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SUPERYACHT #521
Sep 2005

Article selected from our quarterly magazine dedicated to the largest and most luxurious boats with information, interviews, technical articles, images and yachting news


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Article by
Angelo Sinisi

THE PHENOMENON OF CAVITATION

The phenomenon of cavitation was anticipated by Eulero in 1750, a century before it was ascertained experimentally. The great scientist clearly described the conditions under which it could occur. The phenomenon of cavitation in marine propellers is of extreme importance. Throughout the ages the tendency has always been to achieve increasingly higher speeds. The specific thrust per cm2 of blade, with the increase of shaft power, was aimed at overcoming the total static pressure (atmosphere + impeller) developed on the propeller blades. Under such extreme load conditions anomalies in propeller functioning occur, with falls in performance and yield. These anomalies go by the name of cavitation.

The phenomenon of marine propeller cavitation manifested itself for the first time in 1856 in experiments carried out by the Frenchman George Rennie. It was noted that thrust increased with an increase in propeller immersion. But the true nature of the phenomenon was not identified at the time and the researcher restricted himself to concluding that propeller yield increases with immersion.

In 1892 the French shipbuilder A. Normand made experiments similar to Rennie's, but he further demonstrated that thrust does not increase with an increase in revs, because there is a critical number of revs, which increases with increase in immersion, beyond which thrust drops. After just a few years, the phenomenon of cavitation - a word suggested by R. E. Froude - was studied in the first water circulation tunnel, created by the shipbuilder Parsons. Subsequently a bigger cavitation tunnel was built, for the first time with a stroboscopic observation system for photographing the phenomenon.

The stroboscopic system, by means of an adjustable intermittent light, creates an apparent slowing down until state of rest and change of direction.


Figure 1 - Pressure curve of an immersed body

When a body immersed in water meets with a current the fluid threads alter their course, so speed and consequently pressure assume different values from point to point. In Figure 1 the body immersed at depth h meets a flow with speed V0. Ph and Pa are, respectively, the pressure corresponding to immersion h and atmospheric pressure. Therefore the total static pressure is P0 = Ph + Pa.

Having reached point I, the fluid threads open up and thicken at point II which is at the widest part of the body, and then diverge and resume their initial progress after having passed the body. At point I the speed is nil and pressure PI by Bernoulli's theorem is PI = P0 + ½V0².

At point II the velocity of the fluid is VII and we have PII = P0 + ½ρV0² - ½ρVII². If VII is greater than V0, PII is less than P0 and therefore inversely proportional to VII.

At the most it could be PII = 0, as a result of which the fluid threads detach themselves from the walls of the body, forming a cavity. With the diminution of pressure we have tension "e" of the saturated water vapour at surrounding temperature, which is to say we have cold evaporation. The vapour bubbles thus formed are drawn by the current towards zones where pressure increases and they are once more condensed. So the phenomenon of cavitation, as described above, modifies the fluid-dynamic field around the immersed body.

 



Figure 2 - Cavity caused by a fluid band that detaches from the body wall



Figure 3 - Cavitation downstream of narrowing in a tube



Figure 4 - Diagram of molecular structure of water



Figure 5 - Bubble cavitation



Figure 6 - Sheet cavitation



Figure 7 - Section of a super-cavitating propeller blade

 
The fluid thread (Figure 2), instead of following the course OABEDFG, leaves the surface at point B, passing by way of C and violently clashing with the body at point D where due to increased pressure the cavity dissolves, and continuing to points F and G. In the zone around point D there is rapid condensation of the vapour bubbles into very small droplets which violently strike the surface of the immersed body. The material of the body is unlikely to stand up to this bombardment. The first erosions increase the phenomenon, which spreads and leads to cracking and even breakages. Obviously this phenomenon causes sound and ultrasound waves and possibly vibrations.

Such a tormented course acts negatively on the coefficient of lift and resistance of a hydrofoil or a propeller blade, and therefore on yield.

The cavitation threshold increases with augmentation of pressure and increased water purity, and also rises as water temperature falls. In fact with a fall in temperature the solubility of gases in water increases, whereas the number and sizes of the free gas bubbles diminish.

The molecular structure of water is not uniform. In fact, as illustrated in Figure 4, the distance between the molecules is not equal, so neither is the reciprocal force of attraction. Moreover, there are vacuum zones or points, sacs of gas and foreign bodies which are sometimes not wholly wet. As pressure diminishes the air sacs dilate, the liquid evaporates and the vapour fills them. The presence of impurities carries greater weight at the beginning of cavitation.

Cavitation may be bubble or lamina. In bubble cavitation (Figure 5) large vapour bubbles are formed and dissolve in a pulsating manner, visible to the naked eye. A cavitation bubble at point A, where the pressure is the same as that of saturated water vapour "e", dilates and gradually increases in size, since external pressure becomes less than "e". This increase continues up to point B where external pressure has risen to value "e". The bubble diminishes to the point of complete condensation at point C .

After implosion of the bubble a second bubble is formed which condenses at D etc.. At the implosion points there are very high dynamic pressures and therefore corrosion of the material occurs. Bubble cavitation is facilitated by the presence in the fluid of large cavitation nuclei and the phenomenon has a certain duration, which is to say a relatively slow movement.

If on the other hand the movement increases, which is to say there is a high velocity, the liquid cannot evaporate: the fluid threads detach themselves from the body and create a cavity full of vapour. This type of cavitation is known as "lamina cavitation" ( Figure 6 ). Lamina cavitation is less dangerous from the corrosion point of view because there is a lesser quantity of evaporated and condensed liquid. Indeed, if the cavity completely covers the body and closes far from the trailing edge, condensation takes place at a distance and there is no danger of damage to the material. Cavitation of this kind is called supercavitation.

Supercavitating propellers exploit supercavitation and have sections (Figure 7) designed in such a way as to make the most of the phenomenon. They are therefore propellers suitable for a high advancement speed.

Cavitation is not always negative. In fact to create an engine exhaust outlet beneath the keel one uses, as suction, the depression of a cavitation like that of the Venturi tube (Figure 3) .

It should also be noted that the negative effect of cavitation depends not so much on the greater extent of cavitation as on its intermittent character, which produces vibrations and which originates from non-uniformity of the wake, caused by appendages and the form of the stern. So the problem of anticipating this phenomenon may become of even greater importance with regard to the primary aim of achieving maximum propeller efficiency.