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Before any large vessel can enter a busy harbour, a licensed pilot boards to —guidegovernguardglide the ship through narrow channels and shifting sandbanks. Pilots combine formal certification with years of —accumulatedaccumulatingaccumulateaccumulation knowledge of local tides, currents and traffic patterns. Because a single grounding can —blockblockingblockedblockage a channel for days, ports insist that pilotage remain —mandatoryvoluntarymandatedoptional for all vessels above a set tonnage.