After that preliminary stage and depending upon the severity of the attack, damage may either cease or continue. The propeller’s material is ductile at normal sea water temperature and, usually, the first sign of a problem is the so-called orange peel effect where the surface suffers ductile deformation leaving it looking like the surface of the familiar fruit. When a vessel suffers propeller cavitation the material’s surface is subjected to a continuous bombardment of impacts from a fluctuating pressure field. Figure 1 shows the various types of propeller cavitation Different metals have different resistances to attack from cavitation. Careful examination of the metal surface in way of severe cavitation damage may also reveal shades of colour due to the metal being tempered. Experimentally it has been found that with mild steel temperatures near the cavity have locally risen to as high as 400☌ when the specimen has been deeply submerged in water with a constant ambient temperature of only 25☌. The marine surveyor should also be aware that when the water cavities violently collapse, the local temperature in the vicinity of the cavity may also change. The phenomena experienced in cavitation attack are usually found to be a function of the type of cavitation met, its proximity to the water surface and the rate of change of the cavity’s volume. While the destructive potential of collapsing vapour bubbles is usually the main interest to the small craft marine surveyor, he should also keep in mind that there are also important issues of noise and vibration due to the radiating pressures involved to be considered and taken into account. It is often said that cavitation is analogous to boiling with the former taking place at constant ambient temperature and the latter usually at constant ambient pressure. The effects of cavitation including loss of speed and damage to the propeller blades can be minimised by ensuring that the propeller has sufficient blade area relative to the area of the circle described by the propeller blade tips. Most small craft propellers are usually of constant pitch over the blade length and that regime is accurate enough for 99% of boats but on high speed boats with large propeller loading factors the pitch should vary over the length of the blade i.e., the boat should be fitted with a varying pitch propeller. It can also appear as similar damage on the driving face of the propeller in which case, almost certainly, a further factor has entered the problem in the form of an incorrect pitch distribution along the length of the blade. Cavitation is a highly complex phenomenon and the pitting damage it causes usually – but not necessarily – appears on the back of the blade following a clear radial pattern. That flow breakdown is called cavitation and is strictly analogous to the water hammer often heard in old plumbing systems. If either of these exceeds a certain value which depends upon a complex relationship between the propeller type, the flow in which it works and its mean depth below the water relative to its diameter then the flow pattern of the water over the propeller blades breaks down causing a severe loss of thrust and, eventually, physical damage to the surface of the propeller blades and, also, the rudder and local steelwork of the vessel’s hull.
The ratio of the absorbed power or the delivered thrust to the total blade area of the propeller is called, respectively, the power and the thrust loading. It is the collapse of these bubbles that results in the observed damage to the propeller blade surfaces. These bubbles collapse and can cause hammer like impact loads on the blades often in excess of 7 kg/cm2. The negative pressure causes any gas in solution in the water to evolve into bubbles similar to those found when opening a bottle of lemonade or champagne. It is the resolution of the pressures that results in the torque requirement and the thrust development of the propeller. Jeffrey Casciani-Wood HonFIIMSĪs the propeller turns it absorbs the torque developed by the engine at given revolutions i.e., the delivered horsepower – and converts that to the thrust which, in turn, pushes the vessel through the water. According to Bernoulli’s law the passage of a hydrofoil (propeller blade section) through the water causes a positive pressure on the face of the blade and a negative pressure on its back. Certifying Authority Examiner Resourcesįeature article written by Eur.Marine Surveying Books – Buy IIMS Handy Guides.IIMS Membership Benefits for Marine Surveyors.