Hi,

Can anyone help me with the following problems I have, concerning how to add apostrophes to plural words that already end with an s?

which of the following is correct?

Cyrus's car, or Cyrus' car?

Spectres's existence, or spectres' existence?

Vessels's peace, or Vessels' peace?

I notice there are differences between US English and UK English, it's the UK version I need.

Any help would be appreciated.
Many Thanks.