Ship stability
Ship stability is an area of naval architecture
Naval architecture
Naval architecture is an engineering discipline dealing with the design, construction, maintenance and operation of marine vessels and structures. Naval architecture involves basic and applied research, design, development, design evaluation and calculations during all stages of the life of a...

 and ship design that deals with how a ship behaves at sea, both in still water and in waves. Stability calculations focus on the center of gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...

 and center of buoyancy of vessels and on how these interact.
Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years. Historically, ship stability calculations for ships relied on rule of thumb
Rule of thumb
A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination...

 calculations, often tied to a specific system of measurement.