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 was no yet created.

The conclusions of these research activities were very positive for all the three partners. 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 can extrapolate at full scale model test results with CLT propellers with approximately the same degree of security and accuracy that they can extrapolate at full scale model test results with conventional propellers. This R&D programme also highlighted that the lift forces acting on the CLT propellers/blades have an origin quite different than those corresponding to the conventional propellers.

In the case of conventional propellers, it is commonly accepted that these forces are predicted through the application of the circulation theory. SISTEMAR has insistently claimed that in the case of the CLT propellers the lift forces are better modelled through the momentum theory and predicted as a result of the increment of the momentum that the water receives when it passes through the propeller blades.

In the case of the CLT propellers, the overpressure forces acting on the pressure side of the propeller blades are very considerable in comparison with the underpressure forces acting on the suction sides of the propellers blades. Using the New Momentum Theory it is proved that the high efficiency of the CLT propellers is due to this fact.

Another important goal of this R&D programme has been to stand out the scale effect existing in the epsilon coefficient. As it is very well known the epsilon coefficient defines the balance between the overpressure and underspressure forces acting on the blades of a CLT propeller.

As a consequence of this, it has also been obtained the conclusion that these facts must be taken into account when conducting cavitation tests with CLT propellers. A new method for this kind of tests must be developed.

The above mentioned conclusions decided to IZAR to undertake further research, in order to clarify these results. Following this, a new R&D project has been carried out during 2001-2003 among IZAR, CEHIPAR and SISTEMAR.

The objectives of this project were as follows:

• To develop a new procedure to conduct cavitation test with CLT propellers, in order to assure that inside of the 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 expected:

1. To reduce the underpressure existing on the suction side of the CLT propellers/blades and also, to decrease even more the volume of sheet cavitation, and consequently the levels of hull pressure pulses.
2. To increase the overpressure existing on the pressure side of the CLT propeller/blades. This would lead to an increase in the propeller open water efficiency of the actual CLT propellers.

With the aim to reach the above mentioned goals and once developed the procedure to conduct cavitation tests with CLT propellers, one model of conventional propeller and three models of alternative CLT propellers have been manufactured.

The performance at full scale of the basic model of CLT propeller is already known. With the four propeller models the following tests have been carried out:

1. Propulsion and open water.
2. Cavitation tests and simultaneous pressure pulses measurements.
3. Three-dimensional measurements of water speed with a laser-doppler velocimeter in four axial positions (plans) with regard to each propeller tested. One axial position should be upstream of the propeller disk and the three remaining ones downstream of the propeller disk.

In figures 1 and 2 some examples of the results of the measurements of the axial components of the water speed are shown. They correspond respectively to one conventional and to one CLT propeller. The power delivered by both propellers and the respective rpm were the same.

Figure 2 corresponds to the speed measurements conducted with the model of the CLT propeller. It is observed that the velocities downstream of the propeller disk are higher than in the case of the conventional propeller. The diameter of the CLT propeller is 5% lower than the one corresponding to the conventional propeller, so, in the upper area of Figure 1 outwards of the CLT propeller diameter, the velocity of the conventional propeller is higher than the one given in figure 2.

The induced velocities by the CLT propeller downstream of the propeller disk are noticeably higher than those corresponding to the conventional propellers. Further analysis are needed, taken into account the differences in the areas of the propellers disks, but these results are in principle coherent with the assumptions of reaching a higher efficiency using CLT propeller in comparison with the one corresponding to the conventional propeller.