Staying longer in Europe
Mark, you might want to review the article on VAT located at the same URL I posted above.
Immigration issues are distinct & different from Customs issues, so you always want to tackle the two issues separately. So...it''s about ''you'' and then, separately, it''s about ''the boat''.
The UK is not a Schengen signatory, nor is Ireland. Since they are island nations, they really didn''t face the same border-crossing nuisances that the continental European countries did. That also explains why Schengen restrictions aren''t synonymous with the EU, as e.g. Norway is not an EU Member but still chose to participate in Schengen since they share a common border with Sweden.
Re: VAT, all non-EU boats are allowed 18 months of exclusion from VAT liability while within one or more of the EU countries. Once the boat steps into a non-EU country, the boat''s 18 month ''clock'' is reset which *should* allow them to re-enter the EU for another 18 months. (In addition, some countries routinely allow a boat to be put into bond for up to 6 add''l months - e.g. if you are returning home for a period of time, outside the EU, and the boat isn''t going to be used. Whether that add''l period is honored by a subsequent EU country one visits is iffy). Once you step outside the EU, the customs laws of the non-EU country(ies) you visit will apply.
Sorry for the long-winded answer; this stuff can get confusing and its easy to overly generalize.