PURPOSE: To decrease hull resistance at the time of high speed sailing as well as to decrease rocking, and thereby enable a catamaran to be made larger in size.

CONSTITUTION: Let the length of a hull where each barrel section 2 is immersed under water when the catamaran sails, be L, and let the width of each barrel section 2 at a water level be (b). And let the whole width between both the barrels 2 be B. The type of a ship is thereby conditioned by relation between L and (b) which shall be 15≤L/b≤30, and relation between B and (b) which shall be 3≤B/b≤5. Since L can be made long, hull resistance at high speed sailing can be decreased. And L is long enough as compared with the lengths of waves so as to result in an increase in sea worthness, therefore the catamaran less in rocking can thereby be obtained. A distance between both the barrel sections can be made narrow, so that the weight of a both barrel connecting section can be decreased. This constitution thereby allows strength required to be made much less, and also enables the catamaran to be made larger in size.