Wärtsilä’s new WST-24R retractable steerable thruster beats its predecessor on a lot of levels: it comes at a lower cost, it is more powerful, efficient and reliable, it is easier to install, and requires less maintenance. And there’s more. Read on.
When Vincent Klous started thinking about how to renew Wärtsilä’s retractable steerable thruster family to make it more hydrodynamically efficient, by adding the eight-degree tilted propeller gearbox, he quickly realised it would mean more than simply changing the angle of the propeller shaft.
Tilting the pod, shaft line, propeller, and nozzle by eight degrees implied also a longer retraction length. This meant the thruster unit, if left unchanged, would take up more space on board the vessels such as shuttle tankers, offshore support vessels, and construction vessels, for which the thruster was intended.
This set Klous’ product development team in Drunen, the Netherlands, on a comprehensive redesign. Over a period of 18 months, they replaced the hydraulic retraction and steering mechanism with a ground-breaking electric one, which is currently being patented.
“We had to basically develop a completely new thruster, and change all the inboard parts as well,” Klous says. “So this whole system is as compact as possible to minimise the envelope space in the vessel.”
The new WST-24R thruster was launched at the Marintec China conference in December, last year. According to Michel van Veluw, head of Thrusters & Propulsion Control Systems, it is an improvement on its predecessor, the LMT-FS1510, across the board.
“We have improved on every area: performance, maintenance and operational costs, and also safety for our customers and yards,” he says.
In comparison to a conventional 90-degree thruster with a five-degree tilted nozzle, the eight-degree tilt, which Wärtsilä first introduced in the WST drilling series, significantly reduces the amount of thrust lost when the stream from the propeller hits the underside of the vessel’s hull. Experience and research show that eight degrees is the optimal angle to veer the jet (sufficiently) downwards to clear the hull without notably reducing forward thrust.
The tilt also cuts thruster–to-thruster interaction by deflecting the jet sufficiently to reduce ‘forbidden zones’, where thrusters cannot operate without blowing into the stream from other thrusters.
This means, together with the larger propeller and improved hydrodynamic design of the pod and nozzle, the WST-24 generates more than 10% extra unit thrust compared with the LMT-FS1510 and translates into 23% more efficiency during dynamic positioning operations.
The resulting extra efficiency improves dynamic positioning performance of the vessel, reduces fuel consumption and lowers operating costs.
Throughout the redesign process, Klous kept in close contact with ship owners and yards to incorporate their needs into the design, resulting in a thruster that is easier to install, maintain and repair.
“We can fully assemble and pre-align this thruster in the factory, so it will be a lot like plug-and-play when delivered to the yard,” van Veluw explains.
The thruster comes in a single delivery, and alignment is checked by a 3D laser during installation.
The retraction and steering mechanism, together with their combined sealing package, are also designed to be repaired and serviced on board without the need to go to dry dock. An inflatable seal keeps the water out when repairs are underway.
The composite bearing holding the propeller and gearbox, meanwhile, is designed to be serviced by divers in water, again avoiding expensive dock visits.
The unit’s lean design is also expected to result in greater reliability and less need for maintenance.
“We designed it also with a cost reduction in mind, so we reduced a lot of components and combined functionalities. Therefore, we believe it’s also more reliable and also more maintenance friendly,” Klous explains.
One of the prime ways the designers saved space was by replacing the two hydraulic systems used for retracting and steering the LMT-FS1510 with electrical systems.
Eliminating the hydraulic system means less risk of fluid leakage, which pollutes the environment and leads to a slippery, unsafe environment for crew. The design, then, saves further space by using a single electric drive controlling the two engines responsible for retracting and steering the pod.
“When you are retracting you don’t need to steer, and when you are steering you don’t need to retract. That saves both space and money,” Klous says.
The team worked with a major seal manufacturer to combine the two separate sealing packages used on the LMT-FS1510’s retraction and steering mechanisms into one single seal.
The new rotary seal, which has been made also suitable for linear motions, is considerably more watertight than the stuffing box seals on the LMT-FS1510. Thus, by eliminating one oil-sea interface, the new design further reduces the risk of marine pollution.
Perhaps the most significant advance is the new spindle retraction system Klous’ team developed, which is so innovative that the team filed for a patent last year.
The new system reduces the required diameter of spindles to lift the same load from 330mm to 80mm by keeping them loaded permanently under tension. This prevents buckling and makes the retraction system self-locking, removing the need for external locking hooks.
“We are doing that with a couple of spring sets,” Klous says of the system. “It’s capable of lifting the same load with a much lighter system, so that’s also a major positive.”
The long list of new design ideas in the thruster, together with the increased ease of repair and installation, has already brought attention from ship owners and yards. At the moment, the first potential customer is on the brink of signing an exclusivity agreement.
