Some types of multi-hull ships are researched more or less, and some types of them are applied today. The slamming (wave shocks to hull structure) and the wetness of upper deck are common problems of seaworthiness of all sea-going ships, but all multi-hulls differ by the additional possibility of wet deck slamming. Slamming and wetness are the sufficient restrictors of ship operability at sea, therefore, ship dimensions, which are connected with these phenomenon, must be selected carefully for maximal seaworthiness of multi-hulls. If the wave level exceeds the local height of board, deck wetness is generated. The shock of a structure by wave means one-time exceeding of threshold values by two random processes: the vertical displacement of water level relative to ship and the speed of the displacement. The formula for prediction of coincidence frequency is shown. Standards of number of slamming and wetness are selected for definition of the permissible vertical dimensions of hulls. As an example, the results of some calculation of the minimal clearance (the distance between the design water-plane and wet deck) of two catamarans are shown. A strong dependence of the needed clearance from the relative beam of a hull is shown. The carried out data are recommended for the selection of a catamaran minimal clearance at zero approximation.