·add. ·noun A special class of small yachts developed in Germany under the patronage of Emperor William and Prince Henry of Prussia, and so called because these yachts do not conform to the restrictions for the regular classes established by the rules of the International Yacht Racing Union. In yachts of the sonderclass, as prescribed for the season of 1911, the aggregate of the length on water line, extreme beam, and extreme draft must be not more than 32 feet; the weight, not less than 4,035 pounds (without crew); the sail area, not more than 550 square yards; and the cost of construction (for American boats) not more than $2400. The crew must be amateurs and citizens of the country in which the yacht was built.