Gemini has a simple two part construction, molded hull and molded deck, with a large single interior mold over the bridgedeck giving the dinette seats, steps to each hull, the mold coming back from the forward bulkhead to the aft lockers in the aft cabins. This molding is glassed into the bridgedeck prior to lifting, giving strength by a double skin on the bridgedeck. The hull and deck are mechanically and chemically bonded, overlapping the deck over the hull, bonding with a polyester putty and pulling the seal together with bolts and backing nuts every four inches, then a double gunwhale guard over the bond. The hull is a laminate of woven roving and matt, with polyester resin, over an initial layer of gel coat with vinylester resin and a veil fiberglass fabric for blister protection. The deck has half-inch end grain balsa sandwich on the foredeck, around the cockpit and across the cabin top. No deck fittings are drilled through this sandwich, therefore the integrity of the seal is not destroyed.

The center boards are light, strong mahogany plywood, two inches thick, a very simple rope and drum mechanism has resulted in a negligible number of problems of any kind with the centerboards. The case is molded as part of each hull, with no joins or seals, therefore it is virtually impossible to split or break the centerboard case. The mechanism of the pivoting centerboard allows the board to kick up if run aground, and the construction allows for extremely easy use, positive buoyancy means that the board is locked down, and floats up, unlike regular centerboards on monohulls that are usually weighted and need cranking up into the hull.

The 3400 has two underhung spade rudders, which are also adjustable to the eighteen inches draft of the boat, whilst still being steerable. These rudders will also kick up if necessary.