Thursday, July 2, 2009

Propeller Rake and Blade Thickness Diameter Ratio

by Charles Roring in Manokwari of West Papua Indonesia

Rake in propeller is made to increase to amount of water mass which can be sucked and propelled to drive the ship. Most of the rake is aimed at inclining the blades of the propeller backward relative to the generator line of the propeller. Usually the rake is between 00 and 150.
The larger the rake the higher will be the tensile stress of the blades. As a result, the blade thickness diameter ratio of a propeller with 15 degrees of rake is higher than the propeller without a rake whose overall parameters are the same. As I have said before, the higher the rake, the higher will be the working stress.

ambulu7.jpg
A propeller designer must carefully perform strength calculation for ship's propeller whose blades have high rake degree. The stress calculation is carried out using Taylor strength criteria for various blade thickness diameters until the allowable working stress is achieved. As a guidance, the formulas for calculating the blade thickness diameter ratio for a given rake can be read at my other article entitled: Material and Strength of Marine Screw Propeller. If you want to learn more about this subject, I suggest that you read page 301 of The Design of Marine Screw Propellers written by T.P.O. Brien. Here is a sample of propeller strength calculation whose rake is 10 degrees:
Propeller design data: DHP = 13210.56 hp; Blade number = 4; RPM= 97; diameter= 6.8 meters or 22.309 ft; chord diameter ratio at radius 0.2 = 0.230; the material density w = 480 lb/ft3; t2/D = 3.52; rake = 10 degrees; S2 = 1310, S3= 0.22; S4 = 0.746; S5 = 0.31.
It is suggested that propeller designer avoid applying to high rake degree in his or her newly designed propeller to keep the blade thickness diameter ratio lower and to simplify the manufacturing process.

The Taylor Strength Criteria for Propeller whose material is Ni-Al-Bronze


After the strength criteria calculation has been executed, the figures in the above table can be represented in a graph such as the one shown below:

propeller-strength-criteria.gif

Also read:

Pitch diameter ratio of a propeller and engine performance of a ship;

Ship displacement calculation