The catamaran has two hulls shown fully drawn on fig.2. On fig.2 it is shown that a fully drawn crosssection of one underhull (3) can be replaced by the cross-section of the other underhull (4) shown punched, because it is a mirror image of the other about the middlenormal-plane for the dividing ribbon-surfaces, with the keelplane k_ø at a right angle to the cora k.Because a and b are also on the congruent assembly-ribbons outer lines,the superhulls (1) and (2) and the underhulls will meet at these. A cross-section in the hull in á and b′ show that a correct hulldesign and parallel keels can be achieved for the V-shabed bottom with a rounded bow. For the surfing hulls is α different,but close to β which make a slight unimportant asymmetric ξ of the bottom.A stippled curve is symmetric about the above mentioned middlenormal m, in order to achieve an evenly-drawn line-design of the longest hull astern when exchanging the underhulls.