DEVELOPMENT OF PROTECTIVE COVERING SYSTEMS FOR STEEL AND BRONZE SHIP PROPELLERS

This paper reports on the developments of elastomeric covering systems that are suitable for the protection of the surfaces of steel ship propellers, bronze propellers, rudderhorns, pump impellers, and other such marine surfaces. The development of covering systems for steel propellers is to sharply reduce the capital costs of propellers (by approximately 50 percent) and the cost of cathodic protection (by about 80 percent) by providing the necessary protection against cavitation erosion, erosion, and galvanic corrosion while providing a smooth (slick) propeller surface having lower hydrodynamic drag. For existing bronze propellers, the purpose of covering systems is to sharply reduce cavitation erosion damage and cathodic protection requirements, and to improve propulsion efficiency by reducing propeller drag. Results of extensive laboratory research on covering systems, leading to and including the full-scale testing and evaluation of a covered steel propeller, are discussed. Included are test results of materials, primers, binding agents and their combinations that address the important issues of bond strength, long-term hydrolytic stability, cavitation erosion resistance, covering thickness, and surface drag effects for various finishes. Data from specially devised test equipment and methods, including variable-speed rotating disks, ultrasonic horn, model propellers, soak tests, peel tests, and lap shear tests are provided. The final candidate covering systems established to date for marine use are identified.