The Basic
Propeller Diameter
Is simply the distance across the circle swept by the extreme tips of the propeller blades. Diameter is the most critical factor in relation to power absorbed and delivered thrust. In most cases, the larger the diameter, the greater the propeller efficiency.
Revolution per minute
RPM is the number of full rotations a propeller makes in a single minute. In most cases the shaft RPM is different from engine RPM. In most applications, a reduction gear is fitted between the crankshaft and the tail shaft. The primary purpose of installing a reduction gearbox is to reduce the RPM at the propeller so that a more efficient propeller may be used. Since larger diameter propellers are more efficient in producing thrust, lower RPMs are generally desirable for most installations.
Pitch
Is the theoretical distance the propeller pushes the boat forward in one complete revolution. Since the propeller is cutting through water, the actual distance travelled by the boat is not the same as the indicated propeller pitch. The difference in the theoretical and actual distance travelled is called Propeller Slip.
Blade area
Is the surface area of the propeller blades. Blade area can have direct effect on absorbed power and cavitation. Number of Blades – the number of blades will vary from vessel to vessel. Generally, more blades will produce smoother running.
Leading edge
Is the edge of the blade that cleaves the water. Trailing edge – is the edge of the blade where the water moves away from the propeller blade.
Propeller Rotation
Marine Propellers can either be right hand or left hand. Right handed propellers rotate clockwise as viewed from behind the boat looking towards the stern. Left hand propellers turn anti-clockwise.
Other technical terms associated with marine propellers
Cavitation
Bubbles of partial vacuum caused by excessive propeller speed or loading. Vacuum bubbles implode against the suction side of the propeller causing vibration and even worse is the force of imploding bubbles is so great that it actually suck the metal right off the surface of the propeller. Large amount of slip and excessive tip speeds are likely to cause cavitation. The end-result is uneven wear and vibration. You can see photos of cavitation damage by following this link.
Ventilation
Is the effect of air sucked down by the propeller from the water’s surface. Ventilation will lead to vibration and loss of thrust. One way that Ventilation can be corrected is by just simply putting the propeller deeper under the surface of water.
