TS-111
Study on reliability for marine propeller
Katsuya KAWAMAE*, Shingo NISHIKIDO*, Tomonori FURUKAWA* and Tsuyoshi KAWAZOE**
ABSTRACT
The material of most of the marine propellers used nowadays and also employed after the 1970s is a nickel-aluminum-bronze (JIS: CAC703) alloy containing copper, aluminum, nickel, and iron as the principal composition. The alloy of nickel aluminum bronze as compared with manganese bronze (brass) which was frequently used before is high in corrosion fatigue strength and superior in corrosion resistance and repairability. Therefore, few instances of damages to propellers have been reported in recent years. However, with respect to propellers made of this material (JIS: CAC703), it does not necessarily mean that there have been no cases where damages due to corrosion fatigue occurred to the propellers. On the premises that a propeller is of cast metals and that it is impossible to make a casting without any internal defect, this paper is focused on the prevention of a fracture trouble which would be caused by the development of a cast defect into a fatigue crack. The actual effectiveness and points of problem concerning the non-destructive inspection procedure by which examinations are now under way are summarized. The design and manufacturing method of a skewed propeller are proposed here in the concrete for practicability.
Key Words: Marine Propeller, Skewed Propeller, Ni-Al Bronze, Non-destructive inspection, Casting defect
1. Introduction
Since the 1970's, a nickel-aluminum-bronze (JIS: CAC703) alloy is used for the material of most of the marine propellers. The alloy of nickel-aluminum-bronze as compared with manganese bronze (JIS: CAC301) which was frequently used before is high in corrosion fatigue strength and superior in corrosion resistance and repairability. However, with respect to propeller made of this material, it dose not necessarily mean that there have been no cases where damages to corrosion fatigue occurred to the propeller.
In a propeller under a certain fluctuating stress similar to other structural members, stress concentration happens with an internal defect as crack initiation point even if fatigue strength is high, with the result that while the propellers is in service, a fatigue crack occurs, propagates and lead to the fracture of a blade. Namely, whether or not corrosion fatigue crack occurs depend on the size of the internal defect as crack initiation point and the loading force on the blade.
On the premises that a propeller is of cast metals and that it is impossible to make a casting without any internal defect, this paper is focused on the prevention of a fracture trouble which would be caused by the development of a cast defect into a fatigue crack. The actual effectiveness and points of problem concerning the non-destructive inspection procedure by which examinations are now under way are summarized. The design and manufacturing method of a skewed propeller are proposed here in the concrete for practicability.
2. Designs of Marine Propellers
The designs of marine propellers are classified into three categories: blade design based on hydrodynamics and strength, and keyless boss design. For the purpose of this study, particularly the blade strength design is described in this paper.
2.1 Conventional Blade Strength Design of propeller
The design of a propeller blade has been performed based on beam theory. In the calculation of this design, a tensile stress caused by a centrifugal force is added to a stress caused by a bending moment to calculate a stress acting on each section of the blade such as a root section. In the theory of beams, various types of safety assumptions have been incorporated, as load conditions, in radial circulated distribution, thrust distribution, and blade section modulus for actual operation.
* Mitsubishi Heavy Industries, Ltd. 1-1, Akunoura, Nagasaki 850-91, JAPAN
FAX: +8 1-95-828-5913
** Nagasaki University
