In order to obtain movement of a ship in the sea, some form of reaction force has to be present between the ship and the fluid. When considering ships, this reaction is normally either due to air, such as in the case of sail boats, or due to water, such as in the case of ships using a screw propeller. Propellers however, are far more common and are considered as the main type of propulsor used in ships. The thrust present due to the propeller, the torque which is developed in the shaft and the rotational speed of the propeller shaft and the propeller need to be known such that the open water propeller characteristics can be obtained. These are coefficients which are then used to determine the efficiency of the propulsor.
Fig. 1 – Types of propeller tests
The term ‘open water’ refers to the condition where the propeller is not obstructed by the hull and is fully exposed to the water around it. Open water tests are carried out to obtain different coefficients which are then used to find the open water efficiency of a propeller, ηo. This efficiency can be used to estimate the power that the propeller would require [1]. These tests allow uniform comparison between propellers due to the nature of the dimensionless coefficients.
Towing Tank Tests: A towing tank is a tank containing calm water through which models are towed by a carriage. Propeller models are mounted on shafts which are driven by a motor inside some housing. The carriage tows the housing along the tank with a known velocity. The amount of readings possible in such a facility depends on the length of the tank, thus for a considerable amount of values to be collected at each run, a very long tank is needed. To avoid so much space being consumed, a circulating water tank can be used. This achieves a relative velocity between the propeller and the water while keeping the housing stationary.
Cavitation Tunnel Tests: A cavitation tunnel consists of a water channel which is completely closed, equipped with an impeller to circulate the water in it. Since uniform flow is very difficult to attain, conditions close to uniform are normally accepted. The tunnel is closed so scaling of pressure is possible; this is quite useful when seeking similarity between full-scale and model propellers, which is usually required. When the propeller is mounted with the propeller shaft downstream, the propeller can be regarded as being in open water and therefore an open water test can also be performed in a cavitation tunnel .
This type of test requires that a model hull has a propulsion unit added to it and this setup is then mounted onto a carriage. The model hull is run at the corresponding speed, which can be found after applying laws of similarity. The test would actually require both Froude and Reynolds similarity but propulsion tests conform to Froude similarity since Reynolds Number similarity cannot be achieved simultaneously with the first condition .
Constant Speed Method: A specific speed is chosen and kept constant while varying the load. The test can be repeated for different velocities while taking readings for varying loads.
Constant Loading Method: In this case, the loading is chosen and set before the start of the experiment and the speed is varied throughout the experiment for a range of values to be obtained.
Mixed Loading Method: This method is nothing but the use of both constant speed and constant loading methods in combination.
Cavitation is the physical process which occurs in areas of localized high velocity or low pressure. Due to high velocities, there is a fall in pressure and if this results in a low enough pressure, formation of cavities is witnessed. Air bubbles which come out of solution and vapour then fill the cavities which can consequently grow very quickly [9]. As fluid flows past an aerofoil, there is a pressure reduction over the back face. Bubbles form and move to the trailing edge to collapse at higher pressure areas. This contributes to noise, reduction in efficiency and even physical damage to the propeller blades
Cavitation tests investigate this physical phenomenon and are usually carried out in cavitation tunnels. The propeller is mounted onto a shaft in the cavitation tunnel and the impeller speed can be varied in order to vary the speed of the water. The rotational speed of the propeller can be varied independently. The water pressure can also be varied to obtain a similar bubble formation between the model and the full-scale propeller.
The cavitation number and the velocity of the incoming fluid are set before the test. The rotational speed of the propeller is varied during the test until cavitation can be witnessed visually both on the face and the back of the propeller. Since the result of this method depends on visual inspection, it is not very repeatable. Automatic detection systems have been implemented recently so that results are based on physical phenomena rather than on the visual capacity of the tester .
The International Towing Tank Conference (ITTC) is an association made up of organizations from all over the world. Its members seek to analyse the performance of ships and other related facilities through modelling, and then use the outcomes to predict the behavior of such installations. The ITTC also gives test guidelines and procedures one could follow for reliable results. Amongst the members of the ITTC one can find numerous universities as well as private companies. The last conference was the 25th International Towing Tank Conference which took place in Fukuoka, Japan in September of 2008. The next conference will be held in 2011 in Rio de Janeiro, Brazil. The ITTC has set recommended procedures and guidelines for each of the propeller tests mentioned earlier.
Towing Tanks: There are several towing tanks around the globe; two tanks amongst the longest in Europe are:
QinetiQ Haslar (UK) – this is Europe’s largest towing tank which is 270 m long, 12 m wide and 5.5 m deep. It has been used for different purposes, such as in the development of wave energy devices and to ensure safety of submarines .
SSPA, Göteborg, Sweden – this tank is more generally used for propulsion and resistance tests as part of the optimisation of hull forms. The SSPA tank is 260 m long, 10 m wide and 5 m deep .
Fig. 2 – QinetiQ Haslar tank, UK [15] Fig. 3 – SSPA Towing tank, Sweden
Cavitation Tunnels: Some cavitation tunnels around Europe are the following:
MARIN Wageningen, Netherlands – Some tests carried out at this facility are cavitation observation and performance tests, propeller noise measurement tests and supercavitating propeller tests [16].
HSVA, Hamburg, Germany – has three cavitation tunnels available, the largest one, HYKAT, has its test section 11 m long, 2.8 m wide and 1.6 m deep. Between them, the three tunnels can accommodate propellers having diameters ranging from 230 to 400 mm.
Fig. 4 – MARIN Cavitation tunnel [18] Fig. 5 – HVSA HYKAT, Germany
The choice of test that would eventually be carried out depended on the facilities available in the laboratories. These consisted of:
A cavitation tunnel which needed replacement of parts and maintenance
A wave-making tank
A circulating water channel
The circulating water channel was the most adequate facility since it needed very little maintenance and was also an ideal choice for an open water test.
The aim of an open water test is to obtain values of the open water efficiency of the propeller, ηo, for different values of the advance coefficient, J. Dimensional analysis was carried out after deducing that the thrust, T (N), depends on:
the diameter of the propeller, D (m)
the rate of revolutions of the propeller, n (rps)
the advance speed, Va (m/s)
the kinematic viscosity of the fluid, ν (m2/s)
the density of the fluid, ρ (kg/m3)
the gravitational acceleration, g (m/s2)
Such that: (1)
is a function of the advance coefficient; is a function of the Reynolds Number whose effect is negligible since the viscous effect on the blades is quite small when compared to the other forces acting on the propeller; is a function of the Froude Number which is not required in open water tests since there is no wave-making included in the testing procedure.