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Hi-tech RoRos boast green image

The newly coined combi-carriers, Robin Hood and Nils Dacke, ordered by TT Lines from the Rauma yard of Finnyards in Finland are to boast a wealth of technical innovations intended to make them more efficient, economical and environmentally friendly than other RoRo passenger ferries to date.

With an innovative cargo handling system for three cargo decks and a diesel-electric propulsion system — reportedly a world first for a RoRo each 589-ft. (179.6-m) long vessel has a carrying capacity of 155 trucks, 317 passengers and 35 crew.

Passengers are accommodated on the upper decks to increase safety, also allowing designers to make the best use of space for trucks and trailers. The cargo decks are designated as main, upper and lower, the latter being a large easily accessible cargo space below the main deck. With two large double-lane hoistable ramps to the upper deck and a fixed ramp to the lower hold — at each end of the ship — all three levels can be reached directly from quayside. The decks provide about 7,872-ft. (2,400-m) of 10-ft. (3.1-m) wide trailer lanes in a totally drivethrough arrangement — even to the lower hold thus reducing storage losses dramatically. Other layout advantages include optimized cargo flow and the ability to load and unload all decks simultaneously, a design feature which results in shorter docking periods.

Making the most of the lower hold space, within the restrictions of SOLAS requirements for watertight side compartments of at least one third of the ship's breadth on each side, was given high priority. Space was maximized by: choosing a diesel-electric propulsion system with the diesel engines located in side compartments next to the lower hold; locating the access ramps as close to the ends of the ship as possible; and installing a hydraulically operated end-hinged part-ramp in the front. The result is believed to be the creation of the biggest lower hold to date for vessels of this type with access from both ends of the ship.

Although the diesel-electric propulsion system was chosen to save space, the choice also brings a host of other benefits. Operating with one central power station to serve propulsive and auxiliary purposes has reduced diesel engines by 50 percent. Each vessel is fitted with four Krupp Mak 552C medium-speed diesel generators with a total power of 19,000 kW supplying two STN Atlas Elektronik electrical propulsion systems. The latter are of synchro-converter design, each having an output of 6,500 kW at 120-135 rpm. The system has also obviated the need for auxiliary diesels and reduced ancillary systems such as fuel, lubricating oil and cooling systems accordingly. The diesel package requires less maintenance by having fewer spare parts (all engines are of a single type) and reduced components. The electrical system is said to be much more reliable than conventional diesel-mechanical systems. Control and maneuverability are also enhanced.

The speed range of the electric propulsion motors can be adapted accurately to the range required by the propellers, so no reduction gears are needed. The entire package is designed to run quietly with low vibration levels. The speed of the motors can be controlled continuously forward and backward from zero to maximum rpm, providing constant high torque throughout nearly the entire range. Easy reverse and control of the motors results in excellent maneuverability and allows the use of fixed-pitch propellers. The propulsion system also constitutes part of a package which can genuinely be described as environmentally friendly. Running the diesel engines at constant speed keeps exhaust gas emissions at a low level in all operating conditions. According to TT Lines, constant speed operation means: the emission of nitrogen oxides is up to 50 percent lower under part-load conditions (for example, when maneuvering in harbor and during estuary trading); emissions of carbon monoxide and hydrocarbons are lower; and soot production is greatly reduced.

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