Saturday, June 20, 2009

Propeller Pitch Ratio

Before we proceed to the mean pitch ratio calculation of a propeller, we must know, first, the definition of the terminology. Pitch is the distance a propeller travels along an x axis after one revolution. An ideal pitch can be seen if we imagine the propeller as a cork screw that moves forward through a solid material. So, the speed of the cork screw is V = pitch x revolution. But in reality a ship's propeller always works or moves through sea or fresh water. So, such principle of moving through solid material cannot be applied in the design of marine screw propeller.aremiti_3.jpg

cork-screw.jpg
Source: Basic Principles of Ship Propulsion
When a propeller revolves in the water, the fluid which is the sea water will be accelerated afterward. This happens because the water yields. Technically, ship designers or naval architects call this phenomenon as slip. Slip decreases the speed of a propeller. We will discuss about propeller slip in another article.
propeller-pitch-slip.jpg
Back to the propeller pitch ratio, before calculating this parameter, a propeller designer must determine the tip pitch ratio of the designed propeller at the service speed of the designed ship. The value of propeller pitch-diameter ratio (P/D) can be obtained from Bp- diagram. Bp is influenced by: the revolution of the engine at an optimum condition favorable for the propeller operation N; the delivered horse power PD; and speed of advance VA. Such parameters are also needed in the calculation of with the addition of another parameter, i.e. D which is the diameter of the propeller obtained from stern detail of the designed ship. The followings are the formulas for Bp and


and
After calculating the values of Bp and , the next step is reading them on the Bp- diagram. The diagram which I usually use is propeller Troost / Wageningen B-series. An example of the diagram is presented below:

bp-delta-propeller.jpg
Source of the figure: Marine Propellers and Propulsion written by John Charlton
From the above diagram, we can get the values of open water efficiency 0 and the pitch diameter ratio P/D of the propeller.
For the same delivered power and speed of ship, if we increase the engine revolution, the diameter and the pitch will be lower. To high pitch and diameter of propeller will cause the main engine to operate in over-loading condition. After all the principal dimensions of the ship's propeller have been determined or calculated, the next step which a propeller designer or naval architect has to perform is calculating the strength of the propeller and drawing the propeller. by Charles Roring in Manokwari of West Papua Indonesia.
Also read: Boat Propeller

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