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AdvancedpropellerdesignssuitslowrevvingenginesAdvanced propeller designs such as the NPT, Kappel and CLT find their worth with ultra slow speed diesel engines, writes Wendy Laursen
dvanced propeller designs are sometimes criticised for being expensive and difficult to manufacture, but they are facilitating the move to ultra slow speed
diesel engines which offer improved propulsion efficiency and therefore reduced emissions. Design theory dictates that efficiency improves if low rpm engines are combined with larger diameter propellers but conventional propeller designs can result in an oversized, impractical solution. Advanced designs then, offer an alternative for the new range of longer stroke and lower engine speed engines under development by manufacturers such as Wärtsilä and MAN Diesel and Turbo. Stone Marine Propulsion is currently working on orders and developing designs to combine its ‘new profile technology’ (NPT) propellers with the new technology engines from both of these manufacturers. “For a given rpm the NPT is smaller than a conventional propeller,” says Toby Ramsay, technical manager at Stone. “Or for the same diameter a lower, more efficient rpm can be selected. Gains of up to 10% are achievable if the engine rpm can be optimised to suit the NPT optimum diameter in this way.” The secret to the improved performance of the NPT propeller is in the special blade sections, says Mr Ramsay. The blades were developed jointly in Japan and the UK by Dr Noriyaki Sasaki, formerly chief hydrodynamicist at Sumitomo Heavy Industries in Japan. Much of the research was done at the UK’s Newcastle University. Stone claims the NPT propeller has higher efficiency than conventional propellers (around 4%), but without the complex tip geometry and the high production costs associated with other high efficiency propeller types. A smaller diameter and blade surface area than conventional propellers means a lower weight, which is better for the shafting design, and bigger hull clearances, which result in lower vibrations. “The cost of the NPT is the same or cheaper than a conventional propeller because of this weight advantage and the lack of complex geometry,” says Mr Ramsay. Around 50 NPT propellers have been supplied by Stone Marine Propulsion. Last year, sea trials of a 32,000dwt bulk carrier delivered to DS Norden by Jiangmen Nanyang Shipyard demonstrated power savings of about 6% compared to a sister vessel with a conventional propeller, both ships having a standard, not low revving engine. Stone is currently working on several orders to supply optimal diameter NPT propellers for vessels that will have very low revving engines installed. In these cases, the equivalent conventional propeller would not fit the ship unless it was heavily restricted from its optimum diameter. “We are seeing an increasing number of newbuilds that have a lot less power than corresponding ships built a few years ago and the reason for that is to try and save fuel. They were basically overpowered before so power is being trimmed right down to a minimum now, and these new engines are part of that trend.”
It is possible to install boss cap fins on the NPT propeller for further energy savings, and rudder bulbs can also be used. MAN Diesel and Turbo took ownership of the Kappel propeller, another advanced design, in April this year. The technology will be implemented across a range of fuel saving and energy optimisation solutions including the company’s G‐type ultra‐long stroke low speed engines, rudder bulbs, high‐efficiency rudders, hull flow guiding devices and ducts. MAN Diesel and Turbo claims that by combining a Kappel propeller with a G‐type engine, fuel consumption can be reduced by up to 10%. Fuel savings of up to 6% are possible with just a Kappel propeller and conventional engine compared to conventional propeller designs. This is achieved through a reduction in pressure impulses which in turn means that a bigger and more efficient propeller can be used with reduced clearance between hull and propeller tip. The Kappel design minimises water flow over the blade tip as a result of tip vortices which form due to the difference in pressure between the pressure and suction side of the propeller. With flow over the tip minimised, the outer region of the Kappel propeller maintains a higher efficiency. “Tip clearance is one of the parameters which determine how large a propeller diameter can be fitted to a specific vessel and as a larger propeller will have a better open water efficiency, designers seek to maximise the propeller diameter,” says Karsten Borneman, senior sales manager for propellers and aft ship systems at MAN Diesel and Turbo. “An advantage of the Kappel propeller is that the propeller induced pressure impulses are lower than conventional propellers. Today we normally suggest using the same Kappel propeller diameter as a conventional propeller as the design is often based on a conventional propeller, but for future projects where the initial vessel design can be based on Kappel propellers, the propeller diameter can actually be slightly larger with less tip clearance and still keep same pressure impulses, hereby offering the largest possible propulsion efficiency.”
A
Royal Belgian Institute of Marine Engineers
MAN Diesel and Turbo optimises its propeller designs to suit individual ships and takes a holistic approach that includes specified maximum continuous rating (SMCR) fine‐tuning, turbocharger layout, PTO solutions and boosting solutions. Via its VibraSafe concept, the company also takes a holistic approach to vibration control. This includes main engine turning, shaft line dimensions, propeller blade number and phasing of propeller and firing order. Computerised Optimisation of Propulsion Systems (COPS) is an expert system that ensures more accurate pre‐optimisation of hydrodynamic and mechanical aspects of a propulsion system by integrating statistical data and torsional vibration data. Mr Borneman sees big fuel saving potential in replacing existing propeller designs with the latest state‐of‐the‐art propeller designs especially if the vessel profile is changing, for example due to slow steaming. MAN Diesel & Turbo has established a special department which looks into these retrofit matters and claims the payback time is often less than two years. Sistemar offers another advanced propeller design, the contracted loaded tip (CLT) propeller, which the company claims is tip‐vortex free so the downstream overpressure produced by a CLT propeller is higher than for equivalent conventional propellers. This increases the pressure on the rudder and increases the ship’s response to rudder action. The propellers have end plates fitted to the blade tips on the pressure side of the blade and act as a barrier avoiding the communication of water across the tips. The end plates are positioned so that they cause minimal viscous resistance and therefore are parallel to the incoming flow and shaped to the relative motion of the water, taking into account the fluid vein contraction. CLT propellers, due to their higher efficiency, help reduce fuel consumption, emissions and both the energy efficiency design index and the energy efficiency operational index without requiring any modification to the vessel, says Juan González‐Adalid, managing director of Sistemar. AP Møller‐Maersk and Sistemar are currently involved in the TRIPOD research project (TRIple Energy Saving by Use of CRP, CLT and PODded Propulsion), together with a number of other organisations including ABB, VTT, Cehipar and Cintranaval‐Defcar. Currently most ship propellers installed on cargo vessels are thought to waste about 40% of the energy generated, through rotational losses in the wake,
vortex generation, noise production and cavitation. The project aims to contribute to a more environmentally friendly use of energy by improving propulsion efficiency and is studying the installation of CLT propellers and propulsion pods on large container vessels in a contra‐rotating configuration. These three existing technologies have been used separately and are known to improve
overall ship propulsion efficiency compared to conventional propulsion. However, they have never been combined together in a single propulsion package. Results from the project are expected next year. CRUISE SHIP GAINS 10% EFFICIENCY Rolls‐Royce has designed a special propeller hub for its standard Kamewa propellers which increases thrust by recovering previously wasted energy from the flow of the hub vortex. In conjunction with a rudder bulb, the system can be retrofitted to single or twin screw vessels. This Promas Lite system is claimed to improve propulsive efficiency by 5% to 15%. The company says that vessels that have changed their operating profile so that the propeller blade specification is no longer ideal, or propellers that were designed without computational fluid dynamics techniques, have the most to gain. Norwegian Cruise Lines’ 78,000gt vessel Norwegian Sun was fitted with Promas Light in January. The existing 5.8m twin Kamewa controllable pitch propellers were fitted with new blades and hub caps. Rudder bulbs were also fitted. Efficiency was improved by more than 10 per cent at cruising speeds of between 17 and 21 knots. Norwegian Cruise Lines has now placed an order to upgrade Norwegian Spirit during the vessel’s next drydocking. ICE DESIGN LEADS TO LARGER PROPELLERS Steerprop of Finland is conducting research into the use of larger and more powerful azimuthing dual‐end contra‐rotating propellers (CRP) in both open water and ice‐going applications with the aim of bringing the improved efficiency of CRP propulsion to even larger vessels operating even further north. Steerprop has conducted a series of ice basin model tests together with Aker Arctic Technology involving propulsors with simulated power ratings of 2.5MW, 6.5MW, 15MW and 20MW in a variety of vessel type and propulsion configurations. The development process together with the ice basin tests revealed a number of new possibilities to use the higher thrust in addition to better efficiency of CRP propulsors in comparison to the traditional single‐propeller propulsors currently used in ice‐going applications. In particular the different nature of the CRP propellers’ slipstream has provided novel possibilities in ice‐management operations that will be further tested in upcoming ice‐basin tests. Source: www.motorship.com Aug 2012