The subject line scares me. Home made catamarans are often a disaster waiting to happen, unlike monohulls. If you build a monohull in your backyard, the structure is probably going to hold together. The inherent stiffness of a single monocoque hull makes it hard to screw up too bad. Not so in a catamaran. You have two hulls set widely apart and the strength of the connecting structure and its attachments to the hulls is critical. The stresses at those attachment points are enormous.

David