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.
