![]() This can be due to the propeller being too near the surface (a design fault) and spinning too fast. Ventilation is caused when air is drawn down into the propeller from the surface. Cavitation and ventilationĬavitation is often confused with ventilation as the symptoms are similar: increased shaft revs without any increase in boat speed – or sometimes even a sudden reduction in speed. Higher pitch can be used by selecting higher gear ratios, but this is not really viable for most outdrive powered craft due to the small choice of ratios available and in this case it is usually necessary to work with the supplied ratio. Also the higher the pitch, the greater the efficiency of the propeller. The higher the speed of the boat, the higher the pitch should be. It is in fact a combination of the three elements: diameter, pitch and rpm that provides the thrust at the given speed of the boat. To accommodate the loss of thrust from the smaller diameter, the pitch is increased to give more thrust from the propeller’s movement through the water rather than from the blade area. The fact that many modern sports cruisers have limited propeller clearance space is another reason for fitting smaller diameter propellers. Acceleration is achieved by increasing the rotation speed, which usually means a smaller diameter propeller. A large-diameter slow-revving propeller is the most efficient type but this won’t provide the necessary acceleration to get a sports cruiser onto the plane. Propeller diameter is first decided by the type of boat. The vessel’s ‘duty cycle’, which is simply a judgement as to whether a boat is to be used for heavy or light commercial or pleasure purposes, then also influences a designer’s final specification. Once this resistance is known, engine and gearbox specifications can be combined with maximum propeller diameter (clearance beneath the boat is typically 12% of the prop’s own diameter) to determine the prop’s minimum requirements. Designers work to ensure that the lifting characteristics give the hull the least resistance at normal operating speed. However, the design of the lift surfaces of the hull also have a very significant effect. To the bare hull, the drag of appendages must be added such as rudders, shaft support brackets, shafts, depth sounder transducers, stabiliser fins, etc. Bare hull resistance is produced by skin friction of the hull’s surface, waves created by the hull running through the water and the shape of the hull below the water. Resistance is the hull’s natural tendency to drag through the water. Designers will use the boat’s length, beam, draught and displacement in combination with the boat’s underwater drag characteristics to calculate hull resistance. To specify the correct type and size of propeller, a standard set of boat, engine and gearbox measurements is required. In the first instance, the number of blades is decided by the weight of the vessel: the greater the weight, the greater the blade area required to push it through the water with a minimum of slip and cavitation. ![]() A fine pitch (meaning little angle on the blades) gives a lesser distance than a coarse pitch (greater blade angle).Ī propeller is usually designed with two to five blades (though sometimes more). ![]() The angle at which the propeller blades are set governs the distance travelled. Pitch is the forward distance that a propeller would theoretically travel in a single rotation if there were no slip present – imagine a screw being driven into a piece of wood.
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