From last up to the next Drydocking a vessel needs some UnderWater maintenance works to solves some problems.
Theoretically if a vessel is coated by good antifouling at drydock, these can keep also five years.
Essential precondition is the vessel to be sailing continuously.
If a vessel drop anchor or stay for long time in a harbour, then begins adhesion of some marine micro-organisms at all u/w parts.
Marine micro-organisms growth depends the remaining time and 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.