The class is the main focus of IMOCA. As the names imply, this class is "open", meaning that it does not represent fixed boat designs; instead, they are defined by a "box rule" which permits any design, as long as it meets certain restrictions. The restrictions limit the lengths of the boats and place certain safety requirements on the boats, such as minimum stability requirements. This class is recognised by the International Sailing Federation.
Boats are typically light and high-powered, with large sail-plans. Typically consisting of large carbon-fibre shells, there are few concessions to comfort.