What Does the Ship Designation ‘SS’ Mean?

RMS Titanic. USS Indianapolis. HMS Victory. Often, when you see the name of a boat (or a ship, for that matter), the name of the vessel itself will be prefixed by a short set of letters.

There are so many different forms of these letters that you might think they’re little more than a random license-plate style code number. But each one is actually an initialism used to identify the precise type or purpose of each ship—and in civilian vessels, the most frequently encountered of these prefixes is probably SS.

1. U.S. Ship Designations Throughout History
2. Ship Designations Around the World

U.S. Ship Designations Throughout History

SS dates back to the mid-1800s, when the Age of Sail came to an end and faster coal-powered ships became the norm. The shipbuilders of the day wanted a means of setting their modern vessels apart from the wind-powered ones of the past, and labeling each one SS—meaning “steamship”—did the trick.

As naval technologies continued to change, however, so too did people’s understanding of precisely what SS was intended to mean. Other SS vessels were screw steamers or single-screw steamships (screw is the word for a nautical propeller), and it likewise became common practice to specifically label paddle steamers PS, and larger twin-screw and triple-screw steamships as TSS and TrSS, respectively.

Not all vessels are civilian operated, of course. In the United States, all craft of the U.S. Navy are prefixed with the letters USS, standing for “United States Ship.” It was President Theodore Roosevelt who first introduced the convention to standardize the names of all U.S. naval vessels in an executive order, which decreed that “the official designation of vessels of war, and other vessels of the Navy of the United States, shall be the name of such vessel, preceded by the words, United States Ship, or the letters U.S.S., and by no other words or letters.” Before then, ships could be labeled in line with their class, their type, their arrangement of rigs or propellors, or any other such identifying feature, with very little rhyme or reason.

Ship Designations Around the World

Other nations have their own designations. Ships of the UK’s Royal Navy, for instance, are labeled HMS (“His or Her Majesty’s Ship”), with the nations of the Commonwealth following suit—Canada uses HMCS, Australia HMAS, and New Zealand HMNZS.

As for the Titanic, it was given the designation RMS because, despite being a commercial vessel, it was contracted to deliver items sent via the UK’s Royal Mail postal service. (The acronym at the time stood for “Royal Mail Steamer,” and these days simply means “Royal Mail Ship.”) And British scientific vessels given the royal seal of approval are designated RRS, meaning “Royal Research Ship”—as was the case with Sir Robert Falcon Scott’s Antarctic ship RRS Discovery.

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