Sunday, May 10, 2009

Propeller Strength Calculation

by Charles Roring in Manokwari of West Papua - Indonesia

Propeller strength calculation is one of the many factors which a naval architect or propeller designer must perform when designing the propeller of a ship. A propeller must have an adequate thickness to enable it to rotate and propel the ship without failure.

Ideally, the blades of a propeller should be as thin as possible to have higher efficiency. But it will make the propeller easily to vibrate and brake during its operation. If the propeller is too thick, the efficiency of this propelling device will be low and the cost of manufacturing the propeller will be higher.

Therefore, a propeller designer must compromise all these contradicting factors to obtain the most optimum blade thickness.

Usually propeller designers use Taylor method for calculating the strength of a propeller. This method is simple and effective. The use of Taylor method is suitable for propeller blades with normal forms and whose blade area ratio or B.A.R is not too large. The use of Taylor method on B.A.R. which is more than 0.80 will result in blade stress that is 10 to 15 percent too low.

The calculation of propeller strength is needed to assess the value of design stress so that a safe blade thickness could be determined. Generally, the calculation of propeller strength with Taylor method is carried out at the blade radius of 0.2 R. Taylor provides a number of formulas which propeller designers and naval architects can use to calculate the compressive and tensile stresses of the blade. Some design parameters which we must know prior to performing the calculation are the number of blades, the RPM of the main engine, the diameter of the propeller, the chord diameter of the expanded blade at radius of 0.2R, and the material density of the propeller.

The thickness of the propeller is largely determined by its material. Marine propeller can be made from cast steel, special propeller bronze, Ni-Al-bronze, cast iron, gun metal and etc.

The explanation and example of propeller strength calculation can be read on page 288 of The Design of Marine Screw Propellers which was written by T.P. O'Brien.

To help propeller designers, Taylor has provided a chart which is called Taylor Propeller Strength Criteria. To use the chart we have to calculate the compressive and tensile stresses of the propeller. The chart and the strength calculation have to be used with propeller strength requirements stipulated by classification societies such as LR, GL, DNV, NKK, and BKI. These classification societies provide a table with a list of propeller materials and their minimum ultimate stress in km/mm2, and minimum elongation in percent.

After performing the strength calculation, a propeller designer can continue his work by drawing the propeller which is usually done using Holst method. Such method is well and extensively explained on page 171 of a book entitled, Resistance, Propulsion, and Steering of Ships A Manual for Designing Hull Forms, Propellers and Rudders. The book was written by Prof. W.P.A. van Lammeren, Troost L., and Koning J.G. Also read: Cavitation of Marine Propeller; Resistance and Propulsion Calculation in Ship Design

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