In this paper Taggart claims that the oldest known to him idea about propellers with blade end plates dates back to 1855 and was due to Carles Augustus Holm, who obtained a silver medal at the Paris Exhibition this year for the proposal of folding the propeller blade tips towards the after side. At the United States patent registry office, there can be find dozens of patents some of them dating from 1860, where the idea about placing end plates at the blade tips over cylindrical surfaces which are coaxial to the shaft line or over a revolution surface with decreasing radius down stream is claimed. So it can be concluded that the idea of placing these end plates over the propeller blades has been broadly known for a long time.
Therefore a question arises why such propellers promising the achievement of higher efficiency were not introduced into practical application earlier. The answer is that all these authors didn't have the necessary very high fluid mechanics knowledge required to solve the problem i.e. fixing end plates with proper geometry and properly orientated on the propeller blades.
The first pioneering theories that allowed to carry out a good design of this type of propellers were published by Prof. dr Gonzalo Perez Gomez and his co-workers in Spain in 1978 and in English in 1980 [1].
As it is known the efficiency of a propeller mainly depends on the type of its thrust radial distribution law. According to the New Momentum Theory developed by Prof. Perez Gomez, the propeller efficiency reaches its highest values when the thrust produced by the propeller increases continuously from the hub towards the blade tips.
The New Momentum Theory documents that the open water efficiency of the propeller increases with an increase in pressure difference between the overpressure aft of the screw and the underpressure forward of it. To maintain an appreciably high pressure difference essential is to shape properly the blade geometry and more important to fit barrier elements (and plates) at the blade tips in order to separate the pressure fields forward and aft of the screw.
It is of paramount importance to position the end plates in such a way as to cause minimum viscous resistance. They must be positioned parallel to the incoming flow and shaped to the relative movement of the water through the propeller. Due to this requirement, the trailing edge radius at the blade tip section must be lower then the leading edge radius at the same section. The CLT propeller design developed by SISTEMAR fulfils all the requirements. But simply fitting end plates doesn't guarantee a higher efficiency. The efficiency increase can only be achieved by designing the blade geometry in such a way that the radial circulation distribution doesn't produce a null value at the blade tips. Fitting end plates to a propeller with conventional blade geometry would result in lower efficiency.
In order to design a propeller with finite load at the blade tips it was necessary to generalise Lerb,s Induction Factor Theory with the aim of making the use of radial circulation distributions with non null values at the blade tips feasible [1].
In 1977 the first propeller designs were made for the former generation of these type of propellers called TVF (Tip Vortex Free). These propellers were designed to work in association with a nozzle, with the aim of creating shock-free incoming flow onto the tip plates. The results of the initial experiments with TVF propellers were not encouraging because the model basins at that time still used extrapolation techniques which did not correctly allow for the transformation of propeller open water characteristic curves by taking into account the scale effects off the viscous forces acting on the propeller blades during the tests.
In 1981 [2] the results of the first experiments were published and the extrapolation procedure developed to predict the full scale behaviour of TVF propellers were described. For this procedure, the nozzle was considered as a hull appendix and the open water tests were conducted with the propeller alone. The application of the New Momentum Theory to the design of high efficiency propellers with a finite load at the blade tips was presented at the West European Conference of Marine Technology held in Paris in 1847 [3]. This was a major step forward for the next generation of tip loaded propellers the CLT propellers.
During the 6th Lips Symposium held in May 1986, the theoretical basis of the present generation of CLT propellers was given. The characteristic of these propellers is that the tip plates are adapted to the direction of the fluid flow through the propeller disk. This minimizes the viscous resistance of the tip plates and allows the desired pressure distribution across the propeller blades to be maintained. This arrangement remedies the defect of the TVF propellers which had the tip plates located tangentially to the propeller cylinder, which led to unwanted disturbance in the fluid flow pattern downstream of the screw with a consequent detrimental effect on efficiency. In contrast to the TVF propeller the CLT propeller had the tip plates only extended to the overpressure side while the TVF propeller had the tip plates extended over both sides of propeller i.e. over the pressure and suction side of the blade.
Since SISTEMAR was set up in 1987 until the end of 1999 over 250 CLT propellers have been designed, manufactured and installed on ships of very different types. Its up to date know-how enables SISTEMAR to design CLT propellers on model test results carried out with conventional propellers. There is no need to perform model tests with CLT propellers to develop a design maximum performance. This substantially reduces design costs.
For retro-fit CLT propellers for vessels already in service, SISTEMAR designs the new propeller without the need for further model tests. CLT technology was also adapted to controllable pitch propellers. The first CLT blades were fitted to an oceanic research vessel in March 1988.
CLT propellers offer significant advantages over conventional propellers: better performance, better manoeuvrability, lower noise and vibration levels and lower optimum diameters. As the blade area of the CLT propeller is more efficiently used for supplying thrust, the optimum diameter of a CLT propeller is lower than the optimal diameter of an equivalent conventional propeller.
This characteristic is specially important for those vessels with restricted allowance propeller diameter. For large vessels the reduction in propeller diameter leads to important reductions in propeller weight and inertia, with significant cost savings over conventional equivalents. For any vessel size, the smaller propeller allows the vessel to sail in a lighter ballast condition, with consequent savings in fuel consumption. The downstream overpressure produced by a CLT is higher than for an equivalent conventional propeller. This increases the pressure on the rudder and so increases the speed of the ship's response to rudder action. The effect is a smaller turning circle for any given rudder angle and better course keeping stability. The pressure drop on the suction side of CLT propellers is less than for the conventional. This has two important consequences:
The pressure forces that a CLT propeller exerts on the stern hull structure in non-cavitating conditions should be lower than for a conventional propeller. The extent of the cavity that should develop on the suction side of the CLT propeller should be lower than for a conventional equivalent and more stable.
Additionally, CLT propellers are tip-vortex free and so are not subject to cavitation developing at the tip vortex. The combination of these three factors 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 are lower.
