"H.M.S." is an abbreviation for "His (or Her) Majesty's Ship." It's only applicable to vessels of the British Royal Navy. Canadian warships are referred to as "H.M.C.S.", Australian ones as "H.M.A.S.," and New Zealand ones as "H.M.N.Z.S."
Merchant steamships can quite legitimately be referred to as "S.S." ("Steamship.") British steamers that have been given approval to carry the Royal Mail are referred to as "R.M.S." (Royal Mail Steamer). The Titanic was so designated; she's usually referred to as "R.M.S. Titanic." ("S.S. Titanic" is also technically correct. Photos show that the words "S.S. Titanic" were painted on her lifeboats.
So far as I know, there is no abbreviation for merchant sailing vessels. (British sailing warships are labeled "H.M.S.," American warships "U.S.S.," etc. But not sailing merchantmen.)
Hope that helps a little. Good luck.
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