CODED powers up

Despite pods suffering from their problems, there has been no let up in the advance of this technology.

There have been a number of interesting developments of late, such as the two high-speed conventional ferries ordered by Shin-Nihonkai Ferry and due for delivery form Mitsubishi?s Nagasaki yard in mid-2004. The ferries will utilise a contra-rotating propeller (CRP) pod concept with a Combined Diesel-Electric and Diesel Mechanical (CODED) machinery plant to give a trial speed of 31.5 knots. ABB will see this as the first opportunity to test its concept since research was started by KMY Technology in 2000. This continued with RoPax model tests at Marintek in 2000 ULCV, model tests at Samsung in 2001, cavitation tests at Krylov in 2002 and steering characteristics tests at VTT in 2002. LNG tanker model tests were carried out at SSPA this year

ABB, KMY Technology and Wärtsilä are also involved in the Enviropax programme to develop an environmentally friendly Ropax vessel that will utilise the CRP pod concept. Thomas Hackman, marketing manager for ABB?s marine group, told The Motor Ship that as the total fuel economy was being looked at the main propeller would take about 70 per cent of the load and the pods around 30 per cent. He explained the reason behind the lower load on the pod was that the pod could be turned to rudder angles of 7% without hitting the vortex of the main propeller. He also pointed out that electrical transmission costs more than mechanical transmission. The tank test on the project models have not yet been performed but Hackman expected they would be later this year.

Mika Laurilehto, Wärtsilä marine general manager, highlighted the advantages of using pods in a hybrid CODED propulsion system at The Motor Ship?s propulsion conference this year.

In the case of the cruise ship he pointed out that the next generation of ships are likely to be faster and larger than current vessels and, have higher hotel loads. The service speed of vessels has been steadily rising over the years in an effort to move the home port closer to the customer, increasing the transit distance to and from the cruise area. It is for this reason that higher transit speeds are required. When the home port is closer to the passenger, the air trip before and after the cruise can be avoided and, saving the air fare more than pays for the increased fuel used on the ship.

Panamax vessels over 100,000g have been proposed, which would stretch the main dimensions imposed by the Panama Canal to the limits. A greater emphasis on safety is also likely to mean a greater need for redundancy within the propulsion system. This redundancy has been located in at least two different watertight compartments in some recent vessels and, Wärtsilä believes that this is a trend that should continue. It says the next step is to maintain the propulsion power with any two adjacent compartments damaged by either flooding or fire. This means that the engines and their auxiliary systems have to be located in two non-adjacent compartments.

Laurilehto told delegates that Wärtsilä has developed a new propulsion concept for the next generation of fast cruise ships. The concept, which has been patented by the engine manufacturer, features a Combined Diesel-Electric and Diesel-Mechanical (CODED) power plant with a hybrid propulsion arrangement consisting of two electrical pods and one mechanically driven feathering propeller.

Laurilehto highlighted the advantages of the hybrid system, which he says offers lower fuel consumption and better propulsion efficiency for high-speed ships. This includes lower transmission losses than for diesel electric but higher than diesel- mechanical. However the system does offer the same operating benefits as for the diesel electric ship, such as the engines running at optimum load, more efficient use of the installed power and, flexibility on the location of the engines.

Wärtsilä says the ability to split the load between three propellers instead of two yields better propeller open water efficiency, especially in high power applications.

This also allows smaller and more reliable pods to be used.

The single-skeg hull form without any open shaft lines also has lower resistance than a twin shaft line arrangement. The vessel will have excellent manoeuvring characteristics, says Laurilehto, making the need for side thrusters in the stern obsolete.

The concept uses a mechanically driven feathering, centreline propeller at high speeds, which is feathered at lower speeds and, when feathered, it offers considerably reduced resistance than a windmilling conventional propeller. The vessel is driven by the pods alone at low speeds, which ensures that the diesel engines are never operated at low loads, which is not fuel efficient. Wärtsilä says that the risk of pressure side cavitation is also reduced since the CP propeller is never operated at low pitch and high rpm. The engine manufacturer says that these potential problems have been some of the drawbacks normally associated with conventional diesel-mechanical propulsion for ships operating at a wide range of speeds.

The concept also gets around the problem associated with steering with pods in high seas, which has led to a greater speed loss than with a conventional propulsion system and rudders. The hybrid concept uses a Lips Efficiency Rudder mounted behind the centreline propeller, which is used for steering at high speed, with the pods locked in place. Wärtsilä says that the rudder offers other benefits, such as an improved wake field, which results in lower pressure impulses ? a considerable benefit in a cruise ship where low noise is important. As the rudder is only used at high speeds, it can be kept relatively small to keep resistance low, further improving the propulsion efficiency.

The concept is being showcased on a design for a 90,000g Panamax cruise ship that has a service speed of 25 knots. Wärtsilä says that at 25 knots, the delivered power demand at the propeller, including a 15 per cent sea margin is 41MW. However, it has opted for 49MW, consisting of a 25MW centreline propeller and two 25MW pods to meet the redundancy target, which demands that the vessel maintains its service speed with any one engine out of operation.

Wärtsilä has calculated fuel consumption costs based on a cruise ship operating from Miami to the east Caribbean islands with a 25 knot transit speed. The engine manufacturer?s calculations have shown the CODED hybrid machinery concept to have the lowest fuel cost because its propulsion power demand and transmission losses are lower than for diesel-electric. However, while the diesel-mechanical has the lowest transmission losses, it suffers from the highest power demand. The CODED concept also offers a lower capital cost.

Wärtsilä claims “the new CODED machinery concept together with the hybrid propulsion configuration, with two electrical pods and one mechanical CP propeller, will clearly offer higher performance for the next generation of high-speed cruise ships. Compared with a twin-pod solution, the new concept can offer better hydrodynamic performance, equally good manoeuvring and much better economics. Furthermore, the proposed arrangement, with its safe return to port philosophy, establishes new safety standards for future passenger vessels.”