CLT propellers – Theorical fundaments
The Contracted Loaded Tip propellers (CLT) are screw propellers with important load at the blade tips thanks to the fitting of end plates at the blade tips that actuate as a barrier avoiding the communication of water between both sides of the blades, and the appropiated design of the blade geometry and the radial load distribution. The end plates are positioned so as to cause a minimum 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.
The tip plate is located on the pressure side of the blade with the aim to obtain a higher overpressure downstream.
The underpressure on the suction side caused by a CLT propeller is much lower than for an equivalent conventional propeller whilst the overpressure on the downstream side is much higher.
The first theoretical developments and results of model tests on these type of propellers were published in 1976, reaching to the conception of the so called TVF (Tip Vortex Free) propellers.The CLT concept was developed in 1986 as a further step ahead.
Since 1976 a continuous effort on R&D has been made on tip loaded propellers by SISTEMAR’s technicians. As a result of the same together with the feed-back gained from the results of the numerous sea trials carried out, the quality and the reliability of the CLT propeller designs made by SISTEMAR have been continuously improving.
The theoretical fundaments of CLT propellers are explained in detail in the book entitled “Detailed design of ship propellers” by G.P. Gómez and J.G. Adalid. The developments and theories used for the design of CLT propellers are fully explained and demostrated in said book in a comprehensive way.
CLT propellers – Advantages
Energy saving is a primary objective in the design of marine propellers; the increase of oil price and the more strict regulations in terms of air pollution and NOx/Sox emissions require more and more efficient designs. At the same time, requirements about radiated noise and vibration emissions in order to reduce the negative effects on marine life and the hydroacoustic signature of new commercial and navy vessels are the primary aims of any newbuilding.
CLT propellers represent a further opportunity to increase efficiency, reduce the risk of cavitation (and radiated noise as a consequence) without the employment of completely different geometries and propulsive solutions (ducted, contrarotating, etc.)
CLT propellers, thanks to the higher efficiency, help reducing the fuel consumption, the emissions and then both the Energy Efficiency Design Index (EEDI) and the Energy Efficiency Operational Index (EEOI) without requesting any modification to the vessel.
The main advantages of CLT propellers can be summarised as follows:
- HIGHER EFFICIENCY
- HIGHER SHIP SPEED
- LOWER FUEL CONSUMPTION
- REDUCTION IN CO2 EMISSIONS
- HIGHER RANGE
- BETTER MANOEUVRABILITY CHARACTERISTICS
- LOWER NOISE AND VIBRATION LEVELS
- LOWER OPTIMUM DIAMETER
- MUCH BETTER PERFORMANCE FOR CP BLADES IN OFF-DESIGN
- HIGHER CAVITATION INCEPTION SPEED
- HIGHER KT MARGIN AGAINST FACE CAVITATION
The underpressure on the suction side caused by a CLT propeller is much lower than for an equivalent conventional propeller, while the overpressure on the downstream side is much higher.
Theory demonstrates that the open water efficiency of the propeller increases with an increase in pressure differential between the overpressure aft of the screw and the underpressure forward of it.
The pressure drop on the suction side of CLT propellers is less than for the conventional equivalent and therefore the extent of the cavitation developed on the suction side is lower and hence the pressure forces that a CLT propeller exerts on the stern hull structure are lower. Additionally, CLT propellers have a reduced tip vortex because of the existence of the end plates.
The combination of these circumstances means that the pressure forces exerted by a CLT propeller on the stern structure are of a lower magnitude than for conventional propellers, and so in turn the induced hull vibration and noise levels on board are lower.The noise radiated to the water is also lower for CLT propellers.
As the blade area of the CLT propeller is more efficiently used for supplying thrust, the optimum diameter is lower than for an equivalent conventional propeller.
CLT propellers offer higher efficiency which may be used to achieve fuel savings at constant ship speed or alternatively ship speed increase at constant fuel consumption.
Trawlers and tugs achieve an increase of the pulling force.
The downstream overpressure produced by a CLT propeller is higher than for an equivalent conventional propeller. This increases the pressure on the rudder and so increases the ship’s response to rudder action, leading to a substantial reduction of the turning circle for any given rudder angle; a reduction of both time and distance required to stop the ship in a crash stop manoeuvre and an increase of the ability to mantain a rectilinear course.
Further to the above, CLT-CP blades have additional advantages when CP blades operate at constant rpm because the CLT efficiency, the cavitation performance and the noise and vibration levels in off-design conditions are much better than those of the alternative conventional blades.
The reason is that for conventional blades the radial pitch distribution at design condition is unloaded at the tip to avoid high levels of pressure pulses; when the blade operates in off-design conditions with pitch setting below the design pitch, the blade tip is too much unloaded and a negative pitch is achieved at blade tip. Under these circumstances the upper sections of the blade produce a negative thrust while the lower sections produce a positive thrust and as a consequence the propeller efficiency decreases and the pressure pulse levels increase, because a face cavitation is developed at the pressure side of the conventional blade leading to a broad band energy spectra and the noise and vibration level increases in spite of the reduced power delivered to the propeller in off-design condition.
The situation is not the same for CLT blades because the radial pitch distribution at design conditions bears an important load at the tip due to the existence of the end plates and therefore when the blade operates in off design conditions with pitch setting below the design pitch, the load at the blade tip decreases but still is positive for a wide range of variation of the geometrical pitch angle and so the propeller efficiency remains almost constant and the pressure pulses levels are lower than those of design condition because the power is lower.
The percentage of improvement on efficiency over an alternative optimum conventional propeller depends on the type of vessel, being larger for slow vessels and high block coefficient as tankers, bulkcarriers, etc. for which the thrust coefficient based on advance speed is higher.
CLT propellers – Design
The naval architects of the company have developed their own theoretical procedures to design any type of propellers. These procedures have been endorsed by more than 500 experiences at full scale and the theoretical fundaments are contained in the book entitled “Detailed design of ship propellers” by G.P. Gómez and J.G. Adalid published by FEIN.
The following propulsive solutions may be designed:
- Conventional propellers
- CLT propellers
- Conventional or CLT propellers inside a nozzle
- Contrarotating propellers (conventional or CLT)
- Tandem propellers (conventional or CLT)
The propeller blades of the different alternatives may be designed either in fixed pitch or controllable pitch versions.
The CLT concept is feasible for any type of pod arrangement.
The CLT propeller designs have been approved by most of the majors Classification Societies (LRS, ABS, BV, DNV, RINA, GL, etc.).
Before to perform a detailed design of a CLT or any other type of propeller it is necessary to carry out an optimisation procedure to modelize the ship performance. SISTEMAR carries out this type of calculations free of cost for the client in the case of CLT propellers and provides for each case an evaluation of the fuel saving that can be achieved by the CLT propeller if compared with an alternative conventional propeller.
MODEL TESTS WITH CLT PROPELLERS
Model tests with CLT propellers are performed in the very same way than with conventional propellers. The difference between both types of propellers comes in the scaling of the open water tests results because in the case of CLT propellers there are not only scale effects on the viscous forces of the end plates but also scale effects on the lift forces over the blades. These special scale effects must be taken into account when defining the parameters of the cavitation tests.
A complete extrapolation procedure for CLT propellers has been developed. The procedure has into account the severe scale effects on CL and CD coefficients of the blade annular sections of a CLT propeller, by introducing on the ITTC’78 extrapolation procedure some special corrections on the scaling of the CLT propeller open water curves.
The validation of this extrapolation procedure has been accomplished within the framework of a two years R&D project carried out by SISTEMAR with ASTILLEROS ESPAÑOLES, S.A. (nowadays NAVANTIA) and EL PARDO MODEL BASIN (CEHIPAR). The scaling procedure of CLT propellers model test results was presented at The Motor Ship Conference 2005.
A new R&D project has been developed also with IZAR (nowadays NAVANTIA) and CEHIPAR in order to set up a procedure to conduct accurately cavitation tests with models of CLT propellers.
It has been checked within the framework of this project that the underpressure upstream of the CLT propeller is higher at model field than at full scale and therefore, it is not acceptable to determine the parameters for the cavitation tests of a CLT propeller model following the traditional procedure because then the propeller operates during the cavitation tests under unreal and very pessimistic conditions.
As a consequence of this R&D project, a new procedure for cavitation tests with CLT propellers has been developed.
Both R&D projects above mentioned have been sponsored by the Spanish Ministry of Innovation and Technology.
Traditional theories for propeller design and extrapolation methods of model test results are not valid for tip-loaded propellers. This is the reason why SISTEMAR experts have made considerable efforts to develop their theoretical fundaments and procedures to deal with this type of propellers since 1976, when the thesis that a propeller with positively-loaded blade tips would have a higher efficiency than conventional propellers was proposed by Dr. G.P. Gómez.
As a further step within the development of this type of propellers, SISTEMAR has developed a new type of mean line for the blade annular sections. This mean line reduces the underpressure on the suction side of the propeller blades and increases the overpressure on the pressure side, leading so to a higher propeller efficiency together with a much better cavitation performance.
These improvements of the new mean lines have been ratified by the results of model tests carried out for a same ship with three different alternatives of mean lines.
Numerical calculations based on RANS solders and panel codes are been used nowadays for CLT propellers.
Manufacturing & Reparability
CLT propellers – Manufacturing & Reparability
It may be accomplished just by the following companies homologated and licensed by SISTEMAR :
- FUNDICIONES ADRIO (Vigo,Spain) www.adrio.com
- CASUSO PROPELLERS (Entrambasaguas, Spain) www.casusopropellers.com
- MECKLENBURGER METALLGUSS (Wismar,Germany) www.mmg-propeller.de
- WÄRTSILA IBERICA (Maliaño,Spain) www.wartsila.com
- ELICHE RADICE (Milano, Italy) www.elicheradice.it
- YUANHANG PROPELLERS MANUFACTURING CO. LTD. (Guangzhou, China) www.yhpropeller.com.cn
- FUNDILUSA (Cerveira, Portugal) www.fundilusa.com
- DALIAN MARINE PROPELLER CO LTD. (DALIAN, China). http://www.dmpp.cn/
- MENCAST MARINE PTE. LTD. (Singapore) www.mencast.com.sg
The procedure to manufacture a CLT propeller is similar to the one used for a conventional propeller with some differences concerning the mould, the govern of the flow of metal into the mould, the machining of the end plates, etc.
The ISO specifications for propellers together with the SISTEMAR norms are the criteria for assessment.
SISTEMAR performs a detailed inspection of each CLT propeller at the manufacturer workshop prior to its delivery.
The CLT propellers do not add an extra difficulty for repair works if compared with similar jobs performed to conventional propellers.
The repair jobs on a CLT propeller must be carried out just by those workshops homologated by the Classification Society for the repair of conventional propellers.
The procedures to be applied to the repair of a CLT propeller are the very same than those to be applied in the case of conventional propellers.
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