The more redundant system of m-multi engines is able to give the higher availability to total ship but unavoidable to require the more manpower to maintenance crews.
The following equation as a function of maintenance intensity ρ_i is led to the redundant system of m-multi repairable engines according to the combination of three kinds of trouble conditions of each engine; (ρ₀+ρ_w+ρ_z)^m=ρ^m₀+Σ^m_{l=1 m}C_lρ^m-l₀ρ^l_w+Σ^m_{q=1 m}C_q(ρ₀+ρ_w)^m-qρ^q_z=1l, q≤m, ρ_i= λ_i/μ_i (i=0, w, z) where λ_i; failure rate, μ_i; maintenance rate of maintenance level of S_i, subscript i as follows;
z; emergency failure (x; stoppage of engine, y; slow down of engine)
w; non-emergency failure,
0; normal operation
There were obtained some useful informations concerning the allocation of reliability and maintainability and utilization of its maintenance crew due to a lot of failure field data of marine engine. Furthermore the above equation was confirmed by the single markov model for redundant system of marine repairable engines in appendix.

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The fatigue strength of two kinds of gray cast iron under superimposed stress wave forms in tensile and compressive stress regions at room temperature, 300°C and 450°C were discussed.
Obtained main results through the investigations are following.
(1) In tensile stress region at room temperature the influence of the superimposed secondary stress over the fatigue strength for primary stress wave appears about at not less than 0.1 of stress ratio of secondary stress amplitude to primary stress amplitude.
(2) In compressive stress region at room temperature the influence of superimposed secondary stress over the fatigue strength for the primary stress wave is smaller than that in tensile stress region. For evaluation of mean stress in compressive stress region. Gelber's equation is most suitable.
(3) In tensile stress region at room temperature and elevated temperature the fatigue life under superimposed stress wave form is evaluated by using the modified Miner's rule and Yamada's evaluation equation of mean stress. In the case of elevated temperature the frequency modified S-N curves and the frequency modified evaluation equations of mean stress are used.
(4) In compressive stress region at elevated temperature the superimposed stress wave fatigue life can be estimated by the use of the next equation.
1/N_f=Aσ^α*₁⋅exp(-m^*σ^p_e2)
where N_f: number of cycles to failure for primary stress wave
σ₁: primary stress amplitude
σ_e2: equivalent secondary stress amplitude
A, ρ*, m*, p: material's constants

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For the purpose of making highly skewed propeller practicable, various comparison tests have been carried out on both highly skewed propellers and conventional ones, such as systematic tank tests, cavitation test, dynamic stress measuring in stern wake using model propellers, static stress measuring using actual propeller blade, moreover, measuring of torque and thrust fluctuation on the shaft, the variation of the after body hydraulic pressure, after body vertical vibration of an actual ship in service. These tests results have been described on this paper.

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