A propeller is a device with a rotating hub and radiating blades made of bronze and brass. It transforms rotary power into linear thrust by acting on a working fluid like water or air thus moving a ship forward. The rotational motion of the blades is converted to thrust by creating a pressure difference between the two surfaces. A given mass of working fluid is accelerated in one direction and the vessel moves in the opposite direction.
A propeller used in ships imparts momentum to a fluid which causes a force to act on the ship. The propeller consist of blades made up of sections of helicoidal surfaces which are thought to screw through the water, hence the name screw propeller. The blades are twisted airfoils or hydrofoils and each section contributes to the total thrust.
Another design is the controllable pitch propeller where the blades are rotated normally to drive the shaft by additional machinery using hydraulics and the hub and control linkages running the shaft. This allows the drive machinery to operate at a constant speed while the propeller loading is changed to match operating conditions. It also eliminates the need for reversing gear and allows for more rapid change to thrust as the revolutions are constant.
Cavitation is the formation of vapour bubbles in water near a moving propeller blade in regions of low pressure due to Bernoulli’s principle. It can occur if am attempt is made to transmit too much power through the screw, or if the propeller is operating at very high speed. Cavitation can waste power, create vibration and wear,and cause damage to the propeller. The most common types of propeller Cavitation are suction side surface cavitation and tip vortex cavitation.
Suction side surface cavitation occurs when the propeller is operating at high rotational speeds or under heavy load.
Tip vortex Cavitation is caused by extremely low pressures formed at the core of the tip vortex. The tip vortex is caused by fluid wrapping round tip of propeller, from the pressure side to the suction side.
Types of propellers.
- Controllable pitch propeller
- Skewback propeller
- Modular propeller
- Voith Schneider propeller.
Controllable pitch propeller
The controllable pitch propeller has several advantages over the fixed pitch type, including the least drag depending on the speed used, the ability to move the vessel backwards and the ability to use the vane stance, which gives the least water resistance when not using the propeller such as when the sails are not used.
It is an advanced type of propeller used on German submarines. The blade tips of these type of propellers are swept back against the direction of rotation. In addition the blades are tilted to the rear, along the longitudinal axis, giving the propeller a cup shaped appearance. This design preserves thrust efficiency while reducing cavitation, thus makes for a quiet,stealthy design.
This propeller type provides more control over the boat’s performance. There is no need to change an entire prop, when there is the opportunity to change only the pitch or damaged blades.
Voith Schneider propeller.
These propellers use four untwisted straight blades turning around a vertical axis instead of helical blades and can provide thrust in any direction at any time, at the cost of higher mechanical complexity.
Rudders are equipment used for steering ships and aircraft. In ships rudders are located at the stern of ship. The rudder is attached to the frame with the help of pintle’s called bearing pintle’s and locking pintles. A rudder operates by rediecting the fluid past the hull, thus imparting a turning motion to the craft.
The rudder moves from port to starboard ie 30 to 35 degrees in 28 seconds.It is done to direct the ship at a particular direction or to change it’s course.
Rudder drop is actually a weardown of the rudder carrier bearing. It is measured by a trammel gauge.
The rudder stock connects rudder with steering system. In steering platform the motion is transmitted to rudder stock due to which rudder moves.
These are pins or bolts on which the full rudder turns. These are attached to stern frame and rudder bearing pintle’s and locking pintles.