensuring a short delivery time. While the Innovation is currently operated by Kotug Smit and operating in the Port of Rotterdam, the second vessel in the series, called Bis Viridis, is sold to Damen Marine Services.