Prevent Hull Damage
Once the anti-foulant coating begins to develop a layer of slime, more advanced stages of fouling will begin to appear, including tubeworms and barnacles. If a diver does not intervene, these shellfish can actually penetrate the coating and inflict permanent damage. The staged process to permanent barnacle damage is depicted in this figure.

Stage 1: Hull layers.

Stage 2: Development of slime, grasses, and an immature barnacle.  Cleaning the hull now would allow a softer bristle brush to be used resulting in less paint damage and rejuvenation of anti-foulants.

Stage 3: Continued barnacle growth, now beginning to penetrate the protective coating. Cleaning at this stage will require a combination bristle and wire brush.

Stage 4: The barnacle has grown and penetrated the protective coating, putting the barnacle in direct contact with the steel hull causing permanent hull damage. At this stage the anti-foulant is almost non-existent. If the hull is cleaned during Stage 2 when the immature barnacle is just beginning to develop, permanent hull damage can be avoided.

Propeller Efficiency

The propeller is particularly vulnerable to marine fouling since it is an unpainted surface that must remain clean and shiny for proper operation.  The U.S. Navy determined that propeller fouling, despite its small surface area, can generate energy losses amounting to half that of the hull itself so maintaining a clean propeller is critical. On military ships, the unpainted surfaces such as propellers, rudders, and sonar domes are cleaned twice as often as the hull surfaces. The propellers are also polished routinely to reduce friction and ensure that the propeller operates at optimum efficiency. Even with routine maintenance, surface roughness can occur as a result of erosion, corrosion, or from tubeworm tracings. This roughness alone can increase fuel consumption up to 10 percent.

Hull cleaning intervals:

The optimum interval between the periodic cleanings and inspections that comprise a preventive maintenance program will vary with the type of vessel, the location of the vessel, and its service profile (speed of operation, idle time, etc.). The type and condition of bottom coatings will also have an effect on the cleaning interval. Large vessels typically have several layers of coatings, up to 6 millimeters thick, and generally operate 4 to 6 months between hull cleanings.

The location of the vessel also has a substantial influence on the rate of fouling since marine organisms flourish in warm tropical waters. The U.S. Navy has established geographic fouling zones, indicating the frequency with which the hull and unpainted surfaces (propellers, rudders, and sonar domes) should be cleaned for vessels operating within each geographic zone. In Navy Zones 1 and 3, propeller cleaning is recommended up to six times a year and hull cleaning is recommended up to three times a year.