Propeller is a quintessential component of a ship. It has unique construction, design, material and installation. There are some important terms which are used in the design of propeller.
Hub: The hub of a propeller is the solid center disk that mates with the propeller shaft and to which the blades are attached. Ideally the hub should be as small in diameter as possible to obtain maximum thrust, however there is a tradeoff between size and strength. Too small a hub ultimately will not be strong enough.
Blade: Twisted fins or foils that protrude from the propeller hub. The shape of the blades and the speed at which they are driven dictates the torque a given propeller can deliver.
Blade Face: It’s pressure side (also called as pitch side), which is seen first or in-front when watching from stern side of ship.
Diameter: The diameter (or radius) is a crucial geometric parameter in determining the amount of power that a propeller can absorb and deliver, and thus dictating the amount of thrust available for propulsion. With the exception of high speed (35 Knots+) vehicles
the diameter is proportional to propeller efficiency (ie. Higher diameter equates to higher efficiency). In high speed vessels, however, larger diameter equates to high drag. For typical vessels a small increase in diameter translates into a dramatic increase in thrust and torque load on the engine shaft, thus the larger the diameter the slower the propeller will turn, limited by structural loading and engine rating.
Pitch: The pitch of a propeller is defined similarly to that of a wood or machine screw. It indicates the distance the propeller would “drive forward” for each full rotation. In reality since the propeller is attached to a shaft it will not actually move forward, but instead propel the ship forward. The distance the ship is propelled forward in one propeller rotation is actually less than the pitch. The difference between the nominal pitch and the actual distance traveled by the vessel in one rotation is called slip.
Pitch can be classified as: Fixed pitch, Progressive pitch, Variable pitch & Controllable pitch
Pitch Line: It’s a reference line passing through leading edge and trailing edge of blade.
Root: It’s a fillet area, the region of transition from blade surfaces and edges to the hub periphery. It covers the area where the blade attaches to the hub.
Tip: It’s the maximum reach of the blade from the centre of the hub. It separates the leading and trailing edges.
Edge (Leading & Trailing): Edge of the blade which cuts water first, is Leading edge and edge which encounters water at last, is Trailing edge.
Rake: It’s a degreebthat the blades of propeller slants forward or backwards with hub. There are aft rake and forward rake. It affetcs the flow of water through the propeller. Rake is usually expressed as a percentage of the propeller diameter.
Skew: The transverse sweeping of a blade such that viewing the blades from fore or aft shows an asymmetrical shape. Skew is expressed in terms of the circumferential displacement of the blade tip.
Blade area is defined as a ratio of the total area of the propeller disc. A screw propeller may be regarded as part of a helicoidal surface which ‘screws’ its way through the water.