Fig.1 Principal stress distribution on face side of blade.
2.2 Blade Strength Design of Skewed Propeller
A skewed propeller is characterized in that its blade position is phased in the lateral direction of the blade. Because the lateral center of the blade is deviated from the generator line of the propeller, a thrust produced at a position forward from the calculation position causes a twist force at that position in the section of the blade. This means that it is impossible to apply the simple beam theory to the stress calculation of skewed propeller blade. Therefore, the finite element method (FEM) is practically adopted for the stress analysis.
In order to increase the accuracy of stress analysis by FEM, the following items must be considered beforehand.
(1) Effects of accuracy of pressure distribution on blade surface, which is given as a load condition, i.e., accuracy of theoretical calculation of flow around blade.
(2) Effects of number of positions on the blade to be analyzed in radial and lateral directions on a stress value
It was also found out that, when a blade stress is evaluated, care must be taken with distribution of stress on the blade surface in addition to the evaluation of the absolute value of the stress. Namely, it can be said that it is appropriate to position the maximum stress produced on the blade surface at the root lateral center of the blade. In other words, we consider that a propeller B in Fig.1 is the most suitable for use. This is because, when a skewed propeller is adopted, a stress at the root of the blade obtained by the theory of beams and a stress obtained by the FEM can be compared relatively to each other if a stress distribution is the same as that of a conventional propeller. In addition, this propeller realizes the conventional stress distribution such that the propeller has the most critical stress at the root lateral center of the blade, and also reliability is expected to be given from the nondestructive inspection as described later.
3. Casting Defects
3.1 Types of Casting Defects
The defects produced generally in propeller casting can be classified into five categories.
(1) Blow hole: When gasses are discharged from molten metal at the time of its solidification and remain in castings, this defect may occur. It tends to produce more on the upper surface of the castings.
(2) Shrinkage cavity: When supply of molten metal needed for shrinkage at the time of solidification is not sufficient, cavities may occur. For a propeller, many examples show that large cavities occur concentratedly at the root of the blade at one position.
(3) Hot tearing: Areas where thickness is large and shrinkage cavities tend to be produced are different in temperature distribution and temperature gradient from those exposed to the atmosphere in the process of cooling, and tends to cause cracking because free shrinkage is restricted.
(4) Sand inclusion: When molten metal is poured into a mold, the walls or runners of the mold may be eroded and its sand may be separated and caught into the casting.
(5) Slag inclusion: Slag existing together with molten metal in a ladle or oxidized slag produced in contact with air at the time of pouring may be caught into the molten metal and remain on the surface of or in the casting. Though slag floats on the molten metal because it is lighter than the metal, if adhered to the upper surface (corresponding to rear surface of propeller) of the sand mold, sand inclusion tends to occur.
