Ship drive trains are traditionally constructed with five components assembled in a series: diesel engines, gearboxes, propeller axles, propellers and rudders. This arrangement leads to certain limitations in ship construction.
Large diesel engines are generally optimised for a certain operating speed, set to match the ship’s intended cruising velocity. But many vessels, such as cruise liners and those that often pass through narrow passages, spend a good deal of time running at less-than-optimal machine speeds.
Diesel-electric propulsion, a relatively new way of powering ships, differs by consisting of an electrical power plant, usually with generators powered by diesel engines, and an electrical motor driving the main propeller. Diesel-electric drives provide better propeller torque characteristics making it the system of choice in applications such as icebreaking. The electric propulsion system is effective even at very low speeds, while the diesel engines operate at constant speed and at optimum load.
The Swedish-Swiss engineering concern ABB, the world’s largest maker of electric-propulsion systems, takes the newer drives a step further with its Azipod product line. The Azipod unit is fixed outside the ship in a casing, combining the functions of a propulsion motor, main propeller, rudder and stern thruster. These traditionally separately installed units are no longer needed, vacating space on board for other purposes.
The advantages are many.
Manoeuvrability is greatly improved since the entire casing can rotate 360 degrees.
The Azipod arrangement in a cruise vessel has been shown to reduce fuel consumption by about 10 per cent compared to diesel-electric propulsion systems with a conventional shaft-line arrangement, and further improvements are bringing that figure higher still. Shin Nihonkai, Japan’s largest ferry operator, installed Azipod on two new ferries in 2004 and reports fuel savings of 20 per cent at the same time load capacity is increased.
A less obvious but still important environmental improvement from the Azipod system is the enhanced security provided by better manoeuvrability and crash stop behavior. It’s impossible to measure in exact figures, but reducing the risk of shipping accidents benefits all who care about the sensitive ocean ecosystem.
Published in April 2009