What is the difference between a ship and a boat? It’s one of the questions I am most frequently asked. One would imagine that after several thousand years of seafaring, we’d have a simple and concise answer.

We don’t. Most of the answers I’ve heard are somewhat true some of the time, but never true all of the time.

Ships sometimes ride on larger ships; boats sometimes carry smaller boats. Some boats have multiple decks. Some ships lack permanent captains or crew, while some boats have both. And, like ships, larger multihull boats don’t heel much when they turn. Add to this the variety of new hull types that do not conform well (or at all) to traditional ship design, and the challenge of answering the question seems even more daunting.

The fact that military, commercial and recreational vessels all use different tonnage conventions doesn’t make the task any easier, but those differences are not as problematic for this purpose as one might expect. In trying to ascertain the magic size beyond which a boat is unambiguously a ship, it’s useful to look at where a variety of sources—commercial and military—converge.

The smallest class of military vessels universally considered ships is destroyer escorts. The next smallest military vessels (corvettes and patrol boats) are unambiguously called boats. Pictured on the next page is the USS Buckley, DE-51, the lead ship of the most prevalent U.S. destroyer escort class of World War II. The Buckleys were 306 feet long with a displacement of 1,673 long tons. Those specs are a good benchmark, as the Buckleys are the smallest U.S. naval vessels to be considered ships.

Next, consider the realm of U.S. Coast Guard licensing for the U.S. Merchant Marine. The licensing categories are based on gross register tonnage (GRT). A register ton is a measurement of volume: 100 cubic feet. GRT is the total volume of all the permanently enclosed spaces on a vessel.

The Coast Guard categories for small boats, large boats and ships are based on tonnage. Vessels of 0 to 1,600 gross tons generally fall into the small and large boat categories, while vessels of more than 1,600 gross tons usually are categorized as ships.

While we may seem to be converging on a clear definition—vessels less than 300 feet length overall or less than 1,600 gross tons are considered boats—remember that register tons and displacement tons are different units of measurement. So where does the Convention on the International Regulations for Preventing Collisions at Sea (COLREGS), adopted in 1972, draw this line? While the terms ship and boat are never explicitly defined, the regulations split fairly cleanly among vessels less than 164 feet long, vessels 164 to 300 feet long, and vessels greater than 300 feet.

This breakdown aligns nicely with the unofficial USCG licensing categories of small boat, large boat and ship. So, greater than 1,600 tons, regardless of how that tonnage is calculated, defines a ship. Longer than 300 feet can also define a ship. That figure is entirely consistent with the USS Buckley, which was 306 feet, our starting point.

Thus, if a vessel crosses any one of these thresholds (1,600 tons or 300 feet) it may be considered a ship. This definition accommodates many idiosyncrasies of hull and multihull design.

Now that it appears we’ve settled that, a final note: In almost every discussion delineating ships from boats, someone at some point mentions that submarines are always boats, regardless of size.

This is true. The largest submarines ever built were the Soviet Akulas, which are 24,000 long-ton displacement when surfaced (roughly the same as a Yorktown class aircraft carrier) and 547 feet long. They may be boats to their crews, but they are nonetheless ships by all standards.

So, where did this convention of referring to submarines as boats come from? Almost certainly, World War II. The three largest classes of submarines in the U.S. war fleet were Gato, Balao and Tench. All were 312 feet in length. The most voluminous, Tench, was 1,570 displacement tons surfaced—yet still unambiguously considered a boat.

And so, it seems, the debate continues.