Oh my Angelique, those boats made from corrugated sheets are amazing. Would never have thought of that. Don't think they would last long in salt water.
Ok on what is a multi hull... Technically, simply it has more than one hull in the water separate by platform or area raised above the water. A Catamaran is a two hull multi-hull, a trimaran is has tri-hulls etc... If a hull has multiple wings or appendages it is still a monohull since its cg is center in the one hull. A Multi-hull CG is in the center of the different hulls.