Practical Methods for Predicting Periodic Propeller Loads

A variety of procedures for predicting periodic bearing (shaft) loads and single-blade loads on propellers operating in circumferentially nonuniform flow are systematically evaluated. These procedures include the following: 1. a simple quasi-steady procedure that uses the open water characteristics of the propeller; 2. a simple stripwise two-dimensional unsteady procedure; 3. an unsteady lifting line procedure; and 4. four different numerical procedures based on unsteady lifting surface theory. The ranges of applicability and limitations of the evaluated procedures are assessed. The assessments of applicability are based on comparisons with: 1. model experimental periodic bearing loads over systematic ranges of propeller parameter in idealized wake patterns; 2. model experimental single-blade loads in idealized wake patterns; and 3. model experimental single-blade loads simulating the effects of waves and ship motions. Tradeoffs between accuracy of predictions versus ease and simplicity of the computational procedures for practical applications are discussed. The results show that unsteady lifting surface procedures give the best prediction of periodic bearing loads over practical ranges of propeller parameters. However, quasi-steady and empirical methods are adequate for estimating single blade loads for most practical applications.