Eddy 1: A Slender Hull and Hybrid Podded Propulsion for Efficient Harbour Tug Service
5 May 2014 saw the launching of a revolutionary new type of harbour and terminal tug: the Eddy 1. It is the first embodiment of an entire line of tugs ranging from ten-metre line-handlers to 40-metre MPVs and ERRVs. The concept is entirely new and based on three important innovations: a slender hull with ample sideways drift resistance, a forward-and-aft placement of the thrusters and a hybrid drive system for these thrusters.
Holland Shipyards is keen on innovative projects that challenge an established market. This fact was proven with the Eco-tanker Amulet and with the dynamically positioned accommodation ships DP Gezina and DP Galyna. So when father and sons Hoogendoorn, owners and managers of Holland Shipyards, were presented the concept by creators Baldo Dielen and Walter van Gruijthuijsen, their interest was aroused immediately. It took Dielen and Van Gruijthuijsen five years to work out the concept from the initial sketches. Jointly, the partners set up the company Eddy Tug, in which Eddy stands for Efficient, Double-ended and DYnamic. Eddy 1 is the first fruit of that collaboration and will be presented to the towage industry at the ITS convention in Hamburg from 16 to 20 June.
As there are a lot of tug builders worldwide with a wide range of tug and towage vessels, one could wonder why a Dutch shipyard – which almost by definition will have a hard time competing on price level alone – would want to enter this market. The explanation can be found in their firm conviction of a better product at a competitive price level. Also, this size range is small enough for the penalty of labour costs in steel work to be overcome by efficiencies in supply and project management.
One of the key ingredients of the Eddy 3065 design is its unconventional hull shape. Most escort tugs feature a very full forebody and a shallow aft ship to accommodate the large propellers needed for sufficient bollard pull. The Eddy series however have a more canoe-type hull with the volume concentrated amidships and a significant amount of lateral area extending both to the bow and the stern. A deeper keel on centreline provides directional stability and facilitates docking. The thrusters are placed forward and aft of the keel on centreline.
When this hull shape really comes into play is when the tug is in escort service behind a large vessel. Typically, escort tugs are used as a backup for failing propulsion or steering gear of large ships, or to compensate for those ships’ reduced controllability at low speeds. They do so by sailing at an angle from the stern, while maintaining a certain amount of pull on a line from the large ship’s stern. The Eddy 1 is designed for such service, and therefore has its main towing point on the foredeck. As her thrusters are located on either side of the towing point, one of them can maintain the line pull, while the other one keeps the tug heading in the right direction. The underwater shape of the hull resembles a hydrodynamic foil, and generates significant lift (and therefore line pull) when towed at an angle. This reduces the amount of thrust needed from the thrusters – and therefore the fuel consumption during escort service. Also, more reserve power remains available for the assist. The difference in line-pull mechanics between an Eddy Tug and a tug with azimuthing stern drives is similar to that between a balanced rudder and an unbalanced rudder.
Another contributor to the efficient and safe escort performance is the relatively low position of the towing point, resulting in less heel of the tug for the same amount of pull. Nevertheless, Eddy 1’s freeboard is quite high for safety and from the towing point, the line can be pulled over a range of 270 degrees without touching the superstructure. All openings in the accommodation are located near the centreline, and the ample reserve buoyancy in the bow and stern contribute to the fact that the tug’s stability remains positive (self-righting) at extreme heel angles (over 90 degrees). The superstructure has a tumblehome of 35 degrees, allowing the tug to come very close to ships with a large amount of flare in the bow or stern.
Safety was one of the three key focal points in the design from the very beginning, along with performance and economy, and in some cases the three goals are served by a single design feature. This is the case for the hybrid thrusters, which are housed in separate engine rooms, forward and aft. Not only does this give a healthy degree of redundancy in the propulsion system, it also contributes to the watertight subdivision in five compartments, any one of which can be flooded without causing sinking.
Key to the design are the hybrid azimuthing thrusters of type SRP3000 from Schottel, which feature a horizontal input shaft for a direct-diesel drive and an electric motor on a PTI shaft opposite. During transits, Eddy 1 can be sailed in diesel-electric mode up with either one or two diesel generators running and both large diesel engines switched off. In such case, the forward thruster will be driven at about 20 per cent, as switching off the forward thruster entirely would create unwanted drag. The thrusters are rated at 1,900 kW in total. When sailing on electric power alone (saving running hours on the main engines), the Eddy 1 can achieve a transit speed of ten knots and has full manoeuvrability. During normal towage service, she will be sailing in diesel-direct mode, with each thruster driven by a 1,610 kW main engine.
During the very demanding operational profiles (for example 100 per cent bollard pull profile), the electric motors can give an additional boost to the main engines for extra power. The propellers are with fixed pitch blades, but due to the speed regulation with variable frequency drives and the high initial torque of the e-motors, the desirable properties of controllable pitch propellers are achieved to a great extent. In diesel-direct mode, it is also possible to use the e-motors as generators (the PTI then becomes PTO), with an output of 100 kW. The complete electrical installation was done by Holland Ship Electric, the main engines are from Mitsubishi and were supplied by Koedood. Both engine rooms are remarkably spacious for such a small ship, and everything is easily accessible for service.
Eddy 1 is near symmetrical below the waterline. This also applies to the sailing characteristics. When sailing astern, the pilot seat can be rotated, and the same controls can be used. For this reason, the throttles for the fore and aft thruster (on either side of the seat) are also placed bit further forward and aft respectively, thus allowing for two-hand operation without the risk for confusion.
The accommodation has two double cabins, a mess room and a galley, and stores on the lower deck, and three single cabins, a washing room and a toilet on the main deck. The accommodation, built by Hoogendoorn’s Maritime Boarding and Interior Design, houses seven persons.
The wheelhouse has ample glazing, and the aft part is placed slightly lower, amongst other reasons to avoid obstruction of sight. The navigation and communication equipment includes two radars and an ECDIS, and was supplied by Alphatron.
All around the hull, fendering with large cylindrical sections is installed. These fenders are hollow, allowing it to be compressed, while increasing the contact surface at the same time. A pushbow is provided at the stern for barge operations. The towing winch on deck is a SafeWinch from Kraaijeveld. This winch type features automatic control to keep the line tension within a certain safe range, thereby preventing slack lines and possible overloading of the line. The winch is electrically powered. There is sufficient free space on the aft deck to store a container.
With the hybrid configuration, Eddy 1 has all the power under the hood needed for its main tasks, but the running hours of her main engines will be significantly reduced, and the engines will not suffer from chronic underloading. The innovative propulsion arrangement fore-aft and the canoe-type hull shape ensure economic service, while her stability, reserve buoyance and watertight subdivision ensure safe operations.
This is not the first ‘spec’ build for Holland Shipyards. The company does the same with accommodation modules and is confident that not only Eddy 1 will soon find an owner but that many tugs will follow.