The total unsteady cavitation volume on a given propeller, which generates radiated noise at propeller blade rate and harmonics, can be bounded from basic design considerations. At a vessel's design speed, an upper limit to this unsteady cavitation exists from considerations of excessive ship vibrations, and a lower limit exists from considerations of propeller efficiency. This paper presents a model for blade rate radiated noise from merchant ships which is based on these design limitations for cavity volume, and an elementary radiation model. A sample survey was made of the propeller characteristics of the world fleet, which allows the further derivation of a source model based on physical characteristics of a given ship, including a definition of the uncertainty of the estimate. This model is exercised for the estimated merchant ship population at sea and a stochastic representation is derived for the source levels and frequencies to be expected for this merchant fleet operating at its design speed.

A model is developed for the acoustic source strength of blade rate line energy produced by single-screw merchant vessels. These source strengths are based on observed cavitation time histories on merchant vessels and on limitations imposed by considerations of propeller design procedures and ship vibration criteria. Relationships are presented for the expected value of the blade rate source strength for ships of different lengths, expressed both as a monopole source strength located at a known depth below a free surface and as a dipole source strength that describes the pressure radiated to the farfield. These relationships are based on a small sample of merchant ship characteristics and are exercised for the estimated population of ships at sea. This calculation yields a statistical description of the distribution of source level and frequency of propeller blade rate acoustic energy for the fleet of single-screw merchant vessels.

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