Video: What Differentiates Ships from Boats?

Did you know that submarines are considered boats? Well, there is a variety of definitions that differentiate ships from boats, some are simple, some are obsolete, and some counter-intuitive. You may even find yourself leaning to one side or the other.

The typical definition relates to the size of the vessel. A ship is a much larger vessel compared to a boat, but there is no real universally agreed upon distinction between the two by simply comparing their size or displacement. Under American and British 19th Century Maritime Law, ships and boats both fall into one legal category. So the size definition is intuitive but not very well-defined.

In the age of sail, a full-rigged ship was a sailing vessel that had at least three square rigged masts and a full bowsprit. Essentially, sail span was what differentiated ships from other types of vessels but the sail span is clearly not applicable to many of the modern classes of ships.

Some other definitions refer to the usage and capabilities, and we hear about this all the time, it’s not about how big it is, it’s about what you can do with it and yes we’re still talking about ships! For example, based on Indian law, vessels that can carry goods by sea are legally considered ships, but even sailboats can carry goods and travel the blue waters.

Another common notion is that a ship can carry a boat, but not vice versa, but then what do you call a vessel that carries a ship? For example, Blue Marlin which is from a class of semi-submersible heavy-lift ships, its job is to carry super large cargo like oil platforms and other ships. Blue Marlin has already transported large ships like USS Cole, HMS Adelaide, and a few others.

Unfortunately, all these definitions are somewhat subjective and open to interpretation, but one has to wonder if there is any properties or attributes that differentiate ships from boats, regardless of just size, capabilities, what they are used for, how much they cost, and so on? Something intrinsic, well intrinsic might in fact be the answer to the question. Intrinsic comes from the Latin word Intrinsecus, meaning inwardly or on the inside, and indeed some vessels tend to behave inwardly when it comes to this one specific maneuver, the mighty turn. Let us understand this.

Here is the first category of vessels

It’s what most of us have experienced when riding on a motorboat or even a dinghy, as the vessel turns it leans towards the inside of the turn circle. We also observe this when we are turning while riding a bicycle or motorbike. Although on land we, as the operator are leaning our body and the bike inward so that we don’t fall as our body’s inertia wants to continue moving us forward in a straight line, but on water that’s just the behavior of some vessels while turning.

Contrasting the first category of vessels, the second category lean toward the outside of the turn circle, looks a little bit counter-intuitive, doesn’t it?

A U.S. Navy rule of thumb is that ships lean towards the outside of a sharp turn, while boats lean towards the inside.

Now, you might be wondering what is causing this difference in the leaning direction.

You may have seen these cool balancing birds

That balancing point you see is called the center of gravity. Center of Gravity is the average location of the weight of an object. A seesaw is also balanced on its center of gravity, but by applying pressure to the arm of the seesaw we create a rotational force also known as torque, which causes the seesaw to rotate around its center of gravity.

For our purposes, we can assume the Center of Gravity (COG) of a vessel to be in the center of the vessel, somewhere close to the waterline.

Similar to the seesaw, any pressure applied to the vessel will create a rotation around the vessel’s COG, we will talk about this rotation force shortly.

Now let’s say we want to make a right turn while the vessel is moving pointing the rudder to the right creates resistance in the water which pushes the stern of the vessel out and to the left, effectively pointing the bow to the right, this is how a turn is initiated, but as the vessel keeps moving with its bow pointing to the right, it doesn’t just do a perfect turn, it slides in the water. As the vessel is sliding and turning, the water that’s in front of the vessel pushes against the left hand side of the hull.

So we have these two forces, one pushes the back of the vessel to the left, we’ll call that the rudder force, and the other pushes the left hand side of the hull to the right, we’ll call that the hull force. Both of these forces are being applied by the water, which means the center of each force is below the waterline, lower than the center of gravity of the vessel. This means that the rudder force and the hull force will try to rotate the vessel around its center of gravity, but in opposite directions. In other words, these forces will cause the vessel to lean and the more dominant force will determine the direction of the leaning.

All vessels experiences a good amount of rudder force as they all have to use their rudders to initiate the turn, but that’s not the case for the hull force. Vessels that have a large portion of their hull underwater, typically categorized as ships will experience a significant hull force, simply because there is a lot of resistance caused by their underwater portion as the ship goes through the turn. On the other hand, vessels with flatter hulls and no keels, typically categorized as boats will experience very little hull force as there is not much of the hull underwater for the water to push against.

If the hull force is greater than the rudder force, the vessel leans outwards, otherwise it leans inwards.

By the way submarines are called boats because historically they used to be much smaller and the boat reference kind of stuck with them but during a turn which way does a submarine lean? Or does it even lean?

While you think of the answer, watch this video nicely explaining the differences between a ship and a boat.