Thursday, May 13, 2010

Pitch Ratio of Ship Propeller

In propeller design, pitch ratio is one of the most important parameters that naval architect must determine. Pitch is the distance (measured in meters) a propeller travels along the direction of a moving ship in one propeller revolution. Pitch ratio is propeller pitch divided by diameter. The determination of pitch is usually carried out through the reading of Bp delta diagrams which were created through extensive open water test over a number of propellers. 
A naval architect or marine engineer cannot choose pitch ratio as high as he or she likes. It is a parameter that affects all of the propulsion parameters of ship propulsion. In other words when a pitch is altered it will change all the propulsion performance of a ship. If the pitch of the blades is too high, it will overload the main engine that drives the propeller. If it is too small, the main engine will work under-load. Both conditions are not good for the performance of ship propulsion. Thus the determination of the right pitch ratio is important to ensure that the ship will move fast at sea surface with efficient consumption of fuel oil and without overloading the main engine.
Modern ship propellers (both fixed pitch and controllable pitch propellers) have pitches that are variable. From radius r/R 0.2 to 1, the pitches are not the same. Usually the pitch of a ship or boat propeller that was designed based on B-series standard propeller or also known Troost series propeller will have pitch ratios based on the following table or diagram
We can see that from radius r/R = 0.6 to 1, the pitch diameter ratios are full, i.e. 1 or 100% where as pitch ratios from radius 0.5 to 0.6 are different. After we get the Pitch/Diameter ratio (only one value) from the Bp delta diagram of B-series standard propeller, we have to develop it into variable pitches according to the above table. When we have entered the values of these pitch diameter ratio, we can continue the calculation of the propeller design by conducting Propeller Mean Pitch Calculation. by Charles Roring