Does your boat currently have moulding? Are you talking about redoing your moulding or adding it to a boat that currently does not have it? Moulding typically serves two purposes, it hides the bond line where the top deck meets the hull, and is a bumper rail for docks and such. It would depend on if the boat is a shoebox joint or butt joint bond to determine whether or not it makes the boat more rigid or less rigid.