Game Development Reference
Ships and Boats
The physics of ships is a vast subject. While the same principles govern canoes and super
tankers, the difference between the two scales is not trivial. Our goal in this chapter will
be to explain some of the fundamental physical principles to allow you to develop re‐
alistic simulations. The typical displacement-type ship lends itself well to illustrating
these principles; however, many of these principles also apply to other objects sub‐
merged or partially submerged in a fluid, such as submarines and air balloons. Re‐
member, air is considered a fluid when we are considering buoyancy.
While surface ships or ships that operate on the water's surface (at the air water interface)
are similar to fully submerged objects like submarines or air balloons in that they all
experience buoyancy, there are some very distinct differences in their physical nature
that we'll highlight in this chapter. These differences affect their behavior, so it is im‐
portant to be aware of them if you intend to simulate such objects.
Ships have an entire language of their own, so we'll be spending a lot of time just getting
the vocabulary right. This will allow you to do further research on any topics that are
of particular interest. There are many ways to classify ships and boats, but in regards to
the physics governing them, there are three basic types. Displacement vessels, semi-
displacement vessels, and planing vessels are named after the forces that keep the boat
afloat while it is at cruising speed. When not moving, all vessels are in displacement
The term displacement in this context means that the ship is supported solely by buoy‐
ancy—that is, without dynamic or aerostatic lift as you would see on a high-speed racing
boat or a hovercraft. The word displacement itself refers to the volume of water displaced
or “pushed” out of the way by the ship as it sits floating in the water. We'll discuss this
more in the next section.
A planing vessel is one that is not supported by buoyancy, but by hydrodynamic lift.
This includes the everyday speedboats that most boaters own. When the boat isn't mov‐