European Custom Regs/Flag Etiquette

[multihullsailor6]

Advice needed on the handling of European Customs regulations, please.

I am considering buying a second-hand, currently European registered and VAT-paid yacht in Europe.

I hold the South African citizenship, together with a permanent German (a Schengen treaty country) residency permit . The boat will be registered in South Africa and will be flying the South African flag with the respective curtosy country flag on starboard.

When going from one European Schengen-treaty country to another Schengen-treaty country must I fly the Q flag and check in with Customs? When travelling on the roads there are no custom formalities any more in such a case.

What is the story when I sail from, say, Holland to the UK?

Thanks
Roger