The top hatch is a special problem on an ambient-pressure submarine. On a conventional one-atmosphere submarine, outside water pressure helps hold an outward-opening hatch shut. On an ambient submarine, once the cabin is pressurized, the force works in the opposite direction and tries to push that hatch open.

Argonaut Jr is about five feet from keel to deck. With the bottom hatch open, that represents roughly 2.5 psi at the deck. A 20-by-20-inch top hatch has 400 square inches of area, so the air inside can push upward with about 1,000 pounds of force. With two latches, each must resist roughly 500 pounds before adding a safety margin.

The hatch therefore has to be positively dogged down. Heavy draw latches are a modern solution; ordinary trunk-style latches would have been available to Simon Lake in 1894.

Lake may instead have submerged the original Argonaut Jr as a one-atmosphere boat and pressurized it only after reaching the bottom, immediately before opening the lower diver hatch. That operating method also points toward the separate pressurized diver lock he later used in Argonaut I.