Feature article written by Eur. Ing. Jeffrey Casciani-Wood HonFIIMS
As 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. It is the resolution of the pressures that results in the torque requirement and the thrust development of the propeller. 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. These bubbles collapse and can cause hammer like impact loads on the blades often in excess of 7 kg/cm2. It is the collapse of these bubbles that results in the observed damage to the propeller blade surfaces.
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. 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. That flow breakdown is called cavitation and is strictly analogous to the water hammer often heard in old plumbing systems. 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. 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. 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. 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.
