Why Rodney received unusually frequent hull cleaning

What did cleaning Rodney's hull involve, and how often was that normally done?

Normal intervals were about eighteen months to two years, but Rodney and Nelson were cleaned roughly every six months in wartime to protect performance.

A peacetime warship might receive hull cleaning during dry docking roughly every eighteen months to two years. Rodney and Nelson appear to have had the work done about every six months during the Second World War. Naval performance figures commonly assume a clean or moderately aged underwater hull, because fouling progressively reduces speed and efficiency. Frequent cleaning therefore kept the ships closer to their expected performance despite intensive wartime use. The comparison is to changing racing tires early so that peak performance remains available when needed.