Propellers

Propellers can be either Right or Left handed Right handed propellers rotate clockwise when viewed from astern with the vessel in ahead propulsion. Left handed propellers rotate counterclockwise when viewed from astern with the vessel in ahead propulsion. Most vessels have right handed propellers.

Propellers Propellers develop propulsive thrust by accelerating the water in which they act Propellers work like a pump to drive the water aft

Propellers Terminology Pitch- The distance a propeller will push the vessel in one revolution (disregarding slip). For example: a wood screw will pull itself into wood a certain distance for each complete revolution. If this screw were a propeller, the distance it pulled into the wood would be the pitch. Pitch is measured in feet.

Propellers Terminology (cont’d) Diameter- The overall distance from blade tip to opposite blade tip. Diameter is measured in feet. Slip- Theoretically, a propeller will advance in one revolution a distance equal to the pitch. Slip is the % of lost efficiency due to the fact that the propeller is not operating in a solid. For Example: a propeller that is 90% efficient will have a 10% slip. If the propeller has a 25 foot pitch, 25 x.9= 22.5 feet. The propeller will only push 22 ½ feet instead of 25 feet.

Propellers Terminology (cont’d) Face Of The Blade- This is the AFTER surface of the blade. It is the working or pressure surface since it is the surface that drives the water when the vessel is being propelled in the ahead direction.

Propellers Terminology (cont’d) Back Of The Blade- This is the FORWARD surface of the blade. It is the suction or vacuum surface as it will see a decrease in pressure when the vessel is being propelled in the ahead direction.

Propellers

Terminology (cont’d) The Leading Edge- This is the edge that cuts the water first when going ahead. The Trailing or Following Edge- The edge that is the opposite to the Leading Edge.

Propellers Terminology (cont’d) Tip- The point on the blade furthest from the hub. Root- The point of connection between the blade and the hub.

Propellers Terminology (cont’d) Cavitation- Cavitation is a condition where low pressure is formed on the suction or vacuum side of the propeller blade. The low pressure will cause the water to vaporize and flash to steam. Not only does this affect slip, but can cause damage to the propeller when the bubbles travel to an area of high pressure. The bubbles implode and pit the surface of the propeller. Note Pitting on working surface of the following slide.

Propellers Most propellers are made of manganese bronze This soft metal will bend before damage occurs to more costly components such as the shaft or bearings

Propellers Most large vessel propellers are of the “Solid” or “Fixed” type This means the blades are fixed to the hub as in the background photo

Propellers Some propellers have fixed blades that bolt onto the hub. Each blade is cast separately. This allows for easier replacement of damaged blades or new blades may be fitted to change pitch.

Propellers Adjustable Pitch Propellers- This is a hybrid type of bolt-on blade. In a dry-dock, the blades can be moved one bolt hole to change the pitch.

Propellers Controllable Pitch Propellers- These propellers have blades that change pitch by a mechanism in the hub. The RPM remains constant, but the thrust changes with the pitch. Advantages: Speed can be infinitely adjusted. No Loss of power when backing The engine does not need to be stopped and re-started when backing Disadvantages: Larger hub (less efficiency), greater initial cost, more maintenance,

Propellers Kort Nozzles- A shroud is placed around the propeller. This increases the efficiency and thrust. Primarily used is Tugs.

Propellers Azimuthing Propellers- This relatively new technology is used in passenger vessels, ferries, tugs and in applications where maneuvering without tugs is a possibility. The propeller is mounted in an azimuthing (turning) housing allowing the propeller force to be directed in any direction.