From the last up to the next Drydocking, a vessel needs some UnderWater maintenance works, to solves the relevant problems.
Theoretically if a vessel is coated by good antifouling at drydock, is for five years.
Essential precondition is the vessel to be sailing continuously.
If a vessel drop anchor or stay in a harbour for long time, then begins adhesion of some marine micro-organisms at all u/w parts.
Marine micro-organisms growth, depends the remaining time and the location with parameters like water temperature, water visibility, sea/fresh water, current, swell.
Marine growth has as result to increasing the “slipping”, as consequence is to decreasing the speed and increasing the fuel consumption.
Solution is to carry out a “hull cleaning“.
Anodes applied for the period of 5 years. During intermediate survey ( or later ) anodes wastage appears approx. 90-100%.
Solution is to carry out “anodes replacement” bolted or by welding.
Some overboard valves stuck or leaking but to carry out the valve repair ( cleaning or replacement ) is necessary to carry out “plugging of overboard“.
Same for sea chest,