THIS invention relates to a propeller and more particularly but not exclusively, to a propeller for use in inboard and outboard boat engines.
A propeller is a device that transmits power by converting rotary motion into thrust. A pressure difference is produced between forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade, thus resulting in thrust required to drive a means of transport to which the propeller is attached. One specific type of propeller is a propeller for use as means of propulsion in boat engines, whether outboard or inboard.
Many different propeller designs are known in the trade, and they share many of the same general propeller design characteristics. A propeller comprises a plurality of blades extending radially outwardly from a central rotating hub. Each blade is in the form of an airfoil having two opposite surfaces, being a blade face (which is the pressure side of the blade facing the stern), and the blade back (which is the suction side of the blade facing the bow). Each blade furthermore has a leading edge, which is the edge of the propeller adjacent the forward end of the hub. The leading edge leads the blade into the flow when the propeller is providing forward thrust. The opposing edge is referred to as the trailing edge, and the radially outer zone extending between the leading edge and the trailing edge is referred to as the blade tip. The root of the blade is the fillet area in the region of transition between the blade surface and the hub periphery.
A few other terms that are commonly used in in propeller nomenclature include:    Diameter: Two times the distance from the centre of the hub to the tip of the blade, i.e. equal to the diameter of a circle that the tip of the propeller would make when rotating.    Pitch: Pitch is defined at the theoretical forward movement of a propeller during one revolution, assuming that there is no ‘slippage’ between the propeller blade and the water. In practice some slippage does occur, and the design pitch is therefore more than the actual pitch.    Cupping: Many existing propellers incorporate a cup formation at the trailing edge of the propeller blade. Propeller cup is the deformation of a propeller's trailing edge toward the pressure face. Cupping provides a measure of camber to the blade, and therefore changes the pressure distribution along the blade's chord length, adding lift toward the trailing edge.    Cavitation: Cavitation, (which is often confused with ventilation), is a phenomena of water vaporizing or “boiling” due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller's blade surface due to the collapse of microscopic bubbles on the blade. There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, and physical damage to the blade edges.    Ventilation: Ventilation is a situation where surface air or exhaust gasses are drawn into the propeller blades. When this situation occurs, boat speed is lost and engine RPM climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine.
Propeller design is mostly dictated by performance criteria, and traditionally safety aspects have not always been a major consideration. One of the major safety considerations is the edge and tip design of the blades, as the edges and tip are the areas that a person can potentially be exposed to during rotation of the propeller. The situation is furthermore often exacerbated when the propeller starts to wear, for example when in use strikes rocks or other hard surfaces, which may render the edge at least partially serrated, thus increasing the safety risk posed by the propeller.
There are a limited number of propeller safety devices available on the market such as for example propeller guards, propeller rings and hydro propulsion drive systems. These retrofit devices are not practical solutions to combat propeller safety, and in addition also adversely affect propeller performance and engine fuel consumption. The safety considerations are therefore often ignored when designing and manufacturing propellers for commercial use, in particular when there is a sacrifice in terms of performance.
It is accordingly an object of the invention to provide a propeller that will at least partially alleviate the above disadvantage.
It is also an object of the invention to provide a propeller having enhanced safety characteristics without adversely affecting the performance of the propeller.