Initial stability is the resistance of a boat to small changes in the difference between the vertical forces applied on its two sides. It 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.
Wide mono-hull small boats such as the jonsboat have a great deal of initial stability and allow the occupants to stand upright to engage in fishing activities, and so do narrower small boats such as W-kayaks that feature a twin hull.
Very narrow mono-hull boats such as canoes and kayaks have little initial stability, but twin-hull W-kayaks are considerably stabler 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. It is advantageous to keep the centre of gravity as low as possible in small boats so occupants generally sit down at all times. 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.
After approximately 10 degrees of lateral tilt hull shape gains importance and secondary stability becomes the dominant consideration in boat stability.
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