Barnacle encrusted hull.

Across the hull of the Denman you will see patches of dried barnacles and tube worms. This shows how the copper works to help protects the hull from damage from marine organisms.

The copper acts as a sacrificial layer and because of the copper component slows the rate at which marine organisms will attach themselves to the hull.
In more recent times copper based anti fouling paints were used for the same purpose, but this caused other problems with copper and other chemicals leeching back into the marine environment, this practice has been banned from most developed countries.
Alternatives are available that do the same job but are not bad for other marine organisms.

The problem comes from the gradual buildup slowing the vessels speed and adding greatly to it’s use of fuel.

There was no easy way to remove them, the vessel had to be slipped and the build up of weed, tube worms and barnacles had to be scraped or blasted off with water.