Capsizing (or keeling over) is when a boat or ship is turned on its side or it is upside down. The act of reversing a capsized vessel is called righting.
If a capsized vessel has enough flotation to prevent sinking, it may recover on its own if the stability is such that it is not stable inverted. Vessels of this design are called self-righting.
In dinghy sailing, a practical distinction can be made between being knocked down (to 90 degrees) which is called a capsize, and being inverted, which is called being turtled. Small dinghies frequently capsize in the normal course of use and can usually be recovered by the crew. Some types of dinghy are occasionally deliberately capsized, as capsizing and righting the vessel again can be the fastest means of draining water from the boat.
Capsizing (but not necessarily turtling) is an inherent part of dinghy sailing. It is not a question of "if" but a question of "when".〔(【引用サイトリンク】title=Will I capsize? )〕 For those who do not want the experience, a keelboat monohull has physics on its side.〔〔 But even yachts can capsize and turtle in extraordinary conditions, so design considerations are important. Such events can overcome skill and experience; boats need to be appropriate for foreseeable conditions.
A capsized kayak may be righted with a roll or eskimo rescue. As long as the kayaker knows how to react, the water is not too shallow, and the location is not close to dangers which that require evasive action by the kayaker – which cannot be taken while capsized – capsizing itself is usually not considered dangerous. In whitewater kayaking, capsizing occurs frequently and is accepted as an ordinary part of the sport.
For sailing vessels, the "capsize ratio" is a commonly published number used as a guideline for safe operation, where a ratio of less than 2.0 is considered suitable for offshore operation. However the only variables that go into "capsize ratio" are a vessel's beam and its displacement, and a thorough assessment of ship stability needs to consider various other factors.
A vessel may be designated as "self-righting" if it is designed to be able to capsize then return to upright without intervention (with or without crew on board). Most small craft intended as lifeboats with rigid (rather than inflatable) hulls designed since about the middle of the twentieth century are self-righting.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』