Understanding of Propellers and its definitions 🌀

What is Propeller?

        A propeller is a rotating shaft with a twisted blade attached to it which is used to propel the ship using power generated and transmitted by the main engine.

Types of propeller:

By number of blades:

• 3 Blade propeller

• 4 Blade propeller

• 5 Blade propeller

3 blade and 4 Blade propellers are mostly used on ships

By pitch of the propeller:

• Fixed pitch propeller
• Controlled pitch propeller

Lets see what is Fixed and Controlled pitch propeller:

Fixed Pitch Propeller: 

• Propellers of the Fixed Pitch type are cast in one block and normally made of a copper alloy. The position of the blades, and thereby the propeller pitch is fixed and that cannot be changed in operation.

Controlled Pitch Propeller:

• A controllable pitch propeller (CPP) or variable pitch propeller is a type of propeller with blades that can be rotated around their long axis to change their pitch using a means of mechanical or hydraulic arrangements.
• If the pitch can be set to negative values, the reversible propeller can also create reverse thrust for going backwards without the need of changing the direction of shaft revolutions.

Propellers and definitions:

• Blade Face

Pressure Side, Pitch Side. Aft side of the blade when viewed from aft (surface facing the stern).

• Blade Back

Suction side. Forward side of the blade when viewed from aft (surface facing the bow).

• Blade Number

Equal to the number of blades on the propeller.

• Blade Root

Fillet area. The region of transition from the blade surfaces and edges to the hub periphery. The area where the blade attaches to the hub.

• Blade Tip

Maximum reach of the blade from the centre of the hub. Separates the leading and trailing edges.

• Boss or Hub

Solid cylinder located at the centre of the propeller. Bored to accommodate the engine propeller shaft. Hub shapes include cylindrical, conical, radius, & barrelled. Propeller blades are attached to it.

• Leading Edge

The edge of the propeller blade adjacent to the forward end of the hub. The leading edge leads into the flow when providing forward thrust.

• Trailing Edge

The edge of the propeller adjacent to the aft end of the hub. When viewing the propeller from astern, this edge is closest. The trailing edge retreats from the flow when providing forward thrust.

• Rotation

When viewed from the stern (facing forward): Right-hand propellers rotate clockwise to provide forward thrust. Left-hand propellers rotate counter-clockwise to provide forward thrust.

• Pitch

Pitch is defined as the theoretical forward movement of a propeller during one revolution assuming there is no "slippage" between the propeller blade and the water. Pitch is the second number listed in the propeller description.

• Slip

Slip is the difference between actual and theoretical travel of the propeller blades through water. A properly matched propeller will actually move forward 80 to 90 per cent of the theoretical pitch.

• Diameter

Diameter is the distance from the centre of the hub to the tip of the blade x 2. It can also be looked at as the distance across the circle that the propeller would make when rotating. It is the first number listed when describing a propeller.

• Radius

The distance from the axis of rotation to the blade tip. The radius multiplied by two is equal to the diameter.

• Rake

Rake is the degree that the blades slant forward or backwards in relation to the hub. Rake can affect the flow of water through the propeller.

Aft Rake helps to trim the bow of the boat upward, which often results in less wetted surface area and therefore higher top end speed.

Forward, or negative rake, helps hold the bow of the boat down. This is more common in workboat type applications.

• Skew

The transverse sweeping of a blade such that when viewing the blades from fore or aft shows an asymmetrical shape

Aft Skew: Positive skew. Blade sweep in the direction opposite of rotation

Forward Skew: Negative skew. Blade sweep in the same direction as rotation.

Bow thrusters:

            Bow thrusters are type of propellers, which are smaller in size and which help in better maneuverability of the ships at lower speeds. They are generally used for maneuvering the vessel near the coastal waters or while entering or leaving a port. Bow thrusters help in assisting tug boats in berthing the ship without wasting time.