R & D Activities
R&D JOINT ACTIVITIES OF NAVANTIA, CEHIPAR AND SISTEMAR
In 1997 a strong co-operation among Astilleros Españoles, CEHIPAR and SISTEMAR started with the R&D project entitled "OPTIMIZATION OF SHIP PROPULSION BY MEANS OF INNOVATIVE SOLUTIONS INCLUDING TIP PLATED PROPELLERS" (1997-1999). At such time IZAR and NAVANTIA were not yet created.
One of the goals obtained was to develop a procedure to extrapolate at full scale model test results with CLT propeller models. The necessary empirical know-how was deduced in such a way that nowadays CEHIPAR and SISTEMAR can extrapolate at full scale model test results with CLT propellers with the same degree of accuracy than in the extrapolation at full scale of model test results with conventional propellers.
The extrapolation procedure developed is almost the same than the ITTC’78 extrapolation procedure for conventional propellers but with the introduction of some special correlation coefficients on the scaling of the CLT propeller open water curves. The procedure was presented at The Motor Ship Conference 2005 (“Scale effects in model tests with CLT propellers”. 27th Motor Ship Marine Propulsion Conference. Bilbao, January 2005).
Another important goal of this R&D effort has been to stand out the scale effect existing in the epsilon coefficient, that defines the balance between the overpressure and underpressure forces acting on the blades of a CLT propeller.
At full scale the water flow is fully turbulent and there si not separation of the laminar boundary layer and hence the water leaves the cascade of profiles with direction tangent to the mean lines of the profile. However, the laminar flow separation produced at model field affects not only to the viscous forces acting on the blades (leading to higher CD values) but also to the pressure distribution on the blades and hence to the lift forces (leading to lower CL values).
Because of the above, scale effects not only on viscous forces but also on lift forces have to be taken into account to correct the influence of the laminar separation produced at model field with CLT propellers.
Due to the higher CD values of model field and the lower CL values, the epsilon coefficient at model field is higher than at full scale and therefore the propeller open water efficiency is lower at model field than at full scale and furthermore, the cavitation performance at model field is worse than at full scale.
A new R&D project was carried out during 2001-2003 among IZAR, CEHIPAR and SISTEMAR: “Research on the cavitation performance of CLT propellers, on the influence of new types of propeller blades annular sections and the potential application to POD’s”, with the following objectives:
- To develop a new procedure to conduct cavitation test with CLT propellers, in order to assure that inside of cavitation tunnel a phenomenon similar to the one corresponding to full scale be developed.
- To study the advantages that may be derived from the use in the design of the CLT propellers of a new type of mean line developed by SISTEMAR, looking for a maximum increase of the momentum that the cascades of the propeller blades annular section gives to the water.
- To compare the pressure pulses induced on the hull by a conventional propeller with those induced by a standard CLT propeller and a new CLT propeller designed using the new mean line.
With the new type of mean line the following improvements were confirmed:
- A reduction of the underpressure existing on the suction side of the CLT propellers/blades and also, a higher decrease of the volume of sheet cavitation, and consequently the levels of hull pressure pulses.
- An increase of the overpressure existing on the pressure side of the CLT propeller/blades that leads to an increase in the propeller open water efficiency of the actual CLT propellers.
A new R&D project was carried out during 2003-2005 among IZAR, CEHIPAR and SISTEMAR, with the participation also of ACCIONA TRASMEDITERRANEA and TSI, with the aim to validate at full scale the achievements of the previous programs: “Research on the performance of high loaded propellers for high speed conventional ferries”.
CLT blades were installed on the ferry “Fortuny” replacing the existing conventional blades. The ship speed in ideal trials conditions is close to 24 kn, and there were not significant problems of vibrations operating under combinator mode; however, operating under constant rpm mode in off-design with pitch settings below the design pitch the noise and vibration levels were very high becoming non acceptable in certain spaces.
When the pitch decreases and the rpm remains constant the radial loading distribution of the conventional blades decreases even more at the tip and negative loads are achieved at the upper sections of the blade. As a consequence, face cavitation is developed at the pressure side of the conventional blade and the noise and vibration levels increase because of the higher excitation of the pressure pulses. The situation is not the same with the CLT blades because there is not face cavitation at lower pitch settings.
Both from the cavitation tunnel and the full scale measurements it has been deduced that the non acceptable noise and vibration levels are produced as a consequence of the broad band energy spectra detected in the case of the conventional blades that does not exist in the case of CLT blades. This phenomena explains the vibration problems existing with the conventional blades.
The random excitation or broad band between the excitation harmonics corresponding to 2 and 3xBPF facilitates the appearing of local resonance phenomena that are clearly predominant for all the spectra both of noise and vibrations all along the vessel. The structural noise of low frequency is very difficult to reduce with conventional acoustic dampers and because of that the noise and vibration levels increase for lower pitch settings with conventional blades
Comparative full scale trials were performed before and after the installation of the CLT blades. Results of these trials have shown that the fuel consumption has been reduced for all the range of ship speeds after the installation of the CLT blades and also that the non acceptable noise and vibration levels produced with the conventional blades operating at constant rpm with pitch settings below the design pitch have been totally eliminated.