See full version: What s the difference between a boat and a ship
All dictionaries try to avoid the dread lexicographic condition known as circular defining. This is when one looks up a word such as dictionary, sees that it is defined as “a lexicon,” and, when looking up lexicon, finds that it is defined as “a dictionary.” Given that we spend a considerable amount of time avoiding this sort of defining, it may come to a surprise to some users to discover that one of the definitions for boat is “ship,” and vice versa.
The fact that English usage is messy, and has contributed to a use of boat that is somewhat vague, does not mean that there aren't settings where precision is called for. For instance, when you are sailing on someone else's vessel it is polite to always employ the correct terminology. And if you find yourself at a loss about when a boat becomes a ship you should contact your local maritime authority.
A boat is a watercraft of modest size designed to float or plane, to provide passage across water. Usually this water is inland (lakes) or in protected coastal areas. In naval terms, a boat is something small enough to be carried aboard another vessel (a ship).
Strictly speaking and quite uniquely a submarine is a boat as defined by the Royal Navy. Some boats too large for the naval definition include the Great Lakes freighter, riverboat, narrowboat and ferryboat.
While ships and boats are both watercraft, they are different in size, cargo or passenger capacity, where they operate and their capabilities.
Boats are equipped with motor-driven propellers which are powered by small diesel engines. more
Moreover, boats are generally used in areas that offer low sea conditions with thinner channels of water, which cannot accommodate heavy ships. more
In contrast, ships with an extended cargo capacity usually operate in high seas and correctly used to transport people and goods. more
The hull of a boat is constructed in the traditional method, as the old techniques offer enough quality and strength.
On the other hand, LSTs (Landing Ship, Tank) or LSMs (Landing Ship, Medium) have the very word “ship” in their names. Yet, they have flat bottoms.
PT Boat 109 commanded by LT John F. Kennedy
Occasionally, I’ll hear a different explanation: “A ship has a commander and a crew. A boat just has whomever is on it at the time.” here
The problem with all these explanations is that they are riddled with exceptions. [links]
But even the Navy has a hard time explaining the most fundamental mystery of all: what is the difference between a ship and a boat?
Take the WWII PT Boat. It could carry a lifeboat on its foredeck. It was made for deep water and had a permanent crew with a commanding officer. Yet, no one calls a PT Boat a “ship.” Same goes for so-called Vietnam “swift boats” (Patrol Craft, Fast or PCF). They had crews, but were definitely “boats.” [links]