The short answer
The Oxford Learner's Dictionary calls a ship "a large boat that carries people or goods by sea". It calls a boat "a vehicle (smaller than a ship) that travels on water, moved by oars, sails or a motor". So the first difference is size, and the second is where it goes. A ship is made for the sea.
Collins, on Dictionary.com, gives a test you can use standing on a quay. A ship is "a large vessel that cannot be carried aboard another". A boat is one that "can be carried aboard a larger vessel". If it could be lifted onto the deck of something bigger, it's a boat.
Is there a size where a boat becomes a ship?
Not in the dictionaries. You'll find lengths in feet and weights in tonnes quoted online, but neither Oxford nor Collins gives a figure. Both define the two words against each other: a boat is smaller than a ship, and a ship is too big to carry aboard. That leaves a grey area in the middle, which is why people still argue about it. In October 2026, Google's top UK results for this question included cruise and boating forum threads doing exactly that.
Is the Titanic a boat or a ship?
A ship. Royal Museums Greenwich gives Titanic's overall length as 882.75 feet, and explains that the RMS in front of her name stands for Royal Mail Ship. She also carried 20 lifeboats. Those were boats: small craft carried aboard a larger vessel, just as Collins puts it. RMG adds that the 20 could hold 1,178 people, far fewer than the number on board.
Why is a submarine called a boat?
Because the navy calls it one. Collins lists it as a naval meaning of boat: in the navy, a boat can mean a submarine. So when a submariner talks about "the boat", they're using a meaning the dictionary records, not making a mistake.
Can you call a ship a boat?
In everyday English, yes. Oxford gives "any ship" as a second meaning of boat, with the example of going to France "by boat", meaning by ferry. Collins agrees, but marks that meaning "not in technical use". So a cruise ship is a ship in the strict sense, and a boat only in the loose one.
What does UK law call a ship?
The law reaches further than the dictionary. Section 313 of the Merchant Shipping Act 1995 says that "ship" includes every description of vessel used in navigation. The definition says nothing about size. So in that Act the word covers more than it does in conversation, and a legal "ship" and an everyday ship aren't always the same thing.
Ships and boats in our prints
Our pictures sit on both sides of the line. Full Sail is a three-masted square-rigger with every sail set. Collins calls any square-rigged sailing ship a tall ship, and ours are together in our ship prints. Room to Sail is one small dinghy on wide green water, and that's a boat. The two fishing boats in Crab Boats and the rowing boat in Harbour Steps belong to our coastal prints, among the harbours and coves.
Would you like a print of a boat you own? Our boat portraits page explains both ways: print the photo as it is, or have AI restyle it as a watercolour, marine oil or colour study, with a free preview first. For the names of other boats, see our types of boats guide.
Sources
- Oxford Learner's Dictionaries: ship and boat
- Collins, on Dictionary.com: ship and boat
- Royal Museums Greenwich: RMS Titanic facts
- Merchant Shipping Act 1995, section 313
Craig Fearn runs Still & Wake. He writes the guides and journal.