On the New NK-Rule Requirements for the Strength of Propeller Blade

In the recent years, some propeller blades of liner ships have been damaged due to fatigue, which make a demand to revise the Rule recuirements by taking the evaluation method of the propeller blade strength based on the alternative stress, instead of the past one based on the mean stress. There are a few methods to calculate the alternative stress, however, most of them are of the numerical computer calculation to analyze the flow around the propeller but are unsuitable to use in the Rule requirements. In this paper a formula for blade stress calculation using simple parameters is led from Schoenherr's thrust equation and examined by comparing with the results of a computer calculation using the lifting surface theory and with the measured stresses on actual propellers equipped on ships. Further, considering the conditions of sea state and ship operation and the allowable stress provided from the latest fatigue test results, a basic formula of the new Rule requirements for the blade strength (thickness) is presented.