Initial stability or primary stability is the resistance of a

boat A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carry boats. Small boats are typically found on inl ...

to small changes in the difference between the vertical forces applied on its two sides. The study of initial stability and secondary stability are part of

naval architecture Naval architecture, or naval engineering, is an engineering discipline incorporating elements of mechanical, electrical, electronic, software and safety engineering as applied to the engineering design process, shipbuilding, maintenance, and o ...

as applied to small

watercraft Any vehicle used in or on water as well as underwater, including boats, ships, hovercraft and submarines, is a watercraft, also known as a water vessel or waterborne vessel. A watercraft usually has a propulsive capability (whether by sail, o ...

(as distinct from the study of

ship stability Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on centers of gravity, centers of buoyancy, the ...

concerning large

ship A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguishe ...

s).

Determination

Initial stability is determined by the angle of tilting on each side of the boat as its center of gravity (CG) moves sideways as a result of the passengers or cargo moving laterally, or as a response to an external force (e.g. wave). The wider the boat and the further its volume is distributed away from its center line (CL), the greater the initial stability.

Examples

Wide mono-hull small boats such as the jon boat have a great deal of initial stability and allow the occupants to stand upright to engage in

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activities, and so do narrower small boats such as W-kayaks that feature a twin hull. Very narrow mono-hull boats such as

canoes A canoe is a lightweight narrow water vessel, typically pointed at both ends and open on top, propelled by one or more seated or kneeling paddlers facing the direction of travel and using a single-bladed paddle. In British English, the term ...

and

kayaks A kayak is a small, narrow watercraft which is typically propelled by means of a double-bladed paddle. The word kayak originates from the Greenlandic word '' qajaq'' (). The traditional kayak has a covered deck and one or more cockpits, each ...

have little initial stability, but twin-hull W-kayaks are considerably more stable due to the fact that their buoyancy is distributed at a greater distance from their center line and therefore acts more effectively to reduce tilting. For purposes of stability, it is advantageous to keep the

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as low as possible in small boats, so occupants are generally seated. Flatwater rowing shells, which have length to beam ratios of up to 30:1 are inherently unstable and must be actively balanced by the athletes.

Compared to secondary stability

After approximately 10 degrees of lateral tilt hull shape gains importance and secondary stability becomes the dominant consideration in boat stability.

Other types of ship stability

* Secondary stability * Tertiary stability: For kayak rolling, tertiary stability, or the stability of an upside-down kayak, is also important (lower tertiary stability makes rolling up easier)

Determination

Examples

Compared to secondary stability

Other types of ship stability

See also