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.