Wärtsilä’s new ferry propulsion concept features a direct-driven controllable pitch propeller and a contra-rotating propeller behind it mounted on a pulling mechanical thruster.
New design features pulling, steerable mechanical thruster directly aft of forward propeller
In its attempt to meet the market’s increasing demand for both better fuel economy and more environmentally friendly operation,Wärtsilä is following up its podded contra rotating propeller (CRP) concept, introduced some years ago, with yet another combination of a pod and a main propeller. The new concept features a pulling-type, steerable, mechanical thruster located directly aft of the forward propeller, which is mounted on the center line skeg.
“In this new concept the electric motor has been replaced by a mechanical drive system with two 90° bevel gears,” said Oskar Levander, general manager, Conceptual Design, Ship Power, Wärtsilä. “From outside, the setup looks very similar to the existing CRP arrangement with the electric pod. However, the obvious benefit of this new concept is that it allows for fully diesel-mechanical machinery offering a considerable lowering of the investment cost.”
The main propeller is a directly driven, controllable pitch (CP) propeller. Combined with the mechanically driven propeller behind it, the initial cost will be low by avoiding expensive electric pods and drives. The mechanical drive of the forward propeller using a large-bore main engine would further save in initial cost, according to the company.
“The fuel consumption can be reduced by more than 10% and by this amount also, emissions are reduced,” said Levander. “This new concept offers an attractive economic solution representing state-of-art technology that is still relying on wellproven and reliable components.”
The conventional large displacement ferry machinery used today is usually based on twin shaft lines, with twin CP propellers driven by two or four medium-speed diesel engines. A proven arrangement, yes, but according to Wärtsilä, shortcomings in this setup are the long shaft lines below the hull, which together with the other appendages give rise to high resistance. The new concept’s non-requirements for shaft lines result in lower power demands.
“The appendage resistance of a ferry can be as high as 10 to 15% of the total resistance,” said Levander. “Besides this, the risk of pressure side cavitation at lowspeed operation is always apparent with CP propellers running at a constant rpm, sometimes used in this type of mechanical propulsion. From an engine performance point of view, the operation on low load during maneuvering and slow speed is not optimal.”
Looking at the modern ferry tonnage, twin shaft lines with twin rudders are common and offer quite a good maneuvering performance. In order to satisfy the need for more side thrust than can be generated by the rudders alone, additional thrusters are installed in many cases. Medium-speed, four-stroke diesel engines dominate the ferry market and two-stroke engines are rarely used. The main reason for this is that the large cross-head engines do not fit beneath the main cargo deck. The trunk would interfere with cargo handling over the stern and would reduce the available lane capacity. Space is a design issue for ferries,as they are considered to be volumecritical more than weight-critical vessels.
“In the CRP concept with an electric pod, the pod propeller is of the fixedpitch type, while the main propeller has a controllable pitch. Compared to a conventional vessel fitted with twin screws, the podded CRP configuration offers better hydrodynamic efficiency, mainly because the resistance of the single skeg hull form with a single pod is lower than that of a twin screw hull with two shaft lines,” said Levander. “Then the aft propeller takes advantage of the rotative energy left in the slipstream of the forward propeller, when it rotates in the opposite direction. Eventually the skeg offers a more favorable wake than a shaft line, resulting in a better hull efficiency. Model testing has shown reduction in power demand at the propeller in the range 10 to 17%.”
A transparent side view of the vessel showing the new concept with a large main engine. The narrow and high machinery is well suited for a large main engine and could, for example, be a two-stroke engine or a large, medium-speed engine.
The first two ferries featuring podded CRP propulsion are the Hamanasu and the Akasihia operated by the Shin Nihonkai Ferry Line, Japan. The ferries were built by Mitshubishi Heavy Industries and have a top speed of 32 knots. Two, 12-cylinder, Wärtsilä 46, medium-speed engines in Vee-configuration drive a CP propeller, two similar 12V46 engines are driving the generators and one smaller gen-set is used for in-port operation. The total installed propulsion power is 42.8 MW with 17.6 MW on the pod and 25.2 MW on the forward propeller. Compared to the company’s conventional ferry fleet, a 20% reduction in fuel consumption can be reached. Still, these ferries are faster and take 15% more cargo.
Despite the very good performance of the CRP concept proved in the Japanese ferries, Wärtsilä has not seen any surge in new orders for this type of propulsion — while there have been plenty of orders for conventional ferry concepts.
“Most ferries are ordered in Europe and not all owners have the feedback regarding the performance of the Japanese vessels,” said Levander. “Another reason can be the first cost, which is higher for a CRP ferry than that of a conventional twin shaft ferry. The electric pod, for example, is expensive, but it is easy to show that the higher investment cost is paid back in a reasonable timeframe.”
The new Wärtsilä propulsion concept with a mechanical thruster drive offers a machinery arrangement, making it practically feasible for a ferry and also economically competitive in today’s market. A medium-speed diesel engine located in the center of the vessel, above the shaft line of the forward propeller, drives the aft thruster. This means that the engine is located higher up than what is common in a ferry. The engine compartment penetrates the main deck. The forward propeller is driven in the traditional manner by one or two engines. The engines are located in the center of the vessel at tank top level.
The machinery forms a compact unit — higher than normal, but much narrower. It can also be located further aft than in a twin shaft vessel, since the single skeg hull form allows the reduction gear to be located further aft. And the same is true for the thruster’s engine.“The new machinery arrangement does also provide some new options for engine selections,” said Levander. “The narrow and high machinery is well suited for a large main engine and could, for example, be a two-stroke engine or maybe the very largest medium- speed engine on the market — our Wärtsilä 64.”
With the conventional propulsion arrangement, two large engines would close up the entire beam of the main deck while with the new concept— with the engines behind each other — the two engines will take up only two lanes of the main deck. The shorter engine room will allow for a longer lower cargo hold, which compensates for the lost lane meters.
Shown is a view of a CRP RoPax ferry with a fixed ramp in the center leading to the upper cargo deck. A novel machinery and cargo ramp arrangement was developed to facilitate very large main engines that are too high to fit under the main deck.
Wärtsilä has made a few new ferry designs using different machinery solutions. One design is for the conventional twin-screw machinery, while the others have CRP propulsion arrangements but with different main engines. Five alternative designs were actually studied. All ferries include a 2400 m lane, 5150 ton payload and a speed of 24 knots.The ship designs were kept similar, but with the main dimensions varied to offer an optimized solution.
“The method was to start from the mission of the vessel, and then design a ship to meet this mission,” said Levander. “It would be wrong to start designing with a certain hull and then see how much cargo could be fit into it, after the machinery is placed.” The CRP machinery option shows an advantage when it comes to weight. Even with a low-speed engine the weight would be at the same level as with conventional twin-shaft machinery. The power demand of the CRP propulsion is about 9.5 to 11% lower than that of the twin-screw option. An alternative using the Wärtsilä 46 medium-speed engines, with their 400 ton less weight, offered even larger savings due to the reduced weight and smaller ship size.
Annual machinery-related costs are shown for four alternative machinery configurations. The annual operating costs are based on an assumed operating profile. Investment costs are turned into annual payments over 12 years with 6% interest. This graph was based on a fuel price of €230/ton. With a cost of €320/ton, being closer to today’s price level, the savings should be €1 170 000 instead of the €865 000 indicated in the graph for the CRP thruster with Wärtsilä 16V46.
The fuel consumption of the mechanical thrusters is higher than of a conventional shaft line, due to the two bevel gears. On the other hand, it only represents 25% of the total power. The main engine machinery in the CRP options is of a larger type than in the twin-shaft vessels, which also results in lower specific fuel oil consumption.
“Our comprehensive economic comparison between different options has indicated that the CRP options show clear savings in total annual machinery costs compared to a conventional twin-shaft vessel,” said Levander. “Most economical would be an option with our 16V46 medium-speed engine — an annual saving of about €1.2 million (US$1.7 million) can be expected. All in all, the new concept offers a very competitive solution that is a step in the right direction toward a cleaner ferry business.”