How harbour dues are actually calculated
The headline formula is length overall multiplied by days alongside, and then almost every harbour applies exceptions that matter more than the formula. Multihulls are usually charged on beam as well, because they occupy the water a monohull of the same length does not. Commercial vessels are on a separate tariff entirely. Short stays under four hours are commonly free or nominal to keep the visitor berths turning over. And the annual contract, if you can predict your use, is nearly always cheaper than the sum of the nights you will actually take.