The gunwale (pronounced gunnel) is the topmost outer edge of a vessels structural hull (at the deck edge - i.e., plating may continue above the gunwale, see bulwarks). The gunwale serves to keep water off the deck of the vessel as well as keeping passengers and crew aboard. The term gunwales is derived from their historical role as reinforcement for shipborne artillery, allowing for the mounting of gunnery on the side of the vessel. Gunwales are most common on small boats, particularly canoes. Here, the gunwale is a reinforced structural member which helps maintain a vessels shape (particularly if it is framed). As well, the gunwales provide abrasion protection, hand-holds, a mounting point for thwarts & tie-down points (if scuppered). If of split construction, the inside strip is known as a inwale, the outside strip as a outwale, and any outboard mounted protective member is known as a fender. Gunwale heights and strength requirements as well as requirements for freeing ports are set forth by classification societies.
Due to their high and broad location, the maximum breadth (baring tumblehome) and maximum draught of the vessel will likely be at the gunwales. The term "to the gunwales" derives from a boat loaded to it's maximum displacement, the waterline at the gunwales - an overloaded condition.