Stone Marine Propulsion to launch 'game changing' propulsion device
Adrian Miles, managing director of Stone Marine Propulsion, discusses the potential energy savings offered by the company’s Gate Rudder propulsion device
The MD of UK-based propulsion specialist Stone Marine chose his words with care. “The Gate Rudder could be a bit of a game changer”, Adrian Miles said.
The Gate Rudder is an innovative new propulsion device, which is in effect a ducted propeller system, that Stone Marine plans to launch commercially early in 2020.
“We are still doing development work on the product. The unconventional design of the product has complicated modelling work – the full-scale results are significantly better than the model test results had indicated.”
But the voyage data results after a year of operation aboard a 3,850 dwt containership plying a route along Japan’s coast were highly encouraging. Miles noted that the Gate Rudder had achieved 14% fuel savings in sea trials, and the voyage data savings were even higher at 25%.
Following the first gate rudder application, three Japanese newbuilding featuring Gate Rudder systems are currently underway, and are due to be delivered in the next 18 months. The projects under construction have all been negotiated by Japanese shipowners with the Gate Rudder consortium’s Japanese partners. The sea trial results of a larger size containership, and two coastal cargo vessels would help provide more data.
While the projects under construction are all smaller vessels, a larger ship is currently being planned. “A handysize bulk carrier is currently being designed”, Miles said.
Miles would be interested in discussing potential projects with European ship owners, but work on designing the system for larger vessels was ongoing.
Analysis of the results of the first year of operation of a small containership operating along the coast of Japan confirmed that the device had achieved savings of over 25%, surpassing the 14% achieved in sea trials.
With the support of the Nippon Foundation, the first Gate Rudder system had been fitted to a container ship Shigenobu, which entered service in 2018. The voyage data from the vessel’s first year of service revealed significant energy savings of above 25%, compared with the voyage data of an identical container ship Sakura, fitted with a flap rudder.
The Sakura is one of the best container ships designed by Yamanaka shipyard who developed the hull form not only based on numerous model tests but also based on the recent CFD technology. The Shigenobu was built one year later than Sakura with the same hull form and the same engine, a Hanshin LH46LA (low speed four-stroke 6 cylinder) diesel engine. The difference between the two vessels is only the rudder system, including the propeller.
Miles commented that “The results have exceeded expectations. In fact, nobody could believe how well the system has performed”.
Upon entering service in January 2018, Shigenobu mainly operated along the same Tokyo-Tomakomai route along the north-east coast of Japan as a near-identical sister vessel, Sakura. This permits analysis of the Gate Rudder’s performance based on comparisons between the two vessels’ performance in identical conditions.
The Gate Rudder also recorded superior performance in heavier weather, especially in head sea conditions. Shigenobu was reported to be very strong against wind and waves such that the vessel can enter and berth in a stormy port even when the wind is blowing at over 20m/sec, while other vessels waiting for calmer conditions. Extremely stable stopping and astern motion are also experienced with the gate rudder system on Shigenobu.
“These improvements are additional benefits from the device, which was designed to improve fuel efficiency”, Miles said.
Gate Rudder Concept
The Gate Rudder system replaces existing rudder systems with a completely new ducted propeller system, which produces additional thrust rather than additional drag. This is a new type of ducted propeller, distinct from Closed Type Ducted Propeller systems or Front Type Ducted Propeller systems, such as the Mewis Duct.
“It isn’t really correct to describe the Gate Rudder system as a rudder. It is a combined propulsion device with the effect of a Ducted Propeller and a Stern Thruster”, Miles said.
The gate rudder system concept is very simple: simply substituting a conventional rudder with a rudder blade fitted on each side of the propeller.
The patented device was originally invented by Chairman Sadatomo Kuribayashi of Kuribayashi Steam Co., and long-time Stone Marine collaborator Noriyuki Sasaki, a visiting Professor at the University of Strathclyde (and formerly, senior research director of NMRI Japan).
Noriyuki Sasaki told The Motorship that nobody had noticed this simple idea before because conventional modelling techniques were unsuited to unconventional propeller-rudder configurations.
Sasaki added “The resistance of the gate rudder measured in the towing tank was quite high and 5-10 times, relatively, compared to full scale (scale effect). This gave the wrong conclusion for the model test result.”
Sasaki noted the decision to develop the gate rudder was taken despite the fact that the test results achieved from existing model testing methodologies were unsupportive. Sasaki credited Nobuhiro Asaumi, the chairman of Yamanka Shipbuilding Co. to commission the gate rudder and propeller from Kamome Propeller Co. Ltd.
Lower propeller thrust
The propeller in a gate rudder system requires much smaller propeller thrust than that of a conventional rudder system because the rudder is changed from a resistance device to a device generating thrust. This also reduces the hull interaction force which is known as a thrust deduction factor.
A second factor is also at play. “The gate rudder also works like the sails of a sailing ship in the water. The propeller increases this sail performance using so-called USB (Upper Surface Blowing) technology like an aeroplane wing, while the conventional rudder works in the deflected flow of a propeller slipstream which deteriorates the sail performance,” Sasaki said. “We can expect the same effect of the gate rudder for a rolling motion.”
One of the reasons that the results had exceeded expectations was that conventional modelling techniques were unsuited to unconventional propeller-rudder configurations.
“The resistance of the gate rudder in the model test was quite a bit higher than we have actually seen in full size tests”, Miles said. This was also having an effect on modelling the expected performance of other vessels. In addition, the Gate Rudder’s greater volume than some comparable rudder systems means careful design work is required for larger vessels.
“We still have to do some work before we can think of installing this on a 20,000 TEU containership”, Miles said.
Miles noted that the introduction of a number of other vessels fitted with a Gate Rudder would expand the amount of operational data available for analysis. In addition to three smaller vessels under construction, a 38,000 dwt panamax bulk carrier was currently at the design stage.
The efficiency improvements required to meet criteria for Phase 3 of the Energy Efficiency Design Index (EEDI) mean that the Gate Rudder is likely to attract attention from ship owners and naval architects.
Miles was reluctant to discuss the higher efficiency savings of over 25% seen in the voyage data, as the results were route specific and based on one vessel. He preferred to state that the “guaranteed minimum” of 14% fuel efficiencies seen in the sea trial should interest ship owners.
Miles noted that optimising the hydrodynamic design of the stern of a vessel might permit naval architects to eke out additional fuel efficiencies from the Gate Rudder. “The main savings come from the propulsion device but further flow optimisation into the Gate Rudder is possible”.
Miles added that the Gate Rudder could offer additional operational benefits for ship owners beyond fuel efficiencies. The shorter overall length of the propulsion system means a commercial vessel could increase its cargo carrying capacity by 3-4m without increasing the overall length of the vessel, Miles noted.
The Motorship noted that the improved fuel efficiencies offered by the Gate Rudder might permit derating the main engine size, without impacting on operational performance. This would also permit the use of lighter propeller shaft systems.
One particular advantage of the gate rudder system is that it can eliminate the requirement for a stern thruster. Sasaki had identified added the possibility of replacing the stern thruster, offering wider design capability”. The system is extremely good for coastal vessels which need frequent berthing.
PRINCIPAL PARTICULARS— Shigenobu
Length overall 101.90m
Breadth, moulded 17.80m
Speed (maximum) 16.20 knots
Main engine power 3,309kW
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