Propeller Coatings: A Means By Which To Reduce At-Sea Propeller Roughness?

Elizabeth (Liz) Haslbeck (NSWC Carderock)

Propellers of US Navy surface ships are typically uncoated and are thus unprotected from two types of fouling including mineral deposits (or cathodic chalk) and biological forms. These types of fouling negatively impact operational efficiency and can reduce readiness and capability tied to issues such as top speed, downtime for maintenance, and signature concerns. On most vessels, the Navy currently combats the problem through diver in-water cleaning alone. The efficacy of this approach is influenced by scheduling (timing) relative to underway periods, frequency, and vessel homeport/location.  A baseline propeller roughness study (surface ships only) indicated the approach is not perfect, and vessels routinely go to sea with one or types forms of fouling. As well, a substantial proportion of the propulsive fuel penalty associated with hard biofouling is associated with a relatively small proportion of ships steaming in a heavily biofouled (calcareous forms) condition. To address this problem, we explored the extent to which commercial fouling release coating (FRC) systems, designed for use on hulls but applied to propellers, could more reliably mitigate the risk. FRCs are designed to minimize the adhesion strength between biological glues and the coating surface; the shear stresses that develop on propellers were expected to dislodge the biofouling from the coating.  

We installed a 3-coat FRC system (primer, tie coat, topcoat) to the propellers of a surface ship.  We found that through about the first 5-6 years of service the coating system, despite some degree of physical degradation, reliably mitigated both types of fouling* without routine diver in-water cleaning [*exception: oysters].  Coating physical degradation to bare metal was observed within 6-12 months of service, but was limited to leading edges and at the blade tip in areas of highest cavitation on the suction face.  Despite occasional lengthy in-port periods (and likely going to sea in a fouled condition), upon return from at-sea periods the propellers remained free of impactful accumulations of hard biofouling, even where the coating was missing. Over time, coating physical condition degraded over larger areas of the blade faces, and characteristic roughness elements developed including a) circular pits of varying diameters and depths (tied to the coating layer affected – topcoat, tie coat, and primer) and b) edges or ledges of varying lengths and heights (again tied to the coating layer affected).  We quantified the roughness associated these elements and their associated impact on propeller performance, and integrated our findings into a business case analysis. Despite the added roughness tied to coating system degradation, the fuel savings associated with the coating’s early and reliable biofouling control resulted in a positive return-on-investment over the entire interval between dry-docking (coating replacement).