A member query about the VAT rules led us to this page where FAQs are answered by the European Commission. All the relevant legislation is cited. 


Can the 18 months be extended if the yacht is not used? You may want to go home for Christmas! There is a possibility of extension of that period, but only in exceptional circumstances, as it is established in Article 251(3) of the Union Customs Code (UCC).
https://ec.europa.eu/taxation_customs/sites/taxation/files/rules_for_private_boats-faq_en.pdf


Vice Commodore, OCC