In the case of analyzation of longitudinal strength of ships, still water bending moments have a major part of total loads. Their statistical natures during actual voyages have not been clarified sufficiently. Moreover the long-term distribution of wave loads is affected by statistical character of draft and trim in service. It is important to clear above natures in order to analyse reliability of longitudinal strength of ships.
Authors study the natures of still water conditions by survaying log-books of 10 container ships and 13 tankers. The results of our investigation indicate that their distributions are described by normal distribution approximately. In the case of container ships, they are nearly full load conditions and coefficient of variation of still water bending moment is about 30%. As for tankers, coefficient of variation of still water bending moment at ballast condition is about 100% and that of full load condition is about 35%.
These results mean that the variability of still water load of container ships is not larger than that of tankers. So tankers are required to have larger safety margine in order to keep the same strength reliability as that of containers. This is contrary to present concept of load conditions of these ships.

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In the theoretical estimation of the long term distribution of the ship's wave induced variables, it is well known that the estimated extreme values are significantly affected by the supposed wave conditions and the ship operating conditions. However, there are few reports which investigate the statistical characteristics of the wave conditions. Furthermore, there is no report which concern with the varying of ship operating conditions of every each ship, which may appear as so-called the captain's spectra.
Authors studied the statistical characteristics of the wave conditions which are observed by 35 voluntary ships in the North Pacific Ocean during the period of 6 years (1976-1981). From this study, it was clarified that meteorological phenomena such as winds and waves in the ocean had wider scatter band than which was estimate by the usual simple population with constant parameter. Authors proposed the probabilistic model of the population of such phenomena which had a complex distribution with random variable population parameters.
In this report, authors study the statistical characteristics of the extreme values of wave induced variable of the ship with the supposed wave condition having the population of the complex distribution.
The general formulation of the extreme distribution taking number of N samples from the population of the complex distribution is shown, and the asymptotic distribution is proposed. From the analysis of the distribution of the maximum wave height observed by 35 voluntary ships, it is found that the mean value of the distribution is proportional to the logarithmics of the sample size and the standard deviation is nearly constant. The properties of the statis-tical characteristics of the theoretical maximum distribution agree with the observed distribu-tion. On the other hand, the distribution of the usual simple population with constant parameter is found to be inaccurate.
The maximum wave height and its probability of each wave period interval have direct effects upon the estimation of the extreme values of the wave induced variables. Authors' study on the distribution of the maximum wave height shows that the theoretical maximum distribution of the population with the complex distribution well agree with the observed maximum distribution of each wave period. From above result, it is clarified that the coefficient of variation of the maximum wave load is 14% to 18% and its value increase proportional to the ship length. This value is larger than the value from the usual supposed wave conditions of the population with constant parameters.

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This paper presented the fatigue strength criterion for longitudinal strength members subjected to both still water bending loads and fluctuating wave bending loads under service conditions.
Using the modified Miner rule, exponential distribution for long term distribution of wave bending stress during 20 years (10⁸ stress cycles) and ordinary S-N_c (N_c : crack initiation life) curves for notched specimens of mild steel, the following allowable tensile stress S_AL for fatigue strength criterion was given.
S_AL=S_a+0.29/K_t·S_m=f_L/K_t (8.1f+16.3) kg/mm²
where, S_a : stress amplitude (wave bending stress at a probability level of Q=10^-4)
S_m : mean stress (still water bending stress)
K_t : stress concentration factor (K_t-values are not to be regarded as absolute values, but as relative values calibrated from experience for structural members)
f : usage factor defined as the cumulative fatigue damage ratio summed over a period of 20 years.
f_L : σ_B/σ_BM (σ_BM : tensile strength of mild steel. σ_B : tensile strength of another material)
Applying the above formula to longitudinal strength members of several ships reasonable results were obtained.

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To estimate the extreme value of combined bending moment resulting from wave bending moment and still water moment is the most important problem on analyzation of longitudinal strength of ships.
Authors study on the characters of long term distribution of the combined moment, and clarify the following effects of still water load on the characters.
In the case, where distribution of still water moment and wave bending moment are described by normal distribution and exponential distribution respectively, long term distribution of combined moment is approximated by exponential distribution which have the same parameter as that of wave bending moment where values of the moment is high.
The resulting characteristic extreme of the moment corresponding Q (M) =10^-8, is sum of the value of wave bending moment, mean value of still water moment, and a value which is proportional to variance of still water moment.
The distribution of extremes of the moment is equal to that of wave bending moment, but the mean value increases by the same value as that of characteristic extreme value.
Compering the above extreme value with the one estimated by regarding still water moment is constant, the difference of these values of a tanker is not negligible small.

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