I have checked the 1871 England and Wales census for you and there was
nobody by the surname KLUG on it. Therefore, your ancestor must have still
been in the country of his birth for those 18 missing years.
I believe everyone in those days had to have identity papers which were
stamped everytime (s)he arrived or left an area. Usually a notice was
displayed for several weeks stating the intention to leave which gave every
person a chance to claim any money that was owed. Have you made enquiries
at the Visselh�vede archives?
I think I saw on a webpage the last time I surfed for you that the Steam
Ship Republic was not registered to sail to any German port thus your
ancestor must have sailed on a different ship from Hamburg. The reason most
Europeans used British ships for long journeys is that they were able to
withstand the heavy seas and not break up & sink.
Due to the World War II bombing the Hamburg passenger ship records you seek
might have been destroyed but I wish you luck in your enquiries.
Rena