Wärtsilä 5500 kW transverse thruster for Oasis class cruise vessels of Royal Caribbean International.

New WTT-40 transverse thrusters: more power and efficiency for cruise ships

Responding to market demand for next-generation transverse thrusters operating with greater power and efficiency, Wärtsilä launched the new WTT-40 model for cruise ships.

Text: Joost van Eijnatten Photo: Wärtsilä

Introduced in the spring at the Seatrade Cruise Global convention in Fort Lauderdale, Florida, the WTT-40 is the first Wärtsilä transverse thruster specifically designed for the 4000 kW power segment, and it is available with a controllable pitch propeller or a fixed pitch propeller. It carries a number of improved features, in the propeller diameter, lubrication system, hydraulics system, manoeuvrability, and noise and vibration reduction, all of which increase the thruster’s efficiency while lowering costs for customers.

WTT-40 CP: Wärtsilä 4000 kW transverse thruster
WTT-40 CP: Wärtsilä 4000 kW transverse thruster.


How thrusters work

Transverse thrusters provide hydrodynamic thrust in a direction perpendicular to the ship’s longitudinal axis to counter environmental forces like wind, waves and currents. This kind of thruster propulsion is applied in three primary ways: 1) to manoeuvre a large container, cargo, merchant or cruise vessel in harbour to help it dock; 2) to enable, together with the main propellers, cruise vessels to remain in a fixed location in places where they are not permitted to drop anchor – such as, in the area of a coral reef – while allowing the vessel to lower smaller boats for tourism or diving excursions; and 3) to serve a ship’s dynamic positioning (DP) system out on the open ocean, such as on large offshore support vessels and offshore construction vessels working in heavy sea conditions.

Depending on the overall propulsion configuration, cruise vessels can have between two and six transverse thrusters, which are installed in the bow or in both the bow and stern of the ship. The thrusters are located close to the centreline in tunnels running from portside to starboard, operating in both directions. In cruise and merchant ship applications, the transverse thrusters are used up to a few hours per day for harbour manoeuvring purposes. The thruster usage increases for cruise vessels that have station-keeping capability, while maximum use of the thrusters is made in DP systems with a significant number of running hours per day.

Wärtsilä 5500 kW transverse thruster for Oasis class cruise vessels of Royal Caribbean International.
Wärtsilä 5500 kW transverse thruster for Oasis class cruise vessels of Royal Caribbean International.


New degree of power

Previously, standard transverse thrusters had an available power range of up to 3500 kW. Wärtsilä specially designed a 5500 kW transverse thruster to be used on the world’s largest cruise ships, like the Oasis of the Seas. But the WTT-40 is the first Wärtsilä transverse thruster that extends the standard power range upward, mainly intended for the cruise market, but also suitable for application in DP systems. Due to its smart design and more efficient use of components, the WTT-40 is more reliable and also more competitive than previous transverse thruster models.

As cruise vessels become larger, they require larger thrusters in the bow and stern for manoeuvring; the higher and longer the vessel, and the more mass it has, the greater the wind force and other pressures it faces at sea, requiring stronger thrusters to counter with more propulsion. To meet the growing demand for stronger thrusters, ship designers have two options. They can either install more of the same-sized thrusters on their cruise ships, or they can apply larger thruster units. A standard cruise ship can be equipped with four 3000 kW thrusters in the bow, providing 12 MW of manoeuvring power. Now, by producing a 4000 kW thruster, ship designers can install only three larger thrusters that generate the same power as four smaller ones.

In addition, the WTT-40 thruster highlights the importance of a controllable pitch propeller in this market. Transverse thrusters in cruise vessels can have two types of propellers: a controllable pitch propeller or a fixed pitch propeller. With a controllable pitch propeller, the angle of the blades, or pitch, can be changed using hydraulic pressure to control propeller power and thrust while the propeller rotates at constant speed. By contrast, the blades of a fixed pitch propeller have a fixed angle relative to the water flow. Therefore, a variable frequency drive is needed to control the speed of the propeller and determine the thrust.

Transverse thrusters that use controllable pitch propellers and fixed speed electric motors are most common in cruise vessels for several reasons. They fulfil the propulsion needs while requiring a low capital investment, consume limited space in the vessel, and have a low number of installed components onboard. Wärtsilä encourages cruise ship designers and owner companies to consider investing in transverse thrusters with fixed pitch propellers, due to their greater efficiency and reliability, and the fact that they produce less noise and vibration. In the long term, these benefits may offset the initial higher investments due to the addition of the variable frequency drive. However, hydrodynamic and mechanical system advances in the WTT-40 thruster series bring the transverse thruster with controllable pitch propeller to a higher level, as explained below. 

Back view in the dusk of Oasis of the Seas.
Back view in the dusk of Oasis of the Seas.


Innovative features

Next to the higher power level, the first feature that distinguishes the WTT-40 from previous thruster models is its propeller diameter. Whereas standard 3-3.5 MW transverse thrusters employ propellers with a three-meter diameter, the new thruster uses 3.4-meter propellers for the first time. These larger propellers are more efficient and can provide more thrust than a 4000 kW thruster with a smaller propeller. Furthermore, using only three thrusters, rather than four, means that fewer tunnel openings need to be created in the ship, and less installation work is involved. Even with the larger propeller diameter, the layout for the vessel is more efficient because three can be installed closer together than four thrusters. This is an advantage since the closer they are installed to the bow of the vessel, the more effectively they can counter the impacts of wind, currents and waves.

A second key improvement of the WTT-40 involves the integrated lubrication system. Wärtsilä has designed a new hydraulic system that combines the lubrication of the thruster’s gears and bearings with the hydraulics for the pitch actuation of the propeller. By combining these two systems, the conventional hydraulic power pack – consisting of a tank, with pumps and valves mounted on top of the tank – needed to operate the propeller pitch is redundant. Thus, the extra space and work to arrange the piping from the tank to the thruster is no longer needed. For the new WTT-40 model, Wärtsilä combined the lubrication systems so that the hydraulic setting for the propeller is built directly on the steel construction of the thruster itself. Now, when the company delivers a part of the ship’s tunnel in which the thruster is installed, it also delivers the hydraulic system as part of a compact and integrated system. In addition to saving time and space in the vessel, the integrated lubrication system makes the transverse thrusters easier for shipyards to install. But it is also easier to maintain the system because all of the components are located in one place, meaning that filter changes or inspections happen in one location.

Wärtsilä initially introduced the integrated hydraulics for lubrication and pitch setting on its smaller thruster models in 2014, starting with 400kW thrusters used for cargo vessels and extending through 3500 kW thrusters used for cruise vessels. Now, because the concept proved successful, it has been extended to larger models like the WTT-40, which is the first thruster of its size to employ the integrated hydraulic system.

Another innovation concerns the thruster’s propeller shaft seal. This seal keeps seawater out of the vessel and lubrication oil inside. The function of the propeller shaft seal can be periodically monitored using the small header tank that is connected to the system. Since the propeller shaft seal is subject to wear, it will require replacement after a period of five to 10 years. Now, with the WTT-40 model, the entire seal package can be easily exchanged, as the propeller is designed to be taken off in a simple way. The complex process to replace propeller shaft seals is streamlined, and this results in significant time savings during maintenance of transverse thrusters with controllable pitch or fixed pitch propellers.

Wärtsilä transverse thruster range.
Wärtsilä transverse thruster range.


Additionally, the WTT-40 thrusters are designed to comply with environmental legislation. In the United States, Environmental Protection Agency regulation rules on sea and inland waterway pollution, issued in 2013, require companies to use Environmentally Acceptable Lubricant (EAL) if there is any interface between lubrication oil and seawater, as is the case with the propeller seal for thrusters. This way, in case of an emergency oil spill or leak, the biodegradable lubricant released into the water is deemed environmentally acceptable. In response to these regulations, Wärtsilä has designed all of its new thrusters, including the WTT-40, in such a way that they can be optionally equipped with EALs, instead of the mineral oil normally used in thrusters.

As a further improvement, the WTT-40 thrusters contribute to the reduction of noise and vibration levels. Constant speed propellers turn in the water at nominal speed, at all times, which can result in noise and vibrations whether the thrusters are being used or not. In a cruise vessel, controlling noise and vibrations is extremely important, and Wärtsilä has several options to support ship designers in addressing the issue. The most effective way to reduce noise and vibrations is by switching to a unit with a fixed pitch propeller with variable speed control. For both controllable pitch and fixed pitch propellers, a larger propeller and a careful design of the shape of the tunnel in the vessel to optimize the flow helps manage noise and vibrations. A good tunnel design not only benefits ship owners but also supports the shipyards building the ships. The tunnel optimizations are done using computational fluid dynamics (CFD) analysis. The same tool and accompanying industry leading knowledge has been used for the design of the thruster gearbox shape and propeller, resulting in the most efficient, best-performing transverse thruster series to date. 

Further Reading:

Cruising the world with Wärtsilä

The new Wärtsilä Steerable Thruster family

Wärtsilä’s next generation underwater mountable thrusters: efficient and easy to install

Joost van Eijnatten, Manager, Application Engineering, Wärtsilä Marine Solutions, email: joost.vaneijnatten@wartsila.com

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