What is the difference between SS/HMS/RMS on a boat?

SS = steam ship
HMS = Her/His Majesty’s Ship
Edit – forgot to add RMS = Royal Mail Ship/Steamer

RMS = Royal Mail Steamer

A mail carrying vessel.

great, thanks

HMS ships are vessels for the Navy, by the way. War ships.

Wow. Who knew that the S.S. Minnow was a steam ship?!?!

That, for some reason, really blows my mind.

USS: United States Ship. Commissioned warships of the US Navy crewed by active duty US Navy personnel.

USNS: United States Naval Ship. Commissioned auxiliaries of the US Navy crewed by US Merchant Marine officers and crew.

M/V: Motor vessel (This is what the Minnow should have been called because it was powered by diesel engines. The “M/V Minnow”. To cause further confusion, M.V. also stands for ‘marine vessel’ in Canada.)

TS: Training Ship. Training-Class British War ships of the Royal Navy.

HMS: Her Majesty’s Ship. British War ships of the Royal Navy.

S/V: Sailing vessel.

NS: Nuclear-powered ship.

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Edited by me for readability, sourced from the Wikipedia article foot notes.

Darwin was on the HMS beagle, would you still consider that to be a war ship? It didn’t seem to do anything very warlike…

The HMS Beagle was initially equipped with weapons and intended as a war vessel, but later adapted as a survey ship and took part in three expeditions. Because she retained an armament, she retained her warship class. If needed, the vessel would have been called to arms and participated in battle.

Further reading: http://www.portcities.org.uk/london/server/show/ConFactFile.64/HMS-Beagle.html

From Wikipedia, the HMS Beagle was equipped with “10 guns, reduced to 6 guns for survey voyages”.

Wow, i had no idea. Neat!

Here are some examples of those types of boats.
SS: (this is from Austin Powers) The S.S. Shagitsy
H.M.S. : H.M.S. Hood sunk during World War 2 by the German Battleship Bismarck.
R.M.S. : In April 1912 The “Unsinkable” Titanic hit an Iceberg and sunk off the coast of North America.