Tuesday, June 9, 2009

Material and Strength of Marine Screw Propeller



by Charles Roring in Manokwari of West Papua - Indonesia

The strength and dimensions of a ship's propeller is influenced by various propulsion factors and the material choice. If the resistance and propulsion parameters of a ship has been determined, the next step is determining the propeller dimensions. One of them is the thickness of the blade.

In calculating the thickness of the blade, a propeller designer usually has to perform strength calculation so that he can determine the minimum thickness of the blade at radius 0.2 R of the ship's propeller. The calculation is usually based on Taylor's method which is well explained on pages 288 to 301 of The Design of Marine Screw Propellers written by T.P.O. Brien.

Propeller Materials – The Taylor formulas used in the propeller strength calculation are important in assessing the designed working stress and the safe thickness of propeller blade at 0.2 R. The Classification Societes have provided information about propeller materials and their properties which a naval architect or propeller designer can use to design the required propeler. The following table is the requirements provided by Det Norske Veritas for propeller materials

Propeller Material

Minimum ultimate tensile stress

(kg/mm2)

Minimum Elongation

(%)

Cast steel

41

20

Special propeller bronze

45

20

Ni-Al-bronze

60

16

Nodular cast iron, heat treated

Not heat treated

40

15

3

Special cast iron

55

-

Ordinary cast iron

24

-

Gun metal

14

8

The above information is presented on page 285 of The Design of Marine Screw Propellers by T.P.O. Brien. Besides the minimum tensile stress, other propulsion parameters which we need are delivered horse power PD, blade number, RPM, propeller diameter, chord diameter ratio at 0.2 R, material density, and rake of propeller.

The average designed working stress and material density for marine screw propeller is provided below

Material

Density

Design Stress (lbs/inch2)



Single Screw

Twin Screws


(lb/ft3)

Reciprocating engines

Turbine or diesel electric

Reciprocating engines

Turbine or diesel electric

Manganese bronze

525

6000

6250

6250

6500

Nickel-Al-Bronze

480

6750

7000

7000

7250

Cast iron

450

2500

2600

2600

2700

With the development of research and technology in ship's propulsion new materials have been introduced for marine screw propeller. Students and practicing propeller designers must refer to the latest data provided by various classification societies.

Propeller Strength Calculation - For calculating the compressive stress, the following Taylor's formula is usually used:

Then, for calculating the tensile stress of the propeller, the following Taylor's formula should be used

ST = SC (0.666 + S4 t.2/c)

For further explanation of the application of above Taylor's formulas for propeller strength calculation, I suggest that you read T.P.O. Brien's book, The Design of Marine Screw Propellers. The above formulas cannot be used independently. They have to be used with a graph depicting the Strength Criteria of Propeller formulated by Taylor which is given on page 296 of the book.

After performing the strength calculation, the thickness of the propeller is safe for the operation of the ship at the designed speed.